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IBM Car Amplifier 7220 User's Manual

1. and uses a low noise LN current to voltage converter Input This control has four settings F1t DC The shells of the A and B T connectors are connected to chassis ground via a 1 kQ resistor and the signal channel input is DC coupled FIt AC The shells of the A and B T connectors are connected to chassis ground via a 1 KQ resistor and the signal channel input is AC coupled Note that DC coupling should be used at frequencies of 1 Hz Gnd DC The shells of the A and B T connectors are connected to chassis ground and the signal channel input is DC coupled Gnd AC The shells of the A and B T connectors are connected to chassis ground and the signal channel input is AC coupled Note that DC coupling should be used at frequencies of 1 Hz Device This control allows the selection of the input device when operating in voltage input mode and has two settings FET Uses a FET as the input device for which case the input impedance is 10 MQ This is the usual setting Bipolar Uses a bipolar device in the input stage for the lowest possible voltage input noise In this case the input impedance is 10 kQ Note that this selection is not possible when using the DC coupled input modes 5 3 Chapter 5 FRONT PANEL OPERATION 5 2 02 Reference Setup Menu Left hand LCD Right hand LCD Ref Source Harmonic Demod Mon m INTERNAL 1st OFF vailable control EXT FRONT 2nd
2. 2 2 Instruments are normally shipped from the factory with the line voltage selector set to 110 130 V AC unless they are destined for an area known to use a line voltage in the 220 260 V range in which case they are shipped configured for operation from the higher range The line voltage setting can be seen through a small rectangular window in the line input assembly on the rear panel of the instrument figure 2 1 Ifthe number showing is incorrect for the prevailing line voltage refer to table 2 1 the barrel selector will need to be repositioned as follows Observing the instrument from the rear note the plastic door immediately adjacent to the line cord connector figure 2 1 on the left hand side of the instrument When the line cord is removed from the rear panel connector the plastic door can be opened outwards by placing a small flat bladed screwdriver in the slot on the right hand side and levering gently This gives access to the fuse and to the voltage barrel selector which is located at the right hand edge of the fuse compartment Remove the barrel selector with the aid of a small screwdriver or similar tool With the barrel selector removed four numbers become visible on it 100 120 220 and 240 only one of which is visible when the door is closed Table 2 1 indicates the actual line voltage range represented by each number Position the barrel selector such that the required number see table 2 1 will be visible when t
3. Number reported here is instrument s Dynamic Reserve in dB at current Sensitivity and AC Gain settings Figure 5 15 Sensitivity Control Voltage Input Mode When set to current input mode the instrument s full scale current sensitivity may be set to any value between 20 fA and 1 uA wide bandwidth mode or 20 fA and 10 nA low noise mode in a 1 2 5 sequence Left hand LCD SEN Available LUA control 500nA settings 50 fA 20fA DR Number reported here is instrument s Dynamic Reserve in dB at current Sensitivity and AC Gain settings Figure 5 16 Sensitivity Control Wide Band Current Input Mode Left hand LCD ISEN Available 10nA control DnA settings 50 fA 20fA Figure 5 17 Sensitivity Control DR Number reported here is instrument s Dynamic Reserve in dB at current Sensitivity and AC Gain settings Low noise Current Input Mode 5 21 Chapter 5 FRONT PANEL OPERATION AC Gain If the AC Gain control is set to Manual using the Input Setup menu then this control allows it to be adjusted from 0 dB to 90 dB in 10 dB steps although not all settings are available at all full scale sensitivity settings Left hand LCD IAC GAIN 0dB Available 10dB s 80dB 90dB Figure 5 18 AC Gain Control Time Constant The time constant of the output filters is set using this control The settings between 10 us and 640 us are in a binary sequence and apply only to outputs at the rear p
4. The value of n sets the time constant of the output according to the following table n time constant 0 10 us 1 20 us 2 40 us 3 80 us 4 160 us 5 320 us 6 640 us 7 5 ms 8 10 ms 9 20 ms 10 50 ms 11 100 ms 12 200 ms 13 500 ms 14 ls 15 2s 16 5s 6 14 Chapter 6 COMPUTER OPERATION 17 10s 18 20s 19 50s 20 100 s 21 200 s 22 500 s 23 1 ks 24 2 ks 25 5 ks The TC command is only used for reading the time constant and reports the current setting in seconds Hence if a TC 11 command were sent TC would report 11 and TC would report 1 0E 01 i e 0 1 s or 100 ms SYNC n Synchronous time constant control At reference frequencies below 10 Hz if the synchronous time constant is enabled then the actual time constant of the output filters is not generally the selected value T but rather a value equal to an integer number of reference cycles If T is greater than 1 cycle the time constant is between T 2 and T The parameter n has the following significance n Effect 0 Synchronous time constant disabled 1 Synchronous time constant enabled 6 4 04 Signal Channel Output Amplifiers XOF n n X output offset control The value of n sets the status of the X offset facility according to the following table Ii Selection 0 Disables offset facility 1 Enables offset facility The range of n is 30000 corresponding to 300 full scale YOF n n Y output offset control The value of n sets the status of the
5. vaue 9 999 display f 9 999 range 10 000 Figure 5 40 ADC 2 Input 5 33 Chapter 5 FRONT PANEL OPERATION 5 34 X Offset Right hand LCD XOF fs Available control settings 299 99 300 00 Figure 5 41 X Output Offset Control This control allows the X output offset to be adjusted using the lower ADJUST keys adjacent to the displayed value Note that although this control adjusts the level of the offset it is only applied if the OFFSET control on the left hand LCD is set to X or BOTH If the X offset is ON then the description on the LCD display of all outputs which would be affected such as X fs or MAG fs is alternated with the warning message OFFSET Y Offset Right hand LCD YOF fs Available control settings 209 99 300 00 Figure 5 42 Y Output Offset Control This control allows the Y output offset to be adjusted using the lower ADJUST keys adjacent to the displayed value Note that although this control adjusts the level of the offset it is only applied if the OFFSET control on the left hand LCD is set to Y or BOTH If the Y offset is ON then the description on the LCD display of all outputs which would be affected such as Y fs or MAG fs is alternated with the Chapter 5 FRONT PANEL OPERATION warning message OFFSET The instrument provides a quick way to switch between the following pairs of outputs by simply pressing simultaneously both ends of the S
6. 1 PRINT A 160 WEND 180 PRINT ERROR NO ERR RESUME C 2 RS232 Control Program with Handshakes RSCOM2 BAS is a user interface program which illustrates the principles of the echo handshake The program will run on a PC compatible computer either in a Microsoft GW BASIC or QuickBASIC environment or in compiled form The subroutines in RSCOM2 are recommended for incorporation in the user s own programs 10 RSCOM2 9 Feb 96 20 CLS PRINT Lockin RS232 parameters must be set to 9600 baud 7 data bits 1 stop bit and even parity 30 OPEN COM1 9600 E 7 1 CS DS FOR RANDOM AS 1 40 CR CHR 13 carriage return 50 60 main loop 70 WHILE 1 infinite loop 80 INPUT command 00 to exit B no commas are allowed in B 90 IF B 00 THEN END C 1 Appendix C DEMONSTRATION PROGRAMS 100 B B CR append a carriage return 110 GOSUB 180 output the command B 120 GOSUB 310 PRINT Z read and display response 130 IF A THEN GOSUB 410 GOSUB 470 if prompt fetch STATUS 140 and display message 150 WEND return to start of loop 160 170 180 output the string B 190 ON ERROR GOTO 510 enable error trapping 200 IF LOC 1 gt 0 THEN A INPUT LOC 1 1 clear input buffer 210 ON ERROR GOTO 0 disable error trapping 220 FOR J1 1 TO LEN B LEN B is number of bytes 230 C MID B J196 1 PRINT 1 C send byte 240 WHILE LOC 1 20 WEND wait f
7. Data bits Stop bits Parity Prompt character Character echo Delimiter Address GPIB interface settings Address Terminator SRQ mask byte Test Echo Digital outputs Sample rate adjustment Off Manual Sync 200 Hz On 0 9600 Even On 044 12 CR LF 0 Disabled 0 i e DO D7 are at logic zero 0 Index command 6 18 E 16 N n command 6 28 E 12 2F reference mode 3 7 5 4 8 bit programmable output port 3 8 90 Key 4 5 Absolute accuracy 3 8 AC Gain and full scale sensitivity 3 9 and input overload 3 4 control 5 8 5 22 description of 3 4 AC GAIN n command 6 12 E 1 AC DC input coupling 3 2 Accuracy 3 8 Active cursor 4 3 ADC sample rate control 5 17 sampling frequency 3 4 trigger control 5 10 ADC n command 6 21 E 1 ADCI auxiliary input 3 8 connector 4 8 display 5 33 ADC2 auxiliary input 3 8 connector 4 8 display 5 33 ADF command 6 30 E 1 ADJUST keys 4 2 Analog meter 3 9 4 1 4 6 Analog to digital converter ADC 3 4 Anti aliasing filter 3 4 AQN command 6 13 E 1 AS command 6 11 E 1 ASM command 6 12 E 1 Auto functions Auto Measure 3 9 3 16 5 19 Auto Offset 3 15 5 19 Auto Phase 3 11 3 15 5 18 Auto Sensitivity 3 9 3 15 5 18 introduction 3 14 menu 5 18 AUTO key 4 4 Index AUTO menu 4 4 Auto repeat 4 3 AUTOMATIC n command 6 12 E 1 AUX RS232 connector 4 7 Auxiliary ADCs 3 8 Auxiliary RS232 interface 6 4 AXO command 6 16 E 2 Baseband referenc
8. In floating point mode the LR command reports a number equivalent to log X ADC 1 The response range is 3 000 to 42 079 NHZ Causes the lock in amplifier to respond with the square root of the noise spectral density measured at the Y output expressed in volt VHz or amp V Hz referred to the input This measurement assumes that the Y output is Gaussian with zero mean Section 3 10 The command is only available in floating point mode This command is not available when the reference frequency exceeds 60 kHz 6 17 Chapter 6 COMPUTER OPERATION 6 18 ENBW Equivalent noise bandwidth In fixed point mode reports the equivalent noise bandwidth of the output low pass filters at the current time constant setting in microhertz In floating point mode reports the equivalent noise bandwidth of the output low pass filters at the current time constant setting in hertz This command is not available when the reference frequency exceeds 60 kHz Noise output In fixed point mode causes the lock in amplifier to respond with the mean absolute value of the Y output in the range 0 to 12000 full scale being 10000 If the mean value of the Y output is zero this is a measure of the output noise In floating point mode causes the lock in amplifier to respond in volts STAR n Star mode setup command The star mode allows faster access to instrument outputs than is possible using the conventional commands listed above The mode i
9. of full scale After the Auto Sensitivity function is called Auto Sensitivity operations continue to be made until the required criterion is met In the presence of noise or a time varying input signal it may be a long time before the Auto Sensitivity sequence comes to an end and the resulting setting may not be what is really required AUTO PHASE In an Auto Phase operation the value of the signal phase is computed and an appropriate phase shift is introduced into the reference channel so as to bring the value of the signal phase to zero The intended result is to null the output of the Y channel while maximizing the output of the X channel Any small residual phase error can normally be removed by calling Auto Phase for a second time after a suitable delay to allow the outputs to settle The Auto phase facility usually gives good results when the X and Y channel outputs Chapter 5 FRONT PANEL OPERATION are steady implying the signal phase is stable when the procedure is called If a zero error is present on the outputs such as may be caused by unwanted coupling between the reference and signal channel inputs then the following procedure should be adopted 1 Remove the source of input signal without disturbing any of the connections to the signal input which might be picking up interfering signals from the reference signal In an optical experiment for example this could be done by shielding the detector from the source o
10. 1kQ lt 640 us 10 V 170 kHz 1kQ Signal Monitor Amplitude Impedance Aux D A Output 1 2 Maximum Output Resolution Accuracy 0 1 Output Impedance 8 bit Digital Output Reference Output Waveform Impedance Power Low Voltage Data Storage Data Buffer Size Max Storage Rate From LIA From ADC Interfaces Power Requirements Voltage Frequency Power Appendix A SPECIFICATIONS 10 V FS 1kQ 10 V 1mV 1kQ 8 TTL compatible lines that can be independently set high or low to activate external equipment 0 to 5 V square wave TTL compatible 15 V at 100 mA rear panel DIN connector for powering EG amp G preamplifiers 32k 16 bit data points may be organized as 1x32k 2x16k 3x10 6k 4x8k etc up to 800 16 bit values per second up to 40 000 16 bit values per second RS232 IEEE 488 A auxiliary RS232 port is provided to allow daisy chain connection and control of multiple units from a single RS232 computer port 110 120 220 240 VAC 50 60 Hz 40 VA AS Appendix A SPECIFICATIONS General Dimensions Width Depth Height With feet Without feet Weight A 6 432 mm 17 415 mm 16 4 74 mm 2 9 60 mm 2 4 7 4 kg 16 3 Ib Pinouts Appendix B B 1 RS232 Connector Pinout Figure B 1 RS232 and AUX RS232 Connector Female PIN FUNCTION COMMENT 2 RXD Data In 3 TXD Data Out 5 GND Signal Ground 7 RTS Always at 12 V All other pins are not c
11. 3 2 07 Reference Channel DSP ceci cian tes ew eee hee Ea eee alee 3 6 3 2 08 Internal Oscillator 5 1 cau a eo de de teething tees be vase nee eda eddie des dd 3 7 3 2 09 Demodulator DSP sisi 4 e3 ee cb pr dee sanii dee oe ba eRe Be vale ae ala es 3 7 3 210 Output PIOCESSOF eee dece ug ae eee gue ed Ee ae eq ete ws ai Deen ta e e ci 3 8 35 2 14 Main Microprocessor sus eee testis pa CER ARR hee QUE Cee ud vex Rd NE RR ala 3 8 SiS UACCULACY P 3 8 3 3 01 Absolute Accuracy Specifications llle 3 8 TABLE OF CONTENTS 3 3 02 Relative ACCUTACY ect A e e A a EE E e ENTRE es s 3 9 3 4 Full Scale Sensitivity and AC Gain Control ooocooccccccococon e 3 9 3 2 Dynamic R serve isse id ride a dq bd a AAA 3 9 3 6 System Updates and Reference Frequency Changes 0 0 cece een 3 10 3 7 Reference Phase and Phase Shifter siria exa 00590054008 eR obe eb uh aa 3 10 3 8 Output Channel Filters deis RR Ree et pese dana eire ae d e do 3 11 SERI 3 11 3 8 02 Time Constants and Synchronization 0 0 0 cece es 3 12 3 8 09 Output Offset and Expand xe srs nissan eph eR ere ia cad ncaa a LEGE whades 3 12 3 9 Use of Magnitude and Signal Phase Outputs ooooocococococor teenies 3 12 3 10 Noise Measurements iive ir rer Hee e Hear REC EY E Ra dudes ted ds 3 14 3 11 Power up Defaults 5 2 monere mete Re e eR RE Ru E ea E e e a bor as 3 14 3 12 Auto FunctioBs exis sara ER cde eae EI OG PA
12. FS Phase 1 9 V 180 9 V 180 Phase 2 9 V 360 9 V 0 Noise FS Ratio 1000 x X ADC 1 DA hu Dro n is compulsory and is either 1 for CH1 or 2 for CH2 Input coupling control The value of n sets the input coupling mode according to the following table n Coupling mode AC DC DAC n n Auxiliary DAC output controls Sets the voltage appearing at the rear panel DACI and DAC2 outputs The first parameter n which specifies the DAC is compulsory and is either 1 or 2 The value of n specifies the voltage to be output In fixed point mode it is an integer in the range 12000 to 12000 corresponding to voltages from 12 000 V to 12 000 V In floating point mode it is in volts DC n Dump acquired curve s to computer In fixed point mode causes a stored curve to be dumped via the computer interface in decimal format In floating point mode the SEN curve bit 4 in CBD must have been stored if one or more of the following outputs are required in order that the lock in amplifier can perform the necessary conversion from FS to volts or amps X Y Magnitude Noise One curve at a time is transferred The value of n is the bit number of the required curve which must have been stored by the most recent CBD command Hence n can range from 0 to 15 If for example CBD 5 had been sent equivalent to asserting bits 0 and 2 then the X and Magnitude outputs would be stored The permitted values of n w
13. Figure 3 1 Model 7220 Block Diagram 3 1 Chapter 3 TECHNICAL DESCRIPTION 3 2 3 2 02 Signal Channel Inputs The signal input amplifier may be configured for either single ended or differential voltage mode operation or single ended current mode operation In voltage mode a choice of AC or DC coupling is available and the input may be switched between FET and bipolar devices In current mode two conversion gains are selectable to allow for optimum matching to the signal input In both modes the input connector shells may be either floated or grounded to the instrument s chassis ground These various features are discussed in the following paragraphs Input Connector Selection A A B When set to the A mode the lock in amplifier measures the voltage between the centre and the outer of the A input BNC connector whereas when set to A B mode it measures the difference in voltage between the centre pins of the A and B input connectors The latter differential mode is often used to eliminate ground loops although it is worth noting that at very low signal levels it may be possible to make a substantial reduction in unwanted offsets by using this mode with a short circuit terminator on the B connector rather than by simply using the A input mode The specification defined as the Common Mode Rejection Ratio CMRR defines how well the instrument rejects common mode signals applied to the A and B inputs when operating in d
14. If an 8 bit data word is selected the most significant bit is set to zero on output from the lock in amplifier and ignored on input The result is that either the 8 bit or the 7 bit option may be used but the 7 bit option can result in slightly faster communication 6 3 05 Choice of Parity Check Option Parity checks are not required at the baud rates available in the model 7220 that is up to 19 200 baud with typical cable lengths of up to a few meters Therefore no software is provided in the model 7220 for dealing with parity errors Where long cables are in use it may be advisable to make use of a lower baud rate The result is that any of the parity check options may be used but the no parity option will result in slightly faster communication 6 3 Chapter 6 COMPUTER OPERATION 6 4 Where the RS232 parameters of the terminal or computer are capable of being set to any desired value an arbitrary choice must be made In the model 7220 the combination set at the factory is even parity check 7 data bits and one stop bit fixed because these are the MS DOS default 6 3 06 Auxiliary RS232 Interface The auxiliary RS232 interface allows up to sixteen model 7220s or a mixture of compatible instruments to be connected to one serial port on the computer The first lock in amplifier is connected to the computer in the usual way Additional lock in amplifiers are connected in a daisy chain fashion using null modem cables the AUX RS232
15. In floating point mode causes the lock in amplifier to respond with the signal phase in degrees REFP n Reference phase control In fixed point mode n sets the phase in millidegrees in the range 3360000 In floating point mode n sets the phase in degrees REMOTE n Remote only front panel lock out control Allowed values of n are 0 and 1 When n is equal to 1 the lock in amplifier enters remote only mode in which the front panel control functions are inoperative and the instrument can only be controlled with the RS232 or the GPIB interfaces When n is equal to 0 the front panel control functions normally REV Report firmware revision Causes the lock in amplifier to respond with the firmware revision number This gives a four line response which the controlling program must be able to accept E 13 Appendix E ALPHABETICAL LISTING OF COMMANDS RS n n Set read RS232 interface parameters The values of n set the baud rate of the RS232 interface according to the following table n Baud rate bits per second 0 75 1 110 2 134 5 3 150 4 300 5 600 6 1200 7 1800 8 2000 9 2400 10 4800 11 9600 12 19200 The lowest five bits in n control the other RS232 parameters according to the following table bit number bit negated bit asserted 0 data parity 8 bits data parity 9 bits 1 no parity bit 1 parity bit 2 even parity odd parity 3 echo disabled echo enabled 4 prompt disabled prompt enabled RTT Rat
16. Left hand LCD lOSC 0 001 V rms ae 0 002 V rms settings 4 999 V rms 5 000 V rms Figure 5 22 Internal Oscillator Amplitude Control DAC 1 This control sets the voltage appearing at the rear panel DAC1 output connector to any value between 10 V and 10 V with a resolution of 1 mV Adjustment is faster if use is made of the Active Cursor control see section 4 1 04 Left hand LCD IDAC1 10 000 V ou qute s settings 9 999 V 10 000 V Figure 5 23 DAC 1 Output Control 5 24 Chapter 5 FRONT PANEL OPERATION DAC 2 This control sets the voltage appearing at the rear panel DAC2 output connector to any value between 10 V and 10 V with a resolution of 1 mV Adjustment is faster 1f use is made of the Active Cursor control see section 4 1 04 Left hand LCD IDAC2 10 000 V ole settings z 9 999 V 10 000 V Figure 5 24 DAC 2 Output Control Offset This control allows an output offset to be added to the X output Y output neither or both outputs The actual value of offset applied which may range from 300 to 300 of the selected full scale sensitivity setting is set using the right hand LCD panel or automatically by the Auto Offset function Note that the Auto Offset function automatically switches both X and Y output offsets on Left hand LCD OFFSETS OFF Available X ONLY control settings Y ON LY X amp Y Figure 5 25 Output Offset Status Control 5 25 Chapter 5 FRONT PANEL OPERA
17. shared equally between the specified curves For example if all 16 outputs are stored then the maximum number of storage points would be 2048 i e 32768 16 The LEN command sets the actual curve length which cannot therefore be longer than 32768 divided by the number of curves selected If more curves are requested than can be stored with the current buffer length then the buffer length will be automatically reduced Its actual length can of course be determined by sending the LEN command without a parameter The reason why bit 4 is needed which stores both the sensitivity and the IMODE setting is to allow the instrument to transfer the acquired curves to the computer in floating point mode Without this information the unit would not be able to determine the correct calibration to apply Curves 14 and 15 store the reference frequency in millihertz The calculation needed to translate these two 16 bit values to one 32 bit value is Reference Frequency 65536 x value in Curve 15 value in Curve 14 Note that the CBD command directly determines the allowable parameters for the DC and HC commands It also interacts with the LEN command and affects the values reported by the M command E 3 Appendix E ALPHABETICAL LISTING OF COMMANDS E 4 CP n CH n n Analog output control Defines what outputs appear on the rear panel CH1 and CH2 connectors according to the following table N Signal X FS Y FS Magnitude
18. 10 Hz steps Now we need to specify the curve storage 6 33 Chapter 6 COMPUTER OPERATION 6 34 NC Clear and reset curve buffer CBD 49180 Stores Magnitude Phase Sensitivity and Frequency 1 e bits 2 3 4 14 and 15 LEN 100 Number of points 100 STR 100 Store a point every 100 ms must match SRATE parameter The data may now be acquired by issuing the compound command TD SWEEP 1 Starts sweep and curve acquisition Note that the order of these two commands is important If used as shown then the data will be acquired and then the oscillator frequency will be changed at each data point prior to waiting the time set by the SRATE and STR commands This gives sufficient time for the instrument output to stabilize after each change of frequency If the commands were used in the reverse order i e SWEEP 1 TD then the output s would never have time to settle by the time at which they were recorded The frequency sweep starts and the magnitude and phase outputs are recorded to the curve buffer As it runs the M command reports the status of the acquisition and once this indicates it is complete i e parameter 1 0 parameter 2 1 the acquired data may be transferred to the computer using DC 2 Transfers Magnitude curve DC 3 Transfers Phase curve DC 14 Transfers Reference frequency lower 16 bits DC 15 Transfers Reference frequency upper 16 bits Specifications Measurement Modes In phase Quadrature Magnitude P
19. 22 Default Settings Appendix F Default Setting Function The default setting function sets the model 7220 s controls and output displays as follows Left hand LCD Displays the AC Gain control on the upper line and the full scale sensitivity control on the lower line Right hand LCD Displays the magnitude as a percentage of full scale output on the left hand side and the phase angle in degrees output on the right hand side The ten controls accessed via the left hand LCD are set to the following values Full scale sensitivity 500 mV AC Gain 0 dB Time constant 100 ms Slope 12 dB octave Oscillator frequency 1000 000 Hz Oscillator amplitude 0 500 mV rms DAC output 0 000 V DAC2 output 0 000 V Output offsets OFF Phase 0 00 The two output offset controls accessed via the right hand LCD are set as follows X channel output offset 0 00 96 Y channel output offset 0 00 96 The remaining instrument controls accessed via the setup menus are set as follows Input mode Single ended voltage mode A input connector Coupling AC Input connector shell Floating Input device FET Reference mode Internal Reference harmonic Ist Demodulator monitor Off Output expansion Off CHI analog output X 96 CA2 analog output Y 96 Appendix F DEFAULT SETTINGS F 2 Line frequency rejection filter AC Gain control Time constant mode ADC trigger rate Front panel lights Display contrast RS232 interface settings Baud rate
20. Auto Measure function is most often used as a rescue operation to bring the instrument into a well defined state when it is giving unexpected results The length of the above list demonstrates that one or more items can easily be overlooked if performed manually 5 4 Main Display Mode Left hand LCD 5 20 The Main Display Mode is the default mode on instrument power up Two controls chosen from a possible ten may be adjusted using the left hand display although it is not possible to show the same control on both lines of the display simultaneously The choice of control is made using the SELECT keys adjacent to the left hand side of the left hand display Once selected the control is adjusted using the ADJUST keys adjacent to the right hand side of the left hand display The ten controls are now discussed in turn Note that in the following figures the vertical lines below the heading Left hand LCD depict the horizontal limits of the display to indicate how control information is shown It should be remembered that there are two lines available on the display and the user may choose to show a control on either the upper or the lower line see figure 4 4 Sensitivity When set to voltage input mode the instrument s full scale voltage sensitivity may be set to any value between 20 nV and 1 V in a 1 2 5 sequence Chapter 5 FRONT PANEL OPERATION Left hand LCD ISEN Available 1V control 500mV settings 50nV 20nV DR
21. Control Control Selection Keys Description Setting Adjustment Keys SELECT ADJUST 4 lv SEN 1V DR 6 a v A v TIME CONST 500ms Ta Ty SELECT ADJUST d Es INSTRUMENTS Lower Menu Control Control Control Control Selection Keys Description Setting Adjustment Keys Figure 4 4 Main Display Left hand LCD To select a given control press the left hand up or down SELECT keys repeatedly until it is displayed on the corresponding menu line The current setting of the control is then shown adjacent to the description and may be adjusted using the corresponding left hand up or down ADJUST keys Some controls such as time constant and full scale sensitivity have only a limited range of settings and so the use of the ADJUST keys allows the required value to be chosen with only a few keypresses Other controls such as the internal oscillator amplitude and frequency may be set over a wide range of values and to a high Chapter 4 FRONT AND REAR PANELS precision In these cases a significant number of keypresses are required to make adjustments Adjustment of the latter type of control is made easier by the use of one or other of the two methods described below Auto Repeat If an up or down ADJUST key is pressed and held then its action is automatically repeated such that the displayed control setting will increment or decrement at a rate approximately ten times faster than that which c
22. Front Panel Operation Describes the capabilities of the instrument when used as a manually operated unit and shows how to operate it using the front panel controls Chapter 6 Remote Operation This chapter provides detailed information on operating the instrument from a computer over either the GPIB IEEE 488 or RS232 interfaces It includes information on how to establish communications the functions available the command syntax and a detailed command listing Appendix A Gives the detailed specifications of the unit Appendix B Details the pinouts of the multi way connectors on the rear panel Appendix C Lists three simple terminal programs which may be used as the basis for more complex user written programs 1 1 Chapter 1 INTRODUCTION Appendix D Shows the connection diagrams for suitable RS232 null modem cables to couple the unit to an IBM PC or 100 compatible computer Appendix E Gives an alphabetical listing of the computer commands for easy reference Appendix F Provides a listing of the instrument settings produced by using the default setting function If you are a new user it is suggested that you unpack the instrument and carry out the procedure in chapter 2 to check that it is working satisfactorily You should then make yourself familiar with the information in chapters 3 4 and 5 even if you intend that the unit will eventually be used under computer control Only when you are fully conversant with op
23. Noise in Volts or Amps per Root Hertz Right hand LCD N Hz Vor Output value display 22nV range Figure 5 36 Noise Output in Volts or Amps per Root Hertz Shows the noise accompanying the signal in a noise bandwidth defined by the setting of the output filter time constant where it is assumed that the noise is Gaussian Note that the noise floor of the instrument is 2 nV and so the output value will always be greater than this 5 31 Chapter 5 FRONT PANEL OPERATION Ratio Right hand LCD RATIO 30000 29999 29999 30000 Output value display range Figure 5 37 Ratio Output Shows the ratio where ratio X output ADC1 Input usually used to compensate for source intensity fluctuations in optical experiments Log Ratio Right hand LCD LRATIO 2 079 value 2 078 display 5 999 Output range 3 000 Figure 5 38 Log Ratio Output Shows the logarithm to base 10 of the ratio where ratio X output ADC1 Input usually used to compensate for source intensity fluctuations in optical experiments 5 32 ADCI Volts Shows the voltage applied to the rear panel ADCI auxiliary input ADC2 Volts Shows the voltage applied to the rear panel ADC2 auxiliary input Chapter 5 FRONT PANEL OPERATION Right hand LCD ADC1 V 10 000 vaue 9 999 display f 9 999 range 10 000 Figure 5 39 ADC 1 Input Right hand LCD ADC2 V 10 000
24. Note that the demodulator gain is adjusted automatically when the AC Gain value is changed in order to maintain the SEN value However the user is prevented from setting an illegal AC Gain value i e one that would result in overload on a full scale input signal Similarly if the user selects a SEN value which causes the present AC Gain value to be illegal the AC Gain will change to the nearest legal value In practice this system is very easy to operate However the user may prefer to make use of the AUTOMATIC AC Gain facility which gives very good results in most circumstances Note that when signal channel overload occurs the only action required is to reduce the AC Gain At reference frequencies above 1 Hz the Auto Sensitivity and Auto Measure functions can be used to adjust the full scale sensitivity 3 5 Dynamic Reserve At any given setting the ratio DR z 0 7 INPUT LIMIT FULL SCALE SENSITIVITY represents the factor by which the largest acceptable sinusoidal interference input exceeds the full scale sensitivity and is called the Dynamic Reserve of the lock in amplifier at that setting The factor 0 7 is a peak to rms conversion The dynamic 3 9 Chapter 3 TECHNICAL DESCRIPTION reserve is often expressed in decibels for which DR in dB 20 log DR as a ratio Applying this formula to the model 7220 we may put in the maximum value of INPUT LIMIT 3 V and the smallest available value of FULL SCALE SENSITIVI
25. Off Enable 50 or 60 Hz notch filter Enable 100 or 120 Hz notch filter Enable both filters WNr Os Users may identify which version of the instrument they have by sending the command LF 3 if this is accepted by the instrument it was made after June 1996 but if it generates a command error it was made prior to this date Units made after June 1996 respond in addition to the command LINESO which sets the notch filter centre frequency LINESO n Signal channel line frequency rejection filter centre frequency control The value of n sets the line frequency notch filter centre frequency according to the following table n Notch filter mode 0 60Hz and or 120 Hz 1 50 Hz and or 100 Hz Units made prior to June 1996 generate an Invalid Command bit 1 of the serial poll status byte is asserted to the LINE50 command LOCK System lock control Updates all frequency dependent gain and phase correction parameters LR Log ratio output In integer mode the LR command reports a number equivalent to 1000 log X ADC1 where X is the value that would be returned by the X command and ADC1 is the value that would be returned by the ADCI command The response range is 3000 to 2079 In floating point mode the LR command reports a number equivalent to log X ADC 1 The response range is 3 000 to 2 079 E 10 Appendix E ALPHABETICAL LISTING OF COMMANDS LTS n Lights on off control The value of n controls the front pan
26. SEN curve bit 4 in CBD must have been stored if one or more of the following outputs are required in order that the lock in amplifier can perform the necessary conversion from FS to volts or amps X Y Magnitude Noise One curve at a time is transferred The value of n is the bit number of the required curve which must have been stored by the most recent CBD command Hence n can range from 0 to 15 If for example CBD 5 had been sent equivalent to asserting bits 0 and 2 then the X and Magnitude outputs would be stored The permitted values of n would therefore be 0 and 2 so that DC 0 would transfer the X output curve and DC 2 the Magnitude curve 6 25 Chapter 6 COMPUTER OPERATION 6 26 The computer program s subroutine which reads the responses to the DC command needs to run a FOR NEXT loop of length equal to the value set by the LEN curve length command Note that when using this command with the GPIB interface the serial poll must be used After sending the DC command perform repeated serial polls until bit 7 is set indicating that the instrument has an output waiting to be read Then perform repeated reads in a loop waiting each time until bit 7 is set indicating that a new value is available The loop should continue until bit 1 is set indicating that the transfer is completed DCTn Dump acquired curves to computer in table format This command is similar to the DC command described above but allows transfer of seve
27. Ver 3 0 Available 1 control settings 14 Figure 5 9 RS232 Setup 3 Menu The third setup menu with controls affecting the RS232 interface is shown in figure 5 9 Address This control sets the RS232 address which is used when daisy chaining other compatible instruments Each instrument used in the chain must be set to a unique address in the range O to 15 All instruments receive commands but only the currently addressed instrument will implement or respond to the commands except of course the command to change the instrument to be addressed Information only The right hand LCD displays the version level of the instrument s internal firmware Please be prepared to give this number when contacting EG amp G Instruments with a technical query 5 13 Chapter 5 FRONT PANEL OPERATION 5 14 5 2 09 GPIB Setup 1 Menu Left hand LCD Right hand LCD GPIB Address Terminator SETUP1 Available 1 EOI control settings 30 CR 31 Figure 5 10 GPIB Setup 1 Menu This menu shown in figure 5 10 has two controls affecting the GPIB computer interface as follows Address Each instrument used on the GPIB interface must have a unique address in the range 0 to 31 and this control is used to set this address The default setting is 12 Terminator This control selects the method by which the instrument signals to the controlling computer the completion of transmission of a response to a command Three choices are
28. activate one of the auto functions press the appropriate adjacent key as shown in figure 4 7 The display will immediately change to a message indicating that the selected function is in progress and will revert to the Main Display mode when the function is completed When in the AUTO menu a second press of the MENU key affects both the left and right hand displays and sets these to the setup menu mode It is from this mode that the instrument controls which rarely need adjusting once set for a given experiment are activated see figure 4 8 which shows a typical setup menu Chapter 4 FRONT AND REAR PANELS Setup menu Setup menu Menu item 1 Menu item 2 Menu item 3 selection keys description description description description SELECT ADJUST SELECT SELECT a v REF Ref Source a fv 4 v Harmonic Psd Ref a 7 4 v MENU INTERNAL v GT 1st Mon OFF AJ v SELECT ADJUST ADJUST ADJUST ds EGsG INSTRUMENTS 7220 DSP LOCK IN AMPLIFIER Menu item 1 parameter Menu item 2 parameter Menu item 3 parameter adjustment keys adjustment keys adjustment keys Figure 4 8 Setup Menu Mode Left and Right hand LCD Displays In the setup menu mode the left hand SELECT keys adjacent to the left hand display cycle through a series of twelve setup menus In general each menu allows three controls to be adjusted one via the right hand side of the left hand display and the other two via th
29. control 5 15 Handshaking and echos 6 5 Harmonic measurement selection 5 4 Harmonic measurements 3 7 HC command 6 25 E 9 Highband reference mode 3 6 ID command 6 29 E 9 IE n command 6 13 E 9 IMODE n command 6 10 E 9 Initial checks 2 3 Input connector ground float 5 3 connector selection 5 2 connector shell ground float 3 2 coupling 3 2 5 3 device selection 3 2 5 3 float ground control 3 2 impedance 3 2 5 3 mode 5 2 mode selection 3 3 overload 3 4 overload indicators 4 1 selection 3 2 setup menu 5 2 Inspection 2 1 Installation 2 1 Internal oscillator 3 7 Internal oscillator frequency sweep 3 8 Internal reference 5 4 Internal reference mode 3 6 Key specifications 1 3 LCD contrast control 5 10 LED indicators 4 1 Left hand LCD display panel 4 2 LEN n command 6 24 E 9 LF n command 6 12 E 10 Lights control 5 10 Line cord 2 1 Line frequency rejection filter 3 3 5 8 Line fuses 2 1 2 2 Line power input assembly 4 6 Line power switch 4 6 Line voltage selection 2 1 LINESO n command 3 3 6 12 E 10 LOCK command 3 10 6 14 E 10 Log ratio display 5 32 LR command 6 17 E 10 LTS n command 6 30 E 11 M command 6 25 E 11 MAG output 5 6 MAG fs display 5 28 MAG in volts amps display 5 31 MAG command 6 17 E 11 Main Display mode 5 1 5 20 Main display mode 4 4 Main microprocessor 3 8 MENU key 4 4 Microsoft Windows Terminal program 6 2 INDEX Miscellaneous opti
30. has three controls affecting the line frequency rejection filter AC Gain control and output time constants as follows Linefilt This control sets the mode of operation of the internal line frequency rejection filter Early instruments have two possible settings for this control ON or OFF Note that in these units the filter introduces significant gain and phase errors when measuring signals in the frequency range from 5 Hz to 500 Hz Instruments manufactured after June 1996 which are fitted with the updated filter hardware offer four possible settings for the control as defined by the following table Legend Function OFF Line filter inactive F Enable 50 or 60 Hz notch filter 2F Enable 100 or 120 Hz notch filter F amp 2F Enable both filters AC Gain As discussed in section 3 2 04 the correct adjustment of the AC Gain in a DSP lock in amplifier is necessary to achieve the best results This control allows the user to select whether this adjustment is carried out automatically or remains under manual control Chapter 5 FRONT PANEL OPERATION MANUAL In this setting the AC Gain may be manually adjusted from the main display AUTOMATIC In this setting the AC Gain value is automatically selected by the instrument depending on the full scale sensitivity TC s This control affects the output time constants and has two settings SYNC When set to SYNC the actual time constant used is chosen to be some multiple of the r
31. layout The following sections describe the instrument control menus in a logical sequence from the setup menus which typically only need changing occasionally through to the output display selection 5 2 Setup Menu Mode When in the normal Main Display mode two presses of the MENU key set the left and right hand displays to the setup menu mode see figure 5 1 which shows a typical setup menu Setup menu Setup menu Menu item 1 Menu item 2 Menu item 3 selection keys description description description description SELECT ADJUST SELECT SELECT a fy REF Ref Source J v v Harmonic Psd Ref v DIO MENU INTERNAL La 7 v tst Mon OFF TA v SELECT ADJUST ADJUST ADJUST ds EGsG INSTRUMENTS 7220 DSP LOCK N AMPLIFIER Menu item 1 parameter Menu item 2 parameter Menu item 3 parameter adjustment keys adjustment keys adjustment keys Figure 5 1 Setup Menu Mode Left and Right hand LCD Displays In the setup menu mode the left hand SELECT keys of the left hand display cycle through a series of twelve setup menus In general each menu allows three controls to be adjusted one via the right hand side of the left hand display and the other two via 5 1 Chapter 5 FRONT PANEL OPERATION 5 2 the right hand display The setup menu description is shown on the left hand side of the left hand display Figure 5 1 makes this clear One further press of the MENU key causes
32. list of F 1 F 2 Delimiters 6 7 Demodulator DSP 3 7 gain 3 9 monitor 3 7 5 4 Differential voltage input mode 3 2 4 1 5 2 Digital displays 3 9 Digital low pass filter 3 7 Digital output port 3 8 DIGITAL OUTPUTS connector 4 7 Digital outputs setup menu 5 16 Digital phase locked loop 3 6 Digital phase shifter 3 6 Direct digital synthesizer DDS 3 6 Direct mode 6 2 Display contrast control 5 10 Dynamic range 3 4 Dynamic reserve 3 9 ENBWL command 6 18 E 6 Equivalent noise bandwidth 3 14 EVENT n command 6 24 E 6 EX n command 6 16 E 7 External reference 5 4 External reference mode 3 6 Index 2 FAST X connector 4 8 output 3 7 FAST Y connector 4 8 output 3 7 FET n command 6 10 E 7 FET input device 3 2 5 3 FIR filters 3 12 Firmware version display 5 13 FLOAT n command 6 10 E 7 Floating point command format 6 7 FNF n command 6 13 E 7 Front panel layout 4 1 operation 5 1 FRQ command 6 14 E 7 FSTART n command 6 19 E 8 FSTEP n n2 command 6 20 E 8 FSTOP n command 6 19 E 8 Full scale sensitivity 3 9 Fuses line 2 1 2 2 General purpose reference input 3 6 GP nl n2 command 6 4 6 6 6 28 E 8 GPIB interface address 6 4 address control 5 14 connector 4 7 general features 6 4 handshaking 6 5 microprocessor support of 3 8 service request 5 15 6 9 service request mask byte 5 15 setup 1 menu 5 14 setup 2 menu 5 15 status byte 5 15 terminator control 5 14 test echo
33. mode can only be used for reading the sensitivity which is reported in volts or amps For example if IMODE 0 and the sensitivity is 1 mV the command SEN would report 18 and the command SEN would report 1 0E 03 If IMODE was changed to 1 SEN would still report 18 but SEN would report 1 0E 09 SLOPE n Output low pass filter slope roll off control The value of n sets the slope of the output filters according to the following table n slope O 6dB octave 1 12dB octave 2 18dB octave 3 24dB octave SRATE n Oscillator frequency sweep step rate Sets the sweep rate in time per step in the range 50 ms to 1000 s in 5 ms increments E 15 Appendix E ALPHABETICAL LISTING OF COMMANDS E 16 ST Report status byte Causes the lock in amplifier to respond with the status byte Note this command is not normally used in GPIB communications where the status byte is accessed by performing a serial poll Bit 0 Command complete Bit 1 Invalid command Bit 2 Command parameter error Bit 3 Reference unlock Bit 4 Overload Bit 5 New ADC values available after external trigger Bit 6 Asserted SRQ Bit 7 Data available STAR n Star mode setup command The star mode allows faster access to instrument outputs than is possible using the conventional commands listed above The mode is set up using the STAR command to specify the output s required and invoked by sending an asterisk ASCII 42 to request the data The data return
34. new ADC values available after external trigger 6 64 asserted SRQ fi 128 data available Hence for example if the SRQ mask byte is set to decimal 16 i e bit 4 is asserted a service request would be generated as soon as an overload occurred if the SRQ mask byte were set to O i e no bits asserted then service requests would never be generated Test echo When this control is enabled all transmissions to and from the instrument via the GPIB interface are echoed to the RS232 interface Hence if a terminal is connected to the latter port it will display any commands sent to the instrument and any responses generated which can be useful during program development When disabled echoing does not occur The control should always be disabled when not using this feature since it slows down communications 5 15 Chapter 5 FRONT PANEL OPERATION 5 16 5 2 11 Digital Outputs Setup Menu Left hand LCD Right hand LCD DIGITAL Decimal 6543210 32 OUTPUTS 10 0 00000000 BEIDE 1 00000001 settings 254 11111110 255 11111111 Figure 5 12 Digital Outputs Setup Menu This menu shown in figure 5 12 is used to control the 8 TTL lines of the rear panel digital output port used for controlling external equipment Decimal The bit pattern appearing at the digital output port is set by an unsigned eight bit binary number the decimal equivalent of which can range from 0 to 255 This decimal value may be set using the left hand d
35. of the signal phase to zero The intended result is to null the output of the Y channel while maximizing the output of the X channel Any small residual phase can normally be removed by calling Auto Phase for a second time after a suitable delay to allow the outputs to settle The Auto Phase facility is normally used with a clean signal which is known to be of the required phase It usually gives very good results provided that the X and Y channel outputs are steady when the procedure is called If a zero error is present it must be nulled with an Auto Offset operation before the signal is applied and Auto Phase executed If the error is due to unwanted interaction between signal and reference crosstalk the Auto Phase while actually setting the correct phase will not give a zero value of signal phase The final step is to remove the signal and execute another Auto Offset operation This is a powerful method of removing the effect of crosstalk which is not generally in the same phase as the required signal 3 12 04 Auto Offset In an Auto Offset operation the X offset and Y offset functions are turned on and are automatically set to the values required to give zero values at both the X and the Y outputs Any small residual values can normally be removed by calling Auto Offset for a second time after a suitable delay to allow the outputs to settle The primary use of the Auto Offset is to cancel out zero errors which are usually caused by un
36. onee na E a a RR E ae ee 6 23 6 4 10 Computer Interfaces RS232 and GPIB sseeeeeeeeee es 6 27 6 4 11 Instrument Identification lseeeeeeee I teen eee 6 29 6 4 12 Front Panel ss eese ccr e ee FOS Lee RE a SORE ERE DOES EE dee vee e 6 30 6 4 13 DeraultSet ng exorcista era edie ead ced aed by KU ata Ra CR EE Said e 6 30 6 5 Programming ExampleS oo ooo ee e I eae 6 30 6 5 01 Introd Ois astra cu aer s orae ecd Nub ri aub used a SUC ide SORA Ria 6 30 6 5 02 Basic Signal Recovety cete ette EE pU Rx A ae e e 6 30 6 5 03 Frequency Response Measurement o oo ococcoc e teens 6 31 6 5 04 X and Y Output Curve Storage Measurement 0 0 0 cece ees 6 32 6 5 05 Transient Recorder vicio scher eR Heavens dewi re ERE Fe ER Ry ere E REC E e 6 32 6 5 06 Frequency Response Measurement using Curve Storage and Frequency Sweep 6 33 Appendix A Specifications TABLE OF CONTENTS Appendix B Pinouts B 1 RS232 Connector PinOut cesi ceva cio deve cies toe Tisa E ca a a e e ae B 1 B 2 Preamplifier Power Connector PiNQUt ooooococccococo e B 1 B 3 Digital Output Port Connector 0 0 0 RR ene n eae B 2 Appendix C Demonstration Programs C 1 Simple Terminal Emulator 224 000 iue Rer esr AA RE RR DE mead ee weed C 1 C 2 RS232 Control Program with Handshakes 1 0 0 0 0 cece tenes C 1 C 3 GPIB User Interface Program coire pidriird eiii ee eee eee eee dea e a ho ea eee ed RE te C 3 Appendix D Ca
37. only allows storage of the X and Y outputs There is no need to issue a CBD 3 command to set this up since it happens automatically when acquisition starts If the time constant is set to 5 ms or longer then the actual time constant applied to the stored X and Y output values will be 640 us but if it is set to a shorter value then this will be the time constant actually used Take data Initiates data acquisition Acquisition starts at the current position in the curve buffer and continues at the rate set by the STR command until the buffer is full Take data continuously Initiates data acquisition Acquisition starts at the current position in the curve buffer and continues at the rate set by the STR command until halted by an HC command The buffer is circular in the sense that when it has been filled new data overwrites earlier points EVENT n Event marker control During a curve acquisition if bit 13 in the CBD command has been asserted the lock in amplifier stores the value of the Event variable at each sample point This can be used as a marker indicating the point at which an experimental parameter was changed The EVENT command is used to set this variable to any value between O and 32767 HC Chapter 6 COMPUTER OPERATION Halt curve acquisition Halts curve acquisition in progress It is effective during both single data acquisition initiated by TD command and continuous data acquisition initiated by TDC command
38. output filters is not generally the selected value T but rather a value equal to an integer number of reference cycles If T is greater than 1 cycle the time constant is between T 2 and T The parameter n has the following significance n Effect 0 Synchronous time constant disabled Synchronous time constant enabled E 17 Appendix E ALPHABETICAL LISTING OF COMMANDS E 18 SYNCOSC n Synchronous oscillator demodulator monitor control This control operates only in external reference mode The parameter n has the following significance n 0 1 Effect Synchronous Oscillator Demodulator Monitor disabled Synchronous Oscillator Demodulator Monitor enabled When enabled and in external reference mode the instrument s OSC OUT connector functions as a demodulator monitor of the X channel demodulation function TADC n Auxiliary ADC trigger mode control The value of n sets the trigger mode of the auxiliary ADC inputs according to the following table n Trigger mode 0 Asynchronous 5 ms intervals External rear panel TRIG input 2 Burst mode fixed rate triggered by command ADC1 only 3 Burst mode fixed rate triggered by command ADC1 and ADC2 4 Burst mode variable rate triggered by command ADCI only 5 Burst mode variable rate triggered by command ADC1 and ADC2 6 Burst mode fixed rate External trigger rear panel TRIG input ADC1 only 7 Burst mode fixed rate External trigger rear panel TRIG input ADC1 an
39. port of the first to the RS232 port of the second the AUX RS232 port of the second to the RS232 port of the third etc The address of the lock in amplifier must be set up from the front panel before any communication takes place At power up the RS232 port of each lock in amplifier is fully active irrespective of its address Since this means that all lock in amplifiers in the daisy chain are active on power up the first command must be N n where n is the address of one of the lock in amplifiers This will deselect all but the addressed lock in amplifier When it is required to communicate with another lock in amplifier send a new YN n command using the relevant address Note When programming in C remember that in order to send the character Vin a string it is necessary to type in 6 3 07 GPIB Interface General Features The GPIB is a parallel digital interface with 8 bidirectional data lines and 8 further lines which implement additional control and communication functions Communication is through 24 wire cables including 8 ground connections with special purpose connectors which are constructed in such a way that they can be stacked on top of one another to enable numerous instruments to be connected in parallel By means of internal hardware or software switches each instrument is set to a different address on the bus usually a number in the range 0 to 31 In the model 7220 the address is set using the GPIB SETUP 1 setup menu or b
40. response terminator except when the noprompt bit bit 4 in the RS232 parameter byte is set in which case the terminator is CR The default power up state of this bit is zero 6 3 10 Command Format The simple commands listed in section 6 4 have one of five forms CMDNAME terminator CMDNAME n terminator CMDNAME n terminator CMDNAME n n terminator CMDNAME n n terminator where CMDNAME is an alphanumeric string that defines the command and n n n are parameters separated by spaces When n is not enclosed in square brackets it must be supplied n means that n is optional n n means that n is optional and if present may optionally be followed by n Upper case and lower case characters are equivalent Terminator bytes are defined in section 6 3 09 Where the command syntax includes optional parameters and the command is Chapter 6 COMPUTER OPERATION sent without the optional parameters the response consists of a transmission of the present values of the parameter s Any response transmission consists of one or more numbers followed by a response terminator Where the response consists of two or more numbers in succession they are separated by a delimiter section 6 3 11 Some commands have an optional floating point mode which is invoked by appending a full stop character to the end of the command and before the parameters This allows some parameters to be entered or read in floating point format
41. that the transmitter does not send a byte until the receiver is ready to receive it and in the case of a parallel interface that the receiver reads the data lines only when they contain a valid byte GPIB Handshaking The GPIB interface includes three lines DAV NRFD NDAC which are used to implement a three wire handshake The operation of this is completely defined by the IEEE 488 standard and is fully automatic so that the user does not need to know anything about the handshake when writing programs for the GPIB Note that each command must be correctly terminated RS232 Handshaking In the RS232 standard there are several control lines called handshake lines RTS DTR outputs and CTS DSR DCD inputs in addition to the data lines TD output and RD input However these lines are not capable of implementing the handshaking function required by the model 7220 on a byte by byte basis and are not connected in the model 7220 apart from the RTS and DTR outputs which are constantly asserted Note that some computer applications require one or more of the computer s RS232 handshake lines to be asserted If this is the case and if the requirement cannot be changed by the use of a software switch the cable may be used in conjunction with a null modem A null modem is an adaptor which connects TD on each side through to RD on the other side and asserts CTS DSR and DCD on each side when RTS and DTR are asserted on the other side With mos
42. the LEDs adjacent to the front panel BNC connectors ON or OFF Contrast This control adjusts the contrast of the LCD displays and may be set to a value between 0 and 50 5 10 Chapter 5 FRONT PANEL OPERATION 5 2 06 RS232 Setup 1 Menu Left hand LCD Right hand LCD RS232 BaudRate Format Parity SETUP1 19200bps 7D 1P EVEN SES 9600 bps 8D 1P ODD settings 11 D 0P tting 0 bps 8 0 NONE 75 bps 9D OP Figure 5 7 RS232 Setup 1 Menu This menu shown in figure 5 7 has three controls affecting the RS232 computer interface as follows BaudRate Thirteen values of Baud Rate are available in the range 75 to 19200 bits per second Format There are four data formats available 7D 1P Sets up 7 Data Bits 1 Parity bit 1 Parity ON 8D 1P Sets up 8 Data Bits 1 Parity bit 1 Parity ON 8D 0P Sets up 8 Data Bits 0 Parity bit 0 Parity OFF 9D OP Sets up 9 Data Bits O Parity bit 0 Parity OFF Parity If the Format control is set to a mode with the Parity bit enabled the Parity control offers two modes ODD and EVEN If however the Parity bit is not enabled then the Format control displays NONE Chapter 5 FRONT PANEL OPERATION 5 2 07 RS232 Setup 2 Menu Left hand LCD Right hand LCD RS232 Prompt SETUP2 Delimiter ON cr 013 Available OFF OFF sp 032 control settings 124 125 Figure 5 8 RS232 Setup 2 Menu This menu shown in figure 5 8 has thre
43. the REFP computer command or with the use of the Auto Phase function In basic lock in amplifier applications the purpose of the experiment is to measure the amplitude of a signal which is of fixed frequency and whose phase with respect to the reference input does not vary This is the scalar measurement often implemented with a chopped optical beam Many other lock in amplifier applications are of the signed scalar type in which the purpose of the experiment is to measure the amplitude and sign of a signal which is of fixed frequency and whose phase with respect to the reference input does not vary apart from reversals of phase corresponding to changes in the sign of the signal A well known example of this situation is the case of a resistive bridge one arm of which contains the sample to be measured Other examples occur in derivative spectroscopy where a small modulation is applied to the angle of the grating in optical spectroscopy or to the applied magnetic field in magnetic resonance spectroscopy Double beam spectroscopy is a further common example In this signed scalar measurement the phase shifter must be set after removal of any Zero errors to maximize the X or the Y output of the demodulator This is the only method that will give correct operation as the output signal passes through zero and is also the best method to be used in an unsigned scalar measurement where any significant amount of noise is present 3 8 Output Chann
44. the instrument to leave the setup menu mode and return to the main display mode On leaving the setup menu mode the last menu displayed is held in memory and will be displayed on re entry The following sections describe each menu in sequence 5 2 01 Input Setup Menu Left hand LCD Right hand LCD INPUT Input Mode Input Device FIt DC FET Available A B Volts FIt AC Bipolar control settings FB Amps HB Gnd DC B Amps LN Gnd AC Figure 5 2 Input Setup Menu In this menu shown in figure 5 2 three controls affecting the lock in amplifier s signal channel input are displayed Changes to the setting of these controls can be made by using the ADJUST keys adjacent to the appropriate LCD Input Mode This control has four settings A Volts In this setting the signal channel input is a single ended voltage input connected to the front panel BNC connector marked A A B Volts In this setting the signal channel input is a differential voltage input connected to the front panel BNC connectors marked A and B T B Amps HB In this setting the signal channel input is a single ended current input connected to the front panel BNC connector marked B I and uses a high bandwidth HB current to voltage converter B Amps LN In this setting the signal channel input is a single ended current input connected to Chapter 5 FRONT PANEL OPERATION the front panel BNC connector marked B I
45. working order Technical Description Chapter 3 3 1 Introduction The model 7220 lock in amplifier is capable of outstandingly good signal recovery performance provided that it is operated correctly This chapter describes the design of the instrument enabling the best use to be made of its facilities Of particular importance is the correct adjustment of the AC Gain parameter described in section 3 2 04 3 2 Principles of Operation 3 2 01 Block Diagram The model 7220 utilizes two digital signal processors a microprocessor and a dedicated digital waveform synthesizer together with very low noise analog circuitry to achieve its specifications A block diagram of the instrument is shown in figure 3 1 and the sections that follow describe how each functional block operates and the effect it has on the instrument s performance SIGNAL MONITOR IN PHASE O FAST X OUTPUT MAIN MULTIPLIER SP INPUT LINE FREQUENCY AMPLIFIER REJECTION FILTER SIGNAL O AC GAIN INPUT BiG cH1 O ANALOG OUTPUT B LOWPASS LOWPASS VOLTAGE SONVERTER FILTER FILTER FILTER LOOK UP CRYSTAL TABLE OUTPUT DIGITAL OSCILLATOR PROCESSOR DISPLAYS LOOK UP REFERENCE PHASE SHIFTER DAC INPUT LOWPASS LOW PASS FILTER FILTER TTL REF as REFERENCE lt 60kHz O ANALOG TRIGGER OUTPUT REF MON QUADRATURE MULTIPLIER D S P O FAST Y OUTPUT OSCILLATOR OUTPUT gt 60kHz a MICROPROCESSOR CONTROL RS232 ATTENUATOR
46. would cause a spurious output from the ADC due to the nature of the sampling process Consider the situation when the lock in amplifier is measuring a sinusoidal signal of frequency fsignai Hz which is sampled by the main ADC at a sampling frequency fsampling Hz In order to ensure correct operation of the instrument the output values representing the fsignal frequency must have been uniquely generated by the signal to be measured and not by any other process However if the input to the ADC has in addition an unwanted analog sinusoid with frequency f Hz where f is greater than half the sampling frequency then this will appear in the output as a sampled data sinusoid with frequency less than half the sampling frequency fajas If Mfsamplingl Where n is an integer Chapter 3 TECHNICAL DESCRIPTION This al as signal is indistinguishable from the output generated when a genuine signal at frequency f is sampled Hence if the frequency of the unwanted signal were such that the alias signal frequency produced from it was close to or equal to that of the wanted signal then it is clear that a spurious output would result For example if the sampling frequency were 160 kHz then half the sampling frequency would be 80 kHz Let the instrument be measuring a signal of 55 kHz accompanied by an interfering signal of 100 KHz The output of the ADC would therefore include a sampled data sinusoid of 55 KHz the required signal and app
47. 0 and 90 of full scale 6 11 Chapter 6 COMPUTER OPERATION ASM Perform an Auto Measure operation The instrument adjusts its full scale sensitivity so that the magnitude output lies between 30 and 90 of full scale and then performs an auto phase operation to maximize the X output and minimize the Y output ACGAIN n AC Gain control Sets the gain of the signal channel amplifier Values of n from 0 to 9 can be entered corresponding to the range O dB to 90 dB in 10 dB steps AUTOMATIC n AC Gain automatic control The value of n sets the status of the AC Gain control according to the following table n Status 0 AC Gain is under manual control either using the front panel or the ACGAIN command 1 Automatic AC Gain control is activated with the gain being adjusted according to the full scale sensitivity setting LF n Signal channel line frequency rejection filter control In instruments manufactured prior to June 1996 the value of n sets the mode of the line frequency notch filter according to the following table n Selection 0 Off 1 On i e reject 50 60 Hz and 100 120 Hz In instruments manufactured after June 1996 the value of n sets the mode of the line frequency notch filter according to the following table Selection Off Enable 50 or 60 Hz notch filter Enable 100 or 120 Hz notch filter Enable both filters Utl2 omB Users may identify which version of the instrument they have by sending the command L
48. 000 full scale being 10000 In floating point mode causes the lock in amplifier to respond with the Y demodulator output in volts or amps 6 16 Chapter 6 COMPUTER OPERATION XYL X Y outputs Equivalent to the compound command X Y MAG Magnitude In fixed point mode causes the lock in amplifier to respond with the magnitude value in the range 0 to 30000 full scale being 10000 In floating point mode causes the lock in amplifier to respond with the magnitude value in the range 3 000E0 to 0 001E 9 volts or 3 000E 6 to 0 001E 15 amps PHA Signal phase In fixed point mode causes the lock in amplifier to respond with the signal phase in centidegrees in the range 18000 In floating point mode causes the lock in amplifier to respond with the signal phase in degrees MP Magnitude phase Equivalent to the compound command MAG PHA RT Ratio output In integer mode the RT command reports a number equivalent to 1000 x X ADC1 where X is the value that would be returned by the X command and ADC1 is the value that would be returned by the ADC1 command In floating point mode the RT command reports a number equivalent to X ADC 1 LR Log Ratio output In integer mode the LR command reports a number equivalent to 1000 log X ADC1 where X is the value that would be returned by the X command and ADC1 is the value that would be returned by the ADCI command The response range is 3000 to 2079
49. 2 Connector This connector is used to link other compatible EG amp G equipment together in a daisy chain configuration Up to an additional 15 units can be connected in this way Each unit must be set to a unique address see section 5 2 08 Pinouts for this connector are given in appendix B 4 2 05 GPIB Connector The GPIB interface connector conforms to the IEEE 488 1978 Instrument Bus Standard The standard defines all voltage and current levels connector specifications timing and handshake requirements 4 2 06 DIGITAL OUTPUTS Connector This connector provides eight TTL output lines each of which can be set high or low by the use of a front panel setup menu or via the computer interfaces It is most commonly used for controlling auxiliary apparatus such as lamps shutters and heaters Pinouts for this connector are given in appendix B 4 2 07 PREAMP POWER Connector This connector supplies 15 V at up to 100 mA and can be used for powering any of several optional remote preamplifiers available from EG amp G Instruments Pinouts for this connector are given in appendix B 4 2 08 REF MON Connector The signal at this connector is a TTL compatible waveform synchronous with the reference This output monitors correct reference channel operation but its polarity is not uniquely defined so that it does not necessarily show the correct phase relationship with the SIG MON output 4 2 09 REF TTL Connector This connector is provi
50. 20 E 15 ST command 6 8 6 28 E 16 STAR n command 6 18 E 16 Status byte 6 7 STR n command 6 24 E 17 SWEEP n command 6 20 E 17 SYNC n command 6 15 E 17 Synchronous filters 3 12 Synchronous oscillator 3 7 SYNCOSC n command 6 19 E 18 System updates 3 10 TADC n command 6 21 E 18 TC n command 6 14 E 19 TC command 6 14 E 19 TD command 6 24 E 19 TDC command 6 24 E 20 Technical description 3 1 Terminal emulator 6 2 Terminal mode 6 2 Terminators 6 6 Time constants 3 12 Time Constants control 5 22 Time constants control 5 9 Transient recorder 3 8 TRIG connector 4 8 TTL logic reference input 3 6 Typical experiment description 5 35 Update program 3 8 Vector magnitude 3 8 Ventilation 2 1 VER command 6 29 E 20 VMODE n command 6 10 E 20 Voltage input mode 3 2 What is a lock in amplifier 1 2 X output 3 6 X ts display 5 27 X amp Y demodulation functions 3 7 X in volts amps display 5 30 X output offset level control 5 34 X command 6 16 E 20 XOF n1 n2 command 6 15 E 20 XY command 6 17 E 20 Y 96 output 5 6 Y fs display 5 27 Y in volts amps display 5 30 Y output offset level control 5 34 Y command 6 16 E 20 YOF n1 n2 command 6 15 E 21 Zero error 3 15 INDEX Index 5 INDEX Index 6 WARRANTY EG amp G Instruments Corporation warrants each instrument of its own manufacture to be free of defects in material and workmanship for a period of ONE year
51. 4 2 06 DIGITAL OUTPUTS Connector o oooocooococonconr e rra 4 7 TABLE OF CONTENTS 4 2 07 PREAMP POWER Connector iia dsd e ete prn a ei ee alee Ce RC RT a 4 7 4 2 08 REF MON Connector a 0 34 deca we e Ee ee RR ada wed UR A RR ROS UR RR RR ead eas EROR d 4 7 4 2 00 REF TTL Connector 5 1 eLxa Ed REA DUE EE RUE a a etdadadetaa DEGERE Ua 4 7 4 2 10 SIG MON Connector ves cece as ERR EEE ERG a PE CURE 4 7 ADACHI CH2 Connectors sic ecce CR ERE ed LR Saas etes ea xs 4 8 4 212 TRIG Connector oi ERU RISE PRG He ao e FERE HERR WE e ipid 4 8 42 13 ADGL ADC ConnectotSs esse e v trm boe ye eodem quc bala OR e I Gr ee RD cete d 4 8 42 14 DAC DAC Conhbectots acere IR Ao RR ERRARE No ade de Nor ed 4 8 4 2 15 FAST X FAST Y Connectors RR RR RR RR e es 4 8 Chapter Five Front Panel Operation 5 lIntroducti n 212r A eR er POE Hera de Heard Ead 5 1 52 Setup Menu Mode stc Rer eed recede ad ba pee b M RS ER Eque 5 1 5 2 01 Input Setup Menu eee er Rh eter gr eR RON ette wait dae wi reo pe de eee e 5 2 5 2 02 Reterenc Setup Menu erica reo eda cone CERE veh aad A be XY Ge ER ete 5 4 5 2 03 Output Setup Menu ici Sealed de re ee Rep Y ex borde s e Rd bota eid saad e PRU 5 5 5 2 04 Control Options Menu niue presa eor A a bad eR ada 5 8 5 2 05 Miscellaneous Options Menu sessi 2666 ea eee eee RR e RR Rp RET ERO Ye 5 10 5 2 06 RS232 Setup 1 Menu iL ii RR eR ERG RT peed cbs E PN RH RE EE ede be 5 11 5 2 07 RS232 Setup 2 MEN cesce
52. 5 ms input values being rounded up to a multiple of 5 The longest interval that can be specified is 1000000 s corresponding to about one point in 12 days In addition n may be set to O which sets the rate of data storage to the curve buffer to 800 Hz However this only allows storage of the X and Y outputs There is no need to issue a CBD 3 command to set this up since it happens automatically when acquisition starts If the time constant is set to 5 ms or longer then the actual time constant applied to the stored X and Y output values will be 640 us but if it is set to a shorter value then this will be the time constant actually used SWEEP n Oscillator frequency sweep start Stop Starts stops the internal oscillator frequency sweep depending on the value of n according to the following table n Frequency sweep status 0 Stop Pause 1 Run When a frequency sweep has been defined applying SWEEP 1 will start it The sweep will continue until the stop frequency is reached If during the sweep SWEEP 0 is applied the sweep will stop at the current frequency If SWEEP 1 is then applied the sweep will restart from this point Once the sweep reaches the stop frequency and stops the next SWEEP 1 command will reset the frequency to the start frequency and restart the sweep SYNC n Synchronous time constant control At reference frequencies below 10 Hz if the synchronous time constant is enabled then the actual time constant of the
53. 6 4 Command Descriptions This section lists the commands in logical groups so that for example all commands associated with setting controls affecting the signal channel are shown together Appendix E gives the same list of commands but in alphabetical order 6 4 01 Signal Channel IMODE n Controls whether the instrument input is connected to a current or a voltage preamplifier The value of n sets the input mode according to the following table n Inputmode O0 Current mode off voltage mode input enabled 1 High bandwidth HB current mode enabled connect signal to B input connector 2 Low noise LN current mode enabled connect signal to B inputconnector If n 0 then the input configuration is determined by the VMODE command If n gt 0 then current mode is enabled irrespective of the VMODE setting VMODE n Voltage input configuration The value of n sets up the input configuration according to the following table n Input configuration O Both inputs grounded test mode 1 A input only 3 A B differential mode Note that the IMODE command takes precedence over the VMODE command FET n Voltage mode input device control The value of n selects the input device according to the following table n Selection 0 Bipolar device 10 kQ input impedance 2 nV VHz voltage noise 1 FET 10 MQ input impedance 5 nV VHz voltage noise FLOAT n Input connector shield float ground control The value of n sets the input s
54. CH1 CH2 connector will output a voltage related to the MAG fs front panel display as follows Chapter 5 FRONT PANEL OPERATION MAG fs CHI 2 Voltage 120 12 0 V 100 10 0 V 0 0 0 V 100 10 0 V 120 12 0 V PHASE1 When set to PHASE the corresponding rear panel CH1 CH2 connector will output a voltage related to the PHA deg front panel display as follows PHA deg CHI 2 Voltage 180 9 0 V 90 4 5 V 0 0 0 V 90 4 5 V 180 9 0 V PHASE2 When set to PHASE2 the corresponding rear panel CH1 CH2 connector will output a voltage related to the PHA deg front panel display as follows PHA deg CHI 2 Voltage 360 9 0 V 180 0 0 V 0 9 0 V NOISE When set to NOISE the corresponding rear panel CH1 CH2 connector will output a voltage related to the N fs front panel display as follows N fs CH1 2 Voltage 120 12 0 V 100 10 0 V 0 0 0 V 100 10 0 V 120 12 0 V RATIO When set to RATIO the corresponding rear panel CH1 CH2 connector will output a voltage related to the RATIO front panel display as follows RATIO CH1 2 Voltage 12 12 0 V 10 10 0 V 0 0 0 V 10 10 0 V 12 12 0 V 5 7 Chapter 5 FRONT PANEL OPERATION 5 8 5 2 04 Control Options Menu Left hand LCD Right hand LCD CONTROL Linefilt AC Gain OPTIONS OFF MANUAL SYNC ON AUTOMATIC ASYNC Available or control OFF settings F 2F F amp 2F Figure 5 5 Control Options Setup Menu This menu shown in figure 5 5
55. DE n Controls whether the instrument input is connected to a current or a voltage preamplifier The value of n sets the input mode according to the following table n Input mode Current mode off voltage mode input enabled 1 High bandwidth HB current mode enabled connect signal to B input connector 2 Low noise LN current mode enabled connect signal to B input connector If n 0 then the input configuration is determined by the VMODE command If n gt 0 then current mode is enabled irrespective of the VMODE setting LEN n Curve length control The value of n sets the curve buffer length in effect for data acquisition The maximum allowed value depends on the number of curves requested using the CBD command and a parameter error results if the value given is too large For this reason if the number of points is to be increased and the number of curves to be stored is to be reduced using the CBD command then the CBD command should be issued first E 9 Appendix E ALPHABETICAL LISTING OF COMMANDS LF n Signal channel line frequency rejection filter control In instruments manufactured prior to June 1996 the value of n sets the mode of the line frequency notch filter according to the following table n Selection O0 Off On e reject 50 60 Hz and 100 120 Hz In instruments manufactured after June 1996 the value of n sets the mode of the line frequency notch filter according to the following table Selection
56. Data Bits 2 nett Ea 6 3 6 3 05 Choice of Parity Check Option vio ces cadre edad eee a c Y Sead ae 6 3 6 3 06 Auxiliary RS232 Interface ooo tesis eus a be blew la bade Donate 6 4 6 3 07 GPIB Interface General Features 0 0 0 net enna 6 4 6 3 08 Handshaking and Echoes secet doa ie a See RR A eR RR RR e e Saeed 6 5 6 3 09 Terminators aue 16 3 6 ceret RE exhi Ru sem Meu d UE Xd QURE PAR RR da wate ida 6 6 6 3 10 Command Format ne ys e ore cesser dos pee er Bd Dale Sale a RR ERR e RR a Rob e 6 6 63 1 DehMItETs EMT 6 7 6 3 12 Compound Commands caera a exe Cr ke Hee Phe NR aah RONG Ged ERO DER e 6 7 6 3 13 Status Byte Prompts and Overload Byte 0 eee eh 6 7 6 3 14 Service Requests eie cues tar an dele etd des e Eom daa whey de o a eee tele 6 9 6 4 Command Descriptions err ELDER eS YES A oR ee wee Se ete cs 6 10 6 4 01 Signal Channel eso sineira ree EEG A A adam wee EX ROLE REA e 6 10 6 4 02 Reference Chameleon rer e eae See eee eke EI REC E eeu Oe ees 6 13 6 4 03 Signal Channel Output Filters lee 6 14 6 4 04 Signal Channel Output Amplifiers o oooococococococor cece eee nee 6 15 6 4 05 Instrument Outputs os ccs seep lese eer ERI Y eee eed vr et y Er dde 6 16 6 4 06 Internal Oscillator cui eee tex edet eaten he ced exer eie duel eqs iced era 6 19 6 4 07 Auxiliary Outputs oo 6 20 6 4 08 Auxiliary Inputs sisir ca Res Re led ees es b ER ede ee a 6 21 6 4 09 Output Data Curve Buffer er reese terer
57. E command BURSTRATE n Sets the burst mode sampling rate for ADC1 and ADC2 n sets the sample rate for the Variable Rate burst modes according to the following equations When storing only to ADCI i e TADC 2 TADC 4 TADC 6 and TADC 8 16 000 000 Sample Rate 25 x n 157 When storing to ADC1 and ADC 2 i e TADC 3 TADC 5 TADC 7 and TADC 9 16 000 000 25 x n 1031 Sample Rate Note that these equations apply only to units manufactured after December 1995 Earlier instruments used a 16 384 MHz instead of a 16 0 MHz crystal so the above equations should be modified accordingly by replacing the 16 000 000 figure with 16 384 000 For example when n 20 the sample rate will be 24 353 Hz for ADCI for an instrument with a 16 0 MHz crystal and 24 937 Hz for a unit with a 16 384 MHz crystal Chapter 6 COMPUTER OPERATION 6 4 09 Output Data Curve Buffer CBD n Curve buffer define Defines which data outputs are stored in the curve buffer when subsequent TD take data or TDC take data continuously commands are issued Up to 16 curves or outputs may be acquired as specified by the CBD parameter The CBD is an integer between 0 and 65 535 being the decimal equivalent of a 16 bit binary word When a given bit in the word is asserted the corresponding output is selected for storage When a bit is negated the output is not stored The bit function and range for each output are shown in the ta
58. ELECT keys adjacent to their description X fs 2 X volts or amps Y fs o Y volts or amps Mag fs lt gt Mag volts or amps Noise fs lt gt Noise volts VHz or amps V Hz 5 6 Typical Lock in Amplifier Experiment Having discussed how the instrument s controls may be adjusted and outputs displayed readers may find the following basic checklist helpful in setting up the instrument for manual operation Auto Default Use the Auto Default function on the Control Setup menu to set the instrument to a known state Selection of Signal Input Use the Input Setup menu to select voltage single ended or differential or current input mode and connect your signal source to the relevant A and or B I input connector s Selection of Reference Mode The default setting function will have set the reference mode to internal which assumes that the internal oscillator will be used as a source of excitation to your experiment Use the Internal Oscillator amplitude and frequency controls to set the required oscillator output and connect the output signal from the OSC OUT connector to your experiment If using external reference mode use the Reference Setup menu to select one of the two External modes and connect your reference signal to the specified connector Auto Measure Use the Auto Measure function to set the instrument so that it is correctly displaying your signal Other Adjustments You may now adjust the other controls as re
59. F 3 if this is accepted by the instrument it was made after June 1996 but if it generates a command error it was made prior to this date Units made after June 1996 respond in addition to a new command LINESO which sets the notch filter centre frequency LINESO n Signal channel line frequency rejection filter centre frequency control The value of n sets the line frequency notch filter centre frequency according to the following table 6 12 Chapter 6 COMPUTER OPERATION n Notch filter mode O 60 Hz and or 120 Hz 1 50 Hz and or 100 Hz Units made prior to June 1996 generate an Invalid Command bit 1 of the serial poll status byte is asserted to the LINE50 command SAMPLE n Main analog to digital converter sample rate control The sampling rate of the main analog to digital converter which is nominally 166 kHz may be adjusted from this value to avoid problems caused by the aliasing of interfering signals into the output passband n may be set to 0 1 2 or 3 corresponding to four different sampling rates not specified near 166 KHz 6 4 02 Reference Channel Reference channel source control Internal External The value of n sets the reference input mode according to the following table n Selection O INT internal EXTLOGIC external rear panel TTL input 2 EXT external front panel analog input ENF n Reference harmonic mode control The value of n sets the reference channel to one of the NF modes or
60. G Instruments Model 5182 3 2 03 Line Frequency Rejection Filter Following the signal input amplifier there is an option to pass the signal through a line frequency rejection filter which is designed to give greater than 40 dB of attenuation at the power line frequencies of 50 Hz or 60 Hz and their second harmonics at 100 Hz and 120 Hz Early instruments use a simple single stage band rejection filter which has a relatively broad bandwidth This introduces significant gain and phase errors at least in the range 5 to 500 Hz and this should be taken into account if it is used in conjunction with reference frequencies in or near to this range The filter control settings for these units are simply ON or OFF Instruments manufactured after June 1996 use a more sophisticated type of filter which uses two cascaded rejection stages with notch characteristics This allows the filter to be set to reject signals at frequencies equal to either of or both of the fundamental and second harmonic of the line frequency Hence the filter control settings for these instruments are OFF F 2F or F amp 2F Although instruments are supplied with the line frequency filter set to match the line frequency of the country for which they are destined it should be appreciated that if a unit is moved from a 50 Hz area to a 60 Hz area then the filter will need to be adjusted The later instruments therefore respond to a comput
61. IB M and the first three lines of GPCOM are supplied by the card manufacturer and must be the correct version for the particular version of the interface card in use The interface card may be set up using the program IBCONF EXE to set EOI with the last byte of Write in which case no terminator is required Read operations are automatically terminated on EOI which is always sent by the lock in Normally the options called high speed timing interrupt jumper setting and DMA channel should all be disabled The principles of using the Serial Poll Status Byte to control data transfer as implemented in the main loop of GPCOM are recommended for incorporation in the user s own programs 10 GPCOM 9 Feb 96 20 the following three lines and BIB M are supplied by the 30 manufacturer of the GPIB card must be correct version 40 CLEAR 60000 IBINIT1 60000 IBINIT2 IBINIT 1 3 BLOAD BIB M IBINIT1 50 CALL IBINIT1 IBFIND IBTRG IBCLR IBPCT IBSIC IBLOC IBPPC IBBNA IBONL IBRSC IBSRE IBRSV IBPAD IBSAD IBIST IBDMA IBEOS IBTMO IBEOT IBRDF IBWRTF IBTRAP 60 CALL IBINIT2 IBGTS IBCAC IBWAIT IBPOKE IBWRT IBWRTA IBCMD IBCMDA IBRD IBRDA IBSTOP IBRPP IBRSP IBDIAG IBXTRC IBRDI IBWRTI IBRDIA IBWRTIA IBSTA IBERR IBCNT 80 CLS PRINT DEVICE MUST BE SET TO CR TERMINATOR 90 assign access code to interface board 100 BDNAME GPIBO 110 CALL IBFIND BDNAMES GPIBO 6 120 IF GPIBO lt 0 THEN PRINT board as
62. MHz crystal BYTE n Digital output port control The value of n in the range 0 to 255 determines the bits to be output on the rear panel digital output port When n 0 all outputs are low and when n 255 all are high CBD n Curve buffer define Defines which data outputs are stored in the curve buffer when subsequent TD take data or TDC take data continuously commands are issued Up to 16 curves or outputs may be acquired as specified by the CBD parameter The CBD is an integer between 0 and 65 535 being the decimal equivalent of a 16 bit binary word When a given bit in the word is asserted the corresponding output is selected for storage When a bit is negated the output is not stored The bit function and range for each output are shown in the table below E 2 Appendix E ALPHABETICAL LISTING OF COMMANDS Bit Decimal value Output and range 0 1 X Output 10000 FS 1 2 Y Output 410000 FS 2 4 Magnitude Output 10000 FS 3 8 Phase 18000 180 4 16 Sensitivity setting 4 to 27 IMODE 0 1 2 0 32 64 5 32 ADCI 210000 10 0 V 6 64 ADC2 410000 10 0 V 7 128 Unassigned 8 256 DACI x10000 z 10 0 V 9 512 DAC2 x10000 z 10 0 V 10 1024 Noise 210000 FS 11 2048 Ratio 10000 FS 12 4096 Log ratio 3000 to 2000 13 8192 Last value given to EVENT command 14 16384 Reference frequency bits 0 to 15 mHz 15 32768 Reference frequency bits 16 to 32 mHz 32768 points are available for data storage
63. Model 7220 DSP Lock in Amplifier Instruction Manual 190171 A MNL C Copyright O 1996 EG amp G INSTRUMENTS CORPORATION FCC Notice This equipment generates uses and can radiate radio frequency energy and if not installed and used in accordance with this manual may cause interference to radio communications As temporarily permitted by regulation operation of this equipment in a residential area is likely to cause interference in which case the user at his own facility will be required to take whatever measures may be required to correct the interference Declaration of Conformity This product conforms to EC Directives 89 336 EEC Electromagnetic Compatibility Directive amended by 92 31 EEC and 93 68 EEC and Low Voltage Directive 73 23 EEC amended by 93 68 EEC This product has been designed in conformance with the following IEC EN standards EMC BS EN55011 1991 Group 1 Class A CSPIR 11 1990 BS EN50082 1 1992 IEC 801 2 1991 IEC 801 3 1994 IEC 801 4 1988 Safety BS EN61010 1 1993 IEC 1010 1 1990 A1 1992 Trademarks IBM is a registered trademark of International Business Machines Corporation Microsoft MS DOS GW BASIC QBASIC and QUICKBASIC are registered trademarks and Windows is a trademark of Microsoft Corporation Table of Contents Table of Contents Chapter One Introduction 1 1 How to Use This Manual sois ni e sd A pee apes 1 1 1 2 Whatis a Lock m Amplifier screciiosransio ro o aa a 1 2 1 3 K
64. ON 597095 TLEXT REAR 3rd Figure 5 3 Reference Setup Menu In this menu shown in figure 5 3 there are three controls affecting the reference channel of the instrument They are Ref Source This control allows selection of the source of reference signal used to drive the reference circuitry and has three settings INTERNAL The lock in amplifier s reference is taken from the instrument s internal oscillator Note that this setting gives the best phase and gain measurement accuracy under all operating conditions and is always to be preferred if it is possible to design the experiment so that the lock in amplifier acts as a source of reference signal EXT FRONT In this setting suitable for use with reference frequencies above 300 mHz the lock in amplifier s reference should be applied to the front panel REF IN connector A wide variety of signal waveforms may be employed but at frequencies lower than 1 Hz square waveforms should be used EXT REAR In this setting the lock in amplifier s reference should be applied to the rear panel TTL compatible REF TTL connector The use of this input is preferable to the front panel input when a TTL logic reference signal is available Harmonic This control allows selection of the harmonic at which the lock in amplifier will detect It has three settings but most commonly is set to Ist Note that the 2F setting found on other lock in amplifiers corresponds to setting this
65. ORPORATION INCLUDING BUT NOT LIMITED TO SPECIAL OR CONSEQUENTIAL DAMAGES NO PERSON FIRM OR CORPORATION IS AUTHORIZED TO ASSUME FOR EG amp G INSTRUMENTS CORPORATION ANY ADDITIONAL OBLIGATION OR LIABILITY NOT EXPRESSLY PROVIDED FOR HEREIN EXCEPT IN WRITING DULY EXECUTED BY AN OFFICER OF EG amp G INSTRUMENTS CORPORATION SHOULD YOUR EQUIPMENT REQUIRE SERVICE A Contact your local EG amp G Instruments office agent representative or distributor to discuss the problem In many cases it may be possible to expedite servicing by localizing the problem to a particular plug in circuit board B We will need the following information a copy of which should also be attached to any equipment which is returned for service 1 Model number and serial number of instrument 6 Symptoms in detail including control settings 2 Your name instrument user 7 Your purchase order number for repair charges does not apply to repairs in warranty 3 Your address 8 Shipping instructions if you wish to authorize 4 Address to which the instrument should be shipment by any method other than normal surface returned transportation 5 Your telephone number and extension C If you experience any difficulties in obtaining service please contact EG amp G Instruments Signal Recovery Phone 44 0 118 977 3003 Sorbus House Fax 44 0 118 977 3493 Mulberry Business Park WOKINGHAM RG41 2GY United Kingdom
66. Oscillator Frequency Range Setting Resolution Absolute Accuracy Distortion THD 4 Hz 0 8 Hz at F 40 kHz improving to 0 008 Hz at F 400 Hz 0 040 Hz at F 400 Hz improving to better than 0 0001 Hz at F 1 mHz 2 x 18 bit ADCs driving two DSP elements managed by a powerful 68000 series host processor No zero drift on all settings No zero drift on all settings lt 5 ppm C 90 dB 5 ms to 100 ks in a 1 2 5 sequence 10 us to 640 us in a binary sequence 6 12 18 and 24 dB octave Available for F 20 Hz Auto and Manual on X and Y 300 FS 0 001 Hz to 120 kHz 0 001 Hz 25 ppm 30 uHz 80 dB at 1 kHz A 3 Appendix A SPECIFICATIONS Amplitude Range Setting Resolution 1 mV to 500 mV 501 mV to 2 V 2 001 Vto5 V Accuracy 0 001 Hz to 60 kHz 60 kHz to 120 kHz Stability Output Impedance Auxiliary Inputs ADC 1 and 2 Maximum Input Resolution Accuracy Input Impedance Sample Rate ADC 1 only ADC 1 and 2 Trigger Mode Trigger input Outputs CH1 CH2 Outputs Function Amplitude Impedance Fast X Output Time Constant Amplitude Update Rate Output Impedance Fast Y Output Time Constant Amplitude Update Rate Output Impedance A4 1 mV to5 V 1 mV 4mV 10mV 0 3 96 0 5 96 50 ppm C 50 Q 10 V mV 0 2 1 MQ 30 pF 40 kHz max 13 kHz max Int ext or burst TTL compatible X Y R 0 Noise and aux functions 10 V 1kQ lt 640 us 10 V 170 kHz
67. RE DA KU RA Mq dedu Re e QU KE edd e 3 14 3 12 01 Introduction eb se cabo cee eee eee eee een SEEN DR Eee IS Rea dE e 3 14 3 12 02 Auto Sensitivity ce copo cove eds cero cs Y Kee ED ER Ra OE aur HOES a PES reg EEEE E 3 15 3 12 03 Auto Phase s ssid sees a tege nee erede cede ie a Cee aS 3 15 912 04 AUto OMset eos Sat pin EE eee eee awe eee See eee ae a vedi ue 3 15 3 12 05 Auto Measure ined aia aa id meee da 3 16 3 12 06 Default Seting iii A AA ea aah ee aes 3 16 Chapter Four Front and Rear Panels 4 1 FrontBan l 2 22 24 vex sd e dex A ean dd ee 4 1 4 1 01 A and B I Signal Input Connectors 0 0 II II 4 1 4 1 02 08C OUT Gontlectot i cese a bad eon d ea abla a bt 4 1 4 1 03 REFIN Contiectof us stas heed Hes parias ei a bar FERRE pd TS 4 2 4 1 04 Left hand LCD Display Panel ooooccocccoccccco III 4 2 41 05 MENU m 4 4 SS iiec tree E ee ee hdd ae ee Reano eer e det rA Rer dr 4 5 4 1 07 SET K6y eise er RR E Ree Opa Ree eaae E apo lo muri tania Rupe s 4 5 4 1 08 Right hand LCD Display Panel seleeeeeeeee III I 4 5 42 Rear Panel siae bdo a ed 4 6 4 2 01 Line Power Switch o ooooooooooooor ehh hh has 4 6 4 2 02 Line Power Input Assembly 0 0 eee n 4 6 4203 RS232 CoBnectoF essc dee req ud RR eqs MORD CRESCE ES doe aid e A Ce Re Hoe a e d d 4 7 4 2 04 AUX RS232 Commectol oooooooorrr ehh hrs 4 7 4 2 05 GPIB Connector i522 ota gode aod eR ea eR ea RE RU dh RU RU dd e dub ades 4 7
68. SPACE 32 520 CALL IBRD DEV B RETURN 530 Seral Poll asian iaa 540 CALL IBRSP DEV S RETURN infinite loop terminator is CR write A to bus initialize S96 while command not complete serial poll returns S96 read bus into B and print B is buffer Cable Diagrams Appendix D D 1 RS232 Cable Diagrams Users who choose to use the RS232 interface to connect the model 7220 lock in amplifier to a standard serial port on a computer will need to use one of two types of cable The only difference between them is the number of pins used on the connector which goes to the computer One has 9 pins and the other 25 both are null modem also called modem eliminator cables in that some of the pins are cross connected Users with reasonable practical skills can easily assemble the required cables from parts which are widely available through computer stores and electronics components suppliers The required interconnections are given in figures D 1 and D 2 1 1 225 12 TE 4 4 5 gt 5 6 2 6 7 2 7 G e Xe 9 5 9 54321 12345 9876 6789 9 pin female 9 pin male connector to connector to computer port lock in amplifier Figure D 1 Interconnecting RS232 Cable Wiring Diagram Appendix D CABLE DIAGRAMS 00 002 WNH gt NOM All other pins not connected 13121110987654321 12345 0000000000000 OLE DRETS ee lo 25 24 23 22 2120 19 18 17 16 15 14 6789 25 pi
69. T GIVER ARE GNG TO QUAUFIED PERSE E T Figure 4 10 Model 7220 Rear Panel Layout As shown in figure 4 10 the line power switch line power voltage selector two RS232 connectors a GPIB IEEE 488 connector digital output port preamplifier power connector and twelve BNC signal connectors are mounted on the rear panel of the instrument Brief descriptions of these are given in the following text 4 2 01 Line Power Switch Press the end of the switch marked to turn on the instrument s power and the other end marked O to turn it off 4 2 02 Line Power Input Assembly This houses the line voltage selector and line input fuse To check and if necessary change the fuse or line voltage see the procedure in section 2 1 05 Chapter 4 FRONT AND REAR PANELS 4 2 03 RS232 Connector This 9 pin D type RS232 interface connector implements pins 1 2 3 and 7 Earth Ground Transmit Data Receive Data Logic Ground of a standard DTE interface To make a connection to a PC compatible computer it is normally sufficient to use a three wire cable connecting Transmit Data to Receive Data Receive Data to Transmit Data and Logic Ground to Logic Ground Appendix D shows the connection diagrams of cables suitable for computers with 9 pin and 25 pin serial connectors Pinouts for this connector are given in appendix B 4 2 04 AUX RS23
70. TIO RATIO Figure 5 4 Output Setup Menu This menu shown in figure 5 4 has three controls affecting the output channels of the instrument They are Expand This control allows a x10 output expansion to be applied to the X Y or both output channels or to be switched off 5 5 Chapter 5 FRONT PANEL OPERATION 5 6 OFF Output expansion is turned off X ONLY A x10 output expansion is applied to the X output only Y ONLY A x10 output expansion is applied to the Y output only X 8 Y A x10 output expansion is applied to both the X and Y outputs CH1 OUTPUTS CH This control shown on the right hand LCD allows the two rear panel analog outputs CH1 and CH2 to be connected to the required instrument outputs The left hand ADJUST keys are used to select the output provided at the CH1 connector and the right hand ones select that at the CH2 connector Each output may be set to one of the following settings X amp When set to X the corresponding rear panel CH1 CH2 connector will output a voltage related to the X fs front panel display as follows X tfs CH1 2 Voltage 120 12 0 V 100 10 0 V 0 0 0 V 100 10 0 V 120 12 0 V Y X When set to Y the corresponding rear panel CH1 CH2 connector will output a voltage related to the Y fs front panel display as follows Y fs CH1 2 Voltage 120 12 0 V 100 10 0 V 0 0 0 V 100 10 0 V 120 12 0 V MAG When set to MAG the corresponding rear panel
71. TION Reference Phase This control allows the reference phase to be adjusted over the range 360 to 360 in 10m steps although readers will appreciate that a setting of 180 is equivalent to 180 and that 360 is equivalent to 0 The Auto Phase function also affects the value displayed here Left hand LCD PHASE 360 00 p Rd settings 359 99 360 00 Figure 5 26 Reference Phase Control 5 5 Main Display Mode Right hand LCD 5 26 The right hand display in Main Display Mode is used to display two out of the possible fourteen instrument outputs In addition it is used to set the levels of the X and Y output offsets The panel is divided vertically into two halves with the output description on the upper line and the output value on the lower line Selection of the required output type is made using the upper SELECT keys on each side of the right hand display Each press of the key advances the display to the next output choice with the display wrapping so that repeated presses cycle through those available Note that it is not possible to set both displayed outputs to the same type i e both sides of the display cannot for example be set to X The fourteen possible outputs and two controls are now discussed in sequence Note that in the following figures the horizontal lines below the heading Right hand LCD depict the vertical limits of the display to indicate how information is shown
72. TY 20 nV to reach a value of about 1E8 or 160 dB for the maximum available dynamic reserve Figures of this magnitude are available from any DSP lock in amplifier but are based only on arithmetical identities and do not give any indication of how the instrument actually performs In fact all current DSP lock in amplifiers become too noisy and inaccurate for most purposes at reserves of greater than about 100 dB For the benefit of users who prefer to have the AC Gain value expressed in this form the model 7220 displays the current value of Dynamic Reserve in decibels on the input full scale sensitivity control 3 6 System Updates and Reference Frequency Changes Both the signal channel and the reference channel contain calibration parameters which are dependent on the reference frequency These include corrections to the anti alias filter and to the analog circuits in the reference channel In external reference operation the processor uses the reference frequency meter to monitor the reference frequency and updates these parameters when a change of about 2 percent has been detected All the parameters are also updated when the SET key is pressed or the LOCK command is executed Therefore if the most accurate and reproducible settings are required the SET key should be pressed or the LOCK command executed after every intentional change in reference frequency when in the external reference modes Note that sufficient time must be allowed for
73. The floating point output format is given below 1 234E 01 The number of digits between the decimal point and the exponent varies depending on the number but is a minimum of one and a maximum of eight The input format is not as strict but if a decimal point is used there must be a digit before it An exponent is optional The following are all legal commands for setting the oscillator frequency to 100 1 Hz OF 100 1 OF 1 001E2 OF 1 001E 02 OF 1001E 1 6 3 11 Delimiters Any response transmissions consist of one or two numbers followed by a response terminator Where the response of the lock in amplifier consists of two numbers in succession they are separated by a byte called a delimiter This delimiter can be any printing ASCII character and is selected via the RS232 SETUP 2 setup screen or by the use of the DD command 6 3 12 Compound Commands A compound command consists of two or more simple commands separated by semicolons ASCII 59 and terminated by a single command terminator If any of the responses involve data transmissions each one is followed by an output terminator 6 3 13 Status Byte Prompts and Overload Byte An important feature of the IEEE 488 standard is the serial poll operation by which a special byte the status byte may be read at any time from any instrument on the bus This contains information which must be urgently conveyed from the instrument to the controller The function of the individual bi
74. The user may choose to display this information in either the left or the right half of the display so no horizontal limits are depicted see figure 4 9 Chapter 5 FRONT PANEL OPERATION X fs Right hand LCD X fs 300 00 299 99 299 99 300 00 Output value display range Figure 5 27 X Output as Full Scale Shows the X output as a percentage of the selected full scale sensitivity setting Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied with the instrument s reference phase adjusted for maximum X output the display would read 50 00 Y fs Right hand LCD Y fs 300 00 OPE 14299 99 display f 299 99 300 00 Figure 5 28 Y Output as Full Scale Shows the Y output as a percentage of the selected full scale sensitivity setting Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied with the instrument s reference phase adjusted for maximum Y output the display would read 50 00 5 27 Chapter 5 FRONT PANEL OPERATION 5 28 MAG fs Right hand LCD MAG 4fs 300 00 OPE 14299 99 display f 299 99 300 00 Figure 5 29 Magnitude Output as Full Scale Shows the signal magnitude where magnitude V x output Y output as a percentage of the selected full scale sensitivity setting Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied regardless of the setting of the instrument s refer
75. XT program loop OF XX Set oscillator frequency to new value XX hertz software delay of 250 ms 5 x 50 ms allowing output to stabilize MAG Read Magnitude in volts PHA Read Phase in degrees FRQ Read reference frequency in hertz This would be same as the oscillator frequency since the unit is operating in internal reference mode until the stop frequency is reached 6 31 Chapter 6 COMPUTER OPERATION 6 32 6 5 04 X and Y Output Curve Storage Measurement In this example the lock in amplifier is measuring a current input signal applied to the B input connector and the measured X output and Y output are recorded for 10 seconds at a 100 Hz sampling rate The acquired curves as read back to the computer are required in floating point mode The sequence of commands is therefore as follows IE 2 Set reference mode to external front panel input IMODE 1 High bandwidth current input mode AUTOMATIC 1 AC Gain control automatic FLOAT 1 Float input connector shell using 1 KQ to ground LF 0 Turn off line frequency rejection filter SEN 18 Set sensitivity to 1 nA full scale TC 10 Set time constant to 50 ms AQN Auto Phase Now the curve storage needs to be set up NC Clear and reset curve buffer CBD 19 Stores X output Y output and sensitivity i e bits O 1 and 4 LEN 1000 Number of points 100 Hz x 10 seconds STR 10 Store a point every 10 ms 1 100 Hz The data is acquired by issuing TD Acquires data As the acquisitio
76. Y offset facility according to the following table n Selection 0 Disables offset facility 1 Enables offset facility The range of n is 30000 corresponding to 300 full scale 6 15 Chapter 6 COMPUTER OPERATION AXO Auto Offset The X and Y output offsets are turned on and set to levels giving zero X and Y outputs Any changes in the input signal then appear as changes about zero in the outputs EX n Output expansion control Expands X and or Y outputs by a factor of 10 Changes meter CH1 and CH2 outputs full scale to 10 if X or Y selected The value of n has the following significance n Expand mode 0 Off ExpandX 2 Expand Y 3 Expand X and Y CHn n Analog output control Defines what outputs appear on the rear panel CH1 and CH2 connectors according to the following table N Signal X PFS Y FS Magnitude FS Phase 1 9 V 180 9 V 180 Phase 2 9 V 360 9 V 0 Noise FS Ratio 1000 x X ADC 1 NNBWNFK CO n is compulsory and is either 1 for CH1 or 2 for CH2 6 4 05 Instrument Outputs X X output In fixed point mode causes the lock in amplifier to respond with the X demodulator output in the range 30000 full scale being 310000 In floating point mode causes the lock in amplifier to respond with the X demodulator output in volts or amps YL Y output In fixed point mode causes the lock in amplifier to respond with the Y demodulator output in the range 30
77. alog representation of the sinusoidal signal at the reference input to the X channel phase sensitive detector Consequently it is affected by both the reference phase shifter and harmonic controls of the reference channel For example if an external reference at 1 kHz were applied the unit were set to operate in the 2F mode and the synchronous oscillator were turned on then the signal at the OSC OUT connector would be a 2 kHz sinusoid whose phase could be adjusted using the reference phase shifter When used in the synchronous oscillator demodulator monitor mode OSC OUT is updated at the rate at which the reference channel generates new values for the demodulators Since this occurs approximately once every 6 us this should be taken into account when viewing the waveform on an oscilloscope 3 2 09 Demodulator DSP The essential operation of the demodulator DSP is to multiply the digitized output of the signal channel by data sequences called the X and Y demodulation functions and to operate on the results with digital low pass filters the output filters The demodulation functions which are derived by use of a look up table from the phase values supplied by the reference channel DSP are sinusoids with frequency equal to an integer multiple nf of the reference frequency fref The Y demodulation function is the X demodulation function delayed by a quarter of a period The integer n is called the reference harmonic number and in normal l
78. and is used when the interface parameters are at values other than their default settings then communication will be lost 6 5 Programming Examples 6 30 6 5 01 Introduction This section gives some examples of the commands that need to be sent to the lock in amplifier for typical experimental situations 6 5 02 Basic Signal Recovery In a typical simple experiment the computer is used to set the instrument controls and then to record the chosen outputs perhaps as a function of time At sampling rates of up to a few points per second there is no need to use the internal curve buffer The commands to achieve this would therefore be similar to the following sequence IE 2 VMODE 1 FET 1 AUTOMATIC 1 FLOAT 1 LF 0 ASM TC 12 Set reference to external front panel input Single ended voltage input mode 10 MO input impedance using FET stage AC Gain control automatic Float input connector shell using 1 KQ to ground Turn off line frequency rejection filter Auto Measure assumes reference frequency gt 1 Hz Set time constant to 200 ms since previous ASM changed it Then the outputs could be read as follows X Y MAG PHA FRQ Reads X output in volts Reads Y output in volts Reads Magnitude in volts Reads Phase in degrees Reads reference frequency in hertz Chapter 6 COMPUTER OPERATION The controlling program would send a new output command each time a new reading were required Note that a good rule
79. anel FAST X and FAST Y connectors Settings between 5 ms and 5 ks are in a 1 2 5 sequence and apply only to all the other instrument outputs Left hand LCD TIME CONST oks All output available 2ks otherthan settings 10ms ial I 5ms 640us FAST X 8 320US Fast y 20us ae 10us Figure 5 19 Time Constant Control 5 22 Chapter 5 FRONT PANEL OPERATION Slope The roll off of the output filters is set using this control to any value from 6 dB to 24 dB octave in 6 dB steps Note this control does not affect the roll off of outputs at the FAST X and FAST Y connectors which are fixed at 6 dB octave Left hand LCD ISLOPE 6dB oct Available 12dB oct control settings 18dB oct 24dB oct Figure 5 20 Output Filter Slope Control Oscillator Frequency The frequency of the instrument s internal oscillator may be set using this control to any value between 1 mHz and 120 kHz with a 1 mHz resolution Adjustment is faster if use is made of the Active Cursor control see section 4 1 04 Left hand LCD OSC 0 001Hz ED 0 002Hz settings 119999 999Hz 120000 000Hz Figure 5 21 Internal Oscillator Frequency Control 5 23 Chapter 5 FRONT PANEL OPERATION Oscillator Amplitude The amplitude of the instrument s internal oscillator may be set using this control to any value between 1 mV and 5 V rms with a 1 mV resolution Adjustment is faster if use is made of the Active Cursor control see section 4 1 04
80. ated with the right hand display also allow easy switching of the display between an output expressed as a percentage of full scale and the corresponding output expressed in volts or amps using the simultaneous double 4 5 Chapter 4 FRONT AND REAR PANELS keypress feature To perform such a switch simply press both sides of the SELECT keys simultaneously This feature avoids the need to cycle through a number of outputs thereby reducing the number of keypresses needed The edge indicating analog panel meter is linked to the display on the left hand side of the right hand display with full scale corresponding to a digital reading of 100 96 However the panel meter limits at a few percent above full scale whereas the digital displays limit at 300 full scale Output Display 1 Output Display 2 Selection Keys Selection Keys Description Description SELECT SELECT 4 v X mV Y nV V DIO 0 998 0 000 DO ADJUST ADJUST 7220 DSP LOCK IN AMPLIFIER Output Value Output Value Figure 4 9 Main Display Right hand LCD 4 2 Rear Panel e e CPE PREAMP POWER REF MON REF TTL 18 MON CHI mAT Fane E AUX R8292 pa E AA ae GER qw 1 Ey o o ien kd UNS E e 9 A mor omm TRO apo 002 DAD FAST X ear E gt i PM WAL mm DIGITAL OUTPUTS REM y V p I PEN SEDO ar co o fe fe CON acim wane nae warming 0 00 ee ee ee TO tees s tron MT Aoi see BL FTE FEMME ON
81. available CR LF A carriage return followed by a line feed are transmitted at the end of a response string and in addition the GPIB interface line EOI end of instruction is asserted with the line feed character EOI The GPIB interface line EOI end of instruction is asserted at the end of a response string This gives the fastest possible operation since other termination characters are not needed CR A catriage return is transmitted at the end of a response string and in addition the GPIB interface line EOI end of instruction is asserted Chapter 5 FRONT PANEL OPERATION 5 2 10 GPIB Setup 2 Menu Left hand LCD Right hand LCD GPIB SRQ Mask Test echo SETUP2 0 DISABLED Available 1 ENABLED control settings 254 Figure 5 11 GPIB Setup 2 Menu This menu shown in figure 5 11 has two controls affecting the GPIB computer interface as follows SRQ Mask The instrument includes the ability to generate a Service Request on the GPIB interface to signal to the controlling computer that urgent attention is required The request is generated when the result of a logical bit wise AND operation between the Service Request Mask byte set by this control and the instrument s Status Byte is non zero The bit assignments of the Status Byte are as follows Bit Decimal value Function 0 1 command complete 1 2 invalid command 2 4 command parameter error 3 8 reference unlock 4 16 overload 5 32 auto mode active or
82. ble Diagrams D 1 RS232 Cable Diagrams RR Ree RR ER RE a CR ER eee EE E E P die D 1 Appendix E Alphabetical Listing of Commands Appendix F Default Settings Default Setting Function 6 2246 ccc seed Ree e Ree C Ry S ee eo ees de epe dde ce d F 1 Index WARRANTY ocon a id a E ec Meer S E iam a End of Manual TABLE OF CONTENTS vi Introduction Chapter 1 1 1 How to Use This Manual This manual gives detailed instructions for setting up and operating the EG amp G Instruments Model 7220 Digital Signal Processing DSP dual phase lock in amplifier It is split into the following chapters Chapter 1 Introduction Provides an introduction to the manual briefly describes what a lock in amplifier is and the types of measurements it may be used for and lists the major specifications of the model 7220 Chapter 2 Installation and Initial Checks Describes how to install the instrument and gives a simple test procedure which you may perform to check that the unit has arrived in full working order Chapter 3 Technical Description Provides an outline description of the design of the instrument and discusses the effect of the various controls A good understanding of the design will enable you to get the best possible performance from the unit Chapter 4 Front and Rear Panels Describes the connectors controls and indicators which are to be found on the unit and which are referred to in the subsequent chapters Chapter 5
83. ble below Bit Decimal value Output and range 0 1 X Output 10000 FS 1 2 Y Output 10000 FS 2 4 Magnitude Output 10000 FS 3 8 Phase 218000 X180 4 16 Sensitivity setting 4 to 27 IMODE 0 1 2 0 32 64 5 32 ADCI x10000 10 0 V 6 64 ADC2 10000 10 0 V 7 128 Unassigned 8 256 DACI x10000 z 10 0 V 9 512 DAC2 x10000 z 10 0 V 10 1024 Noise 210000 FS 11 2048 Ratio x10000 FS 12 4096 Log ratio 3000 to 2000 13 8192 Last value given to EVENT command 14 16384 Reference frequency bits 0 to 15 mHz 15 32768 Reference frequency bits 16 to 32 mHz 32768 points are available for data storage shared equally between the specified curves For example if all 16 outputs are stored then the maximum number of storage points would be 2048 i e 32768 16 The LEN command sets the actual curve length which cannot therefore be longer than 32768 divided by the number of curves selected If more curves are requested than can be stored with the current buffer length then the buffer length will be automatically reduced Its actual length can of course be determined by sending the LEN command without a parameter The reason why bit 4 is needed which stores both the sensitivity and the IMODE setting is to allow the instrument to transfer the acquired curves to the computer in floating point mode Without this information the unit would not be able to determine the correct calibration to apply Curves 14 and 15 store the re
84. cale nV fA value sensitivity to display 3x full scale range sensitivity Figure 5 33 X Output in Volts or Amps Shows the X output directly in terms of volts or amps depending on whether voltage or current input mode is selected Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied with the instrument s reference phase adjusted for maximum X output the display would read 50 00 mV Y Volts or Amps Right hand LCD Y V LA Output 3x full scale nV fA value sensitivity to display 3x full scale range sensitivity Figure 5 34 Y Output in Volts or Amps Shows the Y output directly in terms of volts or amps depending on whether voltage or current input mode is selected Hence if the sensitivity setting were 100 mV anda 50 mV signal were applied with the instrument s reference phase adjusted for maximum Y output the display would read 50 00 mV Chapter 5 FRONT PANEL OPERATION MAG Volts or Amps Right hand LCD MAG V LA Output 3x full scale nV fA value sensitivity to display zero range Figure 5 35 Magnitude Output in Volts or Amps Shows the signal magnitude where magnitude V x output Y output directly in terms of volts or amps depending on whether voltage or current input mode is selected Hence if the sensitivity setting were 100 mV and a 50 mV signal were applied regardless of the setting of the instrument s reference phase the display would read 50 00 mV
85. ce ueste a home Pe rer a ld la delay baw eod 5 12 5 2 08 RS232 Setup 3 Men i cere sees ces siea RR Er re E FARE AE ER Er Ee 5 13 5 22 09 GPIB Setup 1 Ment iius eet ee A eee tue e M eR ed 5 14 5 2 10 GPIB Setup 2 MB ne eg erede D Re 8 eee loe od 5 15 5 2 11 Digital Outputs Setup Menu 02 e nee e teenies 5 16 5 212 Control Setup Menu a isset er ona ra E ad ob aci aUe AAA ee seed 5 17 5 3 Auto Functions Men wc cea cde me Ree Ree EY oe eee Ee e re rb e rage 5 18 5 4 Main Display Mode Left hand LCD 0 0 I eens 5 20 5 5 Main Display Mode Right hand LCD 0 cece eee eens 5 26 5 6 Typical Lock in Amplifier Experiment o sse 0 0 0 0 0 cece cette enna 5 35 Chapter Six Computer Operation 6 1 Introduction 25 cise Kee eR eR Kies Bee cabs KR ER Heed MONS AEE RU EY Kowa Ee CERE 6 1 6 2 Capabilities s DT 6 1 0 2 01 General veri coc meno ond tah Se ae dee ae ar CUT eee act ee eid la wack de 6 1 6 2 02 Curve Storage cia ria a Riv Os ewe eee RE EX Rede Dosh AA 6 1 6 2 03 Burst Mode Acquisition 2 0 0 eect ESEE EEE EOE EE DESEE EEA 6 1 6 2 04 Internal Oscillator Frequency Sweep Generator 0 0 0 eee ees 6 2 TABLE OF CONTENTS 6 3 RS232 and GPIB Operation aii a danas mnes 6 2 6 3 01 IMtroductiON 3 coss era repara E ue eb ox ia id a ade dat Ru ned eae e 6 2 6 3 02 RS232 Interface General Features 2 2 ee e e 6 2 6 3 03 Choice of Baud Rate cssc sordas rt eer RR e E E 6 3 6 3 04 Choice of Number of
86. control to 2nd Demod Mon The Demodulator Monitor control has two settings ON and OFF which are only meaningful when the lock in amplifier is operated in external reference mode 5 4 Chapter 5 FRONT PANEL OPERATION ON When the Demodulator Monitor is switched ON and the instrument is operating in External Reference mode the signal at the OSC OUT connector changes from that of the internal oscillator to an analog representation of the drive from the reference channel to the X output demodulator The amplitude of this signal may be controlled by the internal oscillator amplitude controls but the internal oscillator frequency control is inactive since the frequency is related to the external reference If the harmonic mode is set to 1st the signal at the OSC OUT connector will be at the same frequency as the applied reference but 1f set to 2nd or 3rd then the output will be at two or three times the reference frequency respectively OFF When the Demodulator Monitor is switched OFF the OSC OUT connector functions as the output from the internal oscillator The signal provided at it may be adjusted both in amplitude and frequency using the instrument s controls This 1s the most common setting 5 2 03 Output Setup Menu Left hand LCD Right hand LCD OUTPUT Expand CH1 Kd un CH2 SETUP OFF X X X ONLY Y Y Available Y ONLY MAGX FMAGZ settings X amp Y PHASE1 PHASE1 PHASE2 PHASE2 NOISE NOISE RA
87. curve acquisitions The curve may be restarted by means of the TD or TDC command as appropriate Curve acquisition status monitor Causes the lock in amplifier to respond with four values that provide information concerning data acquisition as follows First value Curve Acquisition Status a number with five possible values defined by the following table First Value Significance 0 No curve activity in progress 1 Acquisition via TD command in progress and running 2 Acquisition via TDC command in progress and running 5 Acquisition via TD command in progress but halted by HC command 6 Acquisition via TDC command in progress but halted by HC command Second value Number of Sweeps Acquired This number is incremented each time a TD is completed and each time a full cycle is completed on a TDC acquisition It is zeroed by the NC command and also whenever a CBD or LEN command is applied without parameters Third value Status Byte The same as the response to the ST command The number returned is the decimal equivalent of the status byte and refers to the previously applied command Fourth value Number of Points Acquired This number is incremented each time a point is taken It is zeroed by the NC command and whenever CBD or LEN is applied without parameters DC n Dump acquired curve s to computer In fixed point mode causes a stored curve to be dumped via the computer interface in decimal format In floating point mode the
88. d ADC2 8 Burst mode variable rate External trigger rear panel TRIG input ADCI only 9 Burst mode variable rate External trigger rear panel TRIG input ADC1 and ADC2 In the burst modes data is stored in the curve buffer Use the LEN command to set the number of points required Note that it may be necessary to enter CBD 32 before setting the length if the curve buffer has previously been used for more than one data type The data is read out from the buffer using DC 5 for ADCI and DC 6 for ADC2 If the length is set to more than 16384 and a burst mode which stores both ADC1 and ADC2 is specified then the curve length will automatically be reduced to 16384 points Note also that setting the TADC parameter to any value other than 0 or may affect the CBD parameter as follows TADC parameter Effect on CBD parameter 0 VD oco 1o gUn gun none none automatically set to 32 automatically set to 96 automatically set to 32 automatically set to 96 automatically set to 32 automatically set to 96 automatically set to 32 automatically set to 96 Appendix E ALPHABETICAL LISTING OF COMMANDS The maximum sampling rate depends on the number of ADC inputs used and whether the sampling is timed or simply runs as fast as possible In the modes above described as Fixed Rate sampling runs at the maximum possible rate nominally 20 kHz when sampling both ADC1 and ADC2 or 40 kHz when sampling ADC1 only In the Variable Rate modes th
89. d that the waveform is Gaussian with zero mean The zero mean is usually obtained by using the reference phase control or the Auto Phase function with a comparatively long time constant say 1 s and the time constant is then reduced to say 10 ms for the noise measurement The user is strongly advised to use an oscilloscope attached to the rear panel SIG MON signal monitor output when making noise measurements as this is the best way of ensuring that one is measuring a random process rather than line pick up The indicated value of the noise in V Hz or AA Hz is the square root of the mean spectral density over the bandwidth defined by the setting of the output filter time constant 3 11 Power up Defaults All instrument settings are retained when the unit is switched off When the instrument is switched on again the settings are restored but with the following exceptions a The signal channel reverts to AC coupling b The GPIB mask byte is set to zero c The REMOTE parameter is set to zero front panel control enabled d The curve buffer 1s cleared e Any sweep that was in progress at switch off is terminated f Synchronous time constants are enabled g Display backlights are turned on 3 12 Auto Functions 3 14 3 12 01 Introduction The auto functions are groups of control operations which can be executed by means of a single command or two key presses The auto functions allow easier faster operation in most appl
90. ded to allow TTL compatible pulses to be used as the reference input 4 2 10 SIG MON Connector The signal at this connector is that immediately prior to the main analog to digital converter and after the preamplifier line filter and anti alias filters 4 7 Chapter 4 FRONT AND REAR PANELS 4 8 4 2 11 CH1 CH2 Connectors The signal at these connectors is an analog voltage corresponding to a selected output such as X Y R 9 etc as specified in the Output Setup menu The minimum time constant that can be used is 5 ms The full scale output voltage range is 10 0 V although the outputs remain valid to 12 0 V to provide some overload capability 4 2 12 TRIG Connector This connector accepts a TTL compatible input and can be used for triggering the auxiliary Analog to Digital Converters ADCs The input operates on the positive edge only 4 2 13 ADC1 ADC2 Connectors The input voltages at these connectors may be digitized using the auxiliary ADCs and read either from the front panel or by the use of a computer interface command The input voltages are sampled and held when the ADC is triggered and several different trigger modes are available These modes can be set either from the front panel or by using a remote computer command The input voltage range is 10 0 V and the resolution is 1 mV 4 2 14 DAC1 DAC2 Connectors There are two DAC Digital to Analog Converter output connectors The output voltages at these connec
91. domain response is critical The user is recommended to use 12 dB octave unless there is some definite reason for not doing so Note that the filter slope for the rear panel FAST X and FAST Y outputs is fixed at 6 dB octave 3 8 02 Time Constants and Synchronization The output time constant can be varied between 10 us and 5 ks Values from 10 us to 640 us are available at the rear panel FAST X and FAST Y outputs while values from 5 ms to 5 ks apply to all other outputs including CH1 CH2 and the digital displays The filters are of the Finite Impulse Response FIR type with the averaging time of each section being equal to double the nominal time constant These filters offer a substantial advantage in response time compared with analog filters or digital Infinite Impulse Response IIR filters When the reference frequency is below 10 Hz the synchronous filter option is available This means that the actual time constant of the filter is not generally the selected value T but a value which is equal to an integer number of reference cycles If T is greater than 1 reference cycle the time constant is between T 2 and T Where random noise is relatively small synchronous filter operation gives a major advantage in low frequency measurements by enabling the system to give a constant output even when the output time constant is equal to only 1 reference cycle 3 8 03 Output Offset and Expand The output offset facility enables 300 ful
92. e configurations are used in the two bands transitions between which are made automatically according to the value of the reference frequency These transitions are generally transparent to the user External Reference Mode In external reference mode at frequencies above 300 mHz the reference source may be applied to either a general purpose input designed to accept virtually any periodic waveform with a 50 50 mark space ratio and of suitable amplitude or to a TTL logic level input At frequencies below 300 mHz the TTL logic level input must be used Following the trigger buffering circuitry the reference signal is passed to a digital phase locked loop PLL implemented in the reference DSP This measures the period of the applied reference waveform and from this generates the phase values Internal Reference Mode With internal reference operation in the baseband mode i e at reference frequencies 60 kHz the reference processor is free running at the selected reference frequency and is not dependent on a phase locked loop PLL as is the case in most other lock in amplifiers Consequently the phase noise is extremely low and because no time is required for a PLL to acquire lock reference acquisition is immediate See appendix A for numerical values of phase noise In the internal reference highband mode i e reference frequencies gt 60 kHz the instrument essentially operates as if in external mode except that the reference trig
93. e controls affecting the RS232 computer interface as follows Prompt This function can be switched ON or OFF ON The prompt character is sent out by the lock in amplifier after each command response to indicate that the response is finished and the instrument is ready for a new command It can also be used to signal overload conditions see section 6 3 13 for further information OFF No prompt character is sent Echo This function can be switched ON or OFF see section 6 3 08 for further information ON The lock in amplifier echoes each character received over the RS232 interface back to the controlling computer to indicate it is ready to receive the next one In order for this to work the controlling computer should ensure that it does not send the next character until it has read the echo OFF No character echo occurs Delimiter Some instrument control commands generate more than one output value such as the 5 12 Chapter 5 FRONT PANEL OPERATION MP command report Magnitude and Phase Hence it is necessary for the controlling program to be able to determine when all of the first value has been sent The delimiter is a separator character sent between each response which may be used for this purpose The control allows any ASCII character with decimal value between 32 and 125 or 13 to be used 5 2 08 RS232 Setup 3 Menu Left hand LCD Right hand LCD RS232 Address Information only SETUP3 Firmware
94. e mode 3 6 Baud rate control 5 11 Bipolar input device 3 2 5 3 Burst mode acquisition 6 1 BURSTRATE n command 6 22 E 2 BYTE n command 6 21 E 2 CBD n command 6 23 E 2 CH n1 n2 command 6 16 E 4 CHI connector 4 8 output 3 8 output connector 5 6 output control 5 6 CH2 connector 4 8 output 3 8 output connector 5 6 output control 5 6 Commands alphabetical listing of E 1 E 21 compound 6 7 descriptions of 6 10 for auxiliary inputs 6 21 for auxiliary outputs 6 20 for computer interfaces 6 27 for default setting 6 30 for front panel 6 30 for instrument identification 6 29 for instrument outputs 6 16 for internal oscillator 6 19 for output data curve buffer 6 23 for reference channel 6 13 for signal channel 6 10 for signal channel output amplifiers 6 15 for signal channel output filters 6 14 format 6 6 Common mode rejection ratio CMRR 3 2 Index 1 INDEX Computer control sample programs 6 30 Contrast control 5 10 Control options menu 3 8 Control setup menu 5 17 CP n command 6 11 E 4 Current input mode 3 2 5 2 Current to voltage converter 5 2 Current voltage input mode selection 3 3 Curve storage 6 1 DAC n1 n2 command 6 20 E 4 DACI auxiliary output 3 8 connector 4 8 control 5 24 DAC2 auxiliary output 3 8 connector 4 8 control 5 25 DC n command 6 25 E 4 DCT n command 6 26 E 5 DD n command 6 7 6 28 E 6 Default setting 3 16 Default setting control 5 17 Default settings
95. e right hand display The setup menu description is shown on the left hand side of the left hand display Figure 4 8 makes this clear and the various menus are discussed more fully in chapter 5 When in the setup menu mode a further single press of the MENU key returns the instrument to the normal Main Display mode 4 1 06 90 Key This key increments the reference phase shifter by 90 each time the key is pressed 4 1 07 SET Key This key updates all frequency dependent parameters within the lock in amplifier Press this key after any change to the External Reference frequency 4 1 08 Right hand LCD Display Panel This panel is normally used to display two out of the set of fourteen possible instrument outputs with the keys on each side of it being used to make the selection In this mode it is also used to allow adjustment of the level of the output offsets applied to the X and Y outputs In addition the panel is used in the setup menu mode as described in section 4 1 05 above In the normal mode the panel is divided vertically into two sections with the output descriptions shown on the upper line and the output values on the lower line The output type selection is made using the SELECT keys on each side of the upper line simply press the key repeatedly until the required output type is shown Note that it is not possible to set both sides of the display to the same output type Figure 4 9 makes this clear The SELECT keys associ
96. e sampling speed is set by the BURSTRATE command TC n TC TD Filter time constant control The value of n sets the time constant of the output according to the following table n time constant 0 10 us 1 20 us 2 40 us 3 80 us 4 160 us 5 320 us 6 640 us 7 5ms 8 10 ms 9 20 ms 10 50 ms 11 100 ms 12 200 ms 13 500 ms 14 ls 15 2s 16 5s 17 10s 18 20s 19 50s 20 100 s 21 200 s 22 500 s 23 1 ks 24 2 ks 25 5 ks The TC command is only used for reading the time constant and reports the current setting in seconds Hence if a TC 11 command were sent TC would report 11 and TC 1 0E 01 i e 0 1 s or 100 ms Take data Initiates data acquisition Acquisition starts at the current position in the curve buffer and continues at the rate set by the STR command until the buffer is full E 19 Appendix E ALPHABETICAL LISTING OF COMMANDS E 20 TDC VER Take data continuously Initiates data acquisition Acquisition starts at the current position in the curve buffer and continues at the rate set by the STR command until halted by an HC command The buffer is circular in the sense that when it has been filled current data overwrites earlier points Report firmware version Causes the lock in amplifier to respond with the firmware version number The firmware version number is the number displayed on the front panel RS232 SETUP 3 setup screen VMODE n Voltage input configuration XI The value of n sets up
97. ecifies the voltage to be output In fixed point mode it is an integer in the range 12000 to 12000 corresponding to voltages from 12 000 V to 12 000 V In floating point mode it is in volts 6 20 Chapter 6 COMPUTER OPERATION BYTE n Digital output port control The value of n in the range 0 to 255 determines the bits to be output on the rear panel digital output port When n 0 all outputs are low and when n 255 all are high 6 4 08 Auxiliary Inputs ADC n Read auxiliary analog to digital inputs Reads the voltage appearing at the rear panel ADC1 n 1 and ADC2 n 2 inputs In fixed point mode the response is an integer in the range 12000 to 12000 corresponding to voltages from 12 000 V to 12 000 V In floating point mode it is in volts TADC n Auxiliary ADC trigger mode control The value of n sets the trigger mode of the auxiliary ADC inputs according to the following table Trigger mode Asynchronous 5 ms intervals External rear panel TRIG input Burst mode fixed rate triggered by command ADC1 only Burst mode fixed rate triggered by command ADC1 and ADC2 Burst mode variable rate triggered by command ADC1 only Burst mode variable rate triggered by command ADC1 and ADC2 Burst mode fixed rate External trigger rear panel TRIG input ADC1 only Burst mode fixed rate External trigger rear panel TRIG input ADC1 and ADC2 8 Burst mode variable rate External trigg
98. ed is specified by the value of n as follows Data returned by command X Y MAG PHA ADCI XY MP ADCI ADC2 A DU AhuU0NROp Transfer command This command establishes the high speed transfer mode Use the STAR command to set up the desired response to the command and then send an ASCII 42 without terminator to the instrument The instrument will reply with the selected output as quickly as possible and then wait for another If the computer processes the reply quickly and responds immediately with another then very rapid controlled data transfer is possible The first transfer takes a little longer than subsequent ones because some overhead time is required for the model 7220 to get into the high speed transfer mode When in this mode the front panel controls are inactive and display is frozen The mode is terminated by sending any command other than an when the instrument will exit the mode and process the new command or after a period of 10 seconds following the last command Caution Check that the computer program does not automatically add a carriage Appendix E ALPHABETICAL LISTING OF COMMANDS return or carriage return line feed terminator to the command since these characters will slow down communications STR n Storage interval control Sets the time interval between successive points being acquired under the TD or TDC commands n specifies the time interval in ms with a resolution of
99. een the lock in amplifier and the computer it is essential that each transmission i e command or command response is terminated in a way which is recognizable by the computer and the lock in amplifier as signifying the end of that transmission In the model 7220 there are three input termination options for GPIB communications selected from the front panel under the GPIB SETUP 1 menu or by means of the GP command The lock in amplifier may be set to expect the CR byte ASCII 13 or the lt CR LF gt sequence ASCII 13 followed by ASCII 10 to be appended by the controller as a terminator to the end of each command or alternatively instead of a terminator it may expect the EOI signal line pin 5 on the GPIB connector to be asserted during the transmission of the last character of the command The third option is normally to be preferred with modern interface cards which can easily be set to a wide variety of configurations The selected GPIB termination option applies also to the output termination of any responses sent back by the lock in amplifier to the controller i e the lock in amplifier will send CR or lt CR LF gt or no byte as appropriate In all cases the lock in amplifier asserts the EOI signal line during the transmission of the last byte of a response In RS232 communications the lock in amplifier automatically accepts either CR or lt CR LF gt as an input command terminator and sends out lt CR LF gt as an output
100. eference frequency F 2 Thelock in amplifier measures at 2F 3 The lock in amplifier measures at 3F FRQJI Reference frequency meter If the lock in amplifier is in the EXT or EXT LOGIC reference source modes the FRQ command causes the lock in amplifier to respond with 0 if the reference channel is unlocked or with the reference input frequency if it is locked If the lock in amplifier is in the INT reference source mode it responds with the frequency of the internal oscillator In fixed point mode the frequency is in mHz In floating point mode the frequency is in Hz E 7 Appendix E ALPHABETICAL LISTING OF COMMANDS FSTARTT n Oscillator frequency sweep start frequency Sets the start frequency for a subsequent sweep of the internal oscillator frequency In fixed point mode n is in millihertz In floating point mode n is in hertz FSTEP n n Oscillator frequency sweep step size and type The frequency may be swept either linearly or logarithmically as specified by parameter n The step size is specified by parameter n Log sweep n 0 In fixed point mode n is the step size in thousandths of a percent In floating point mode n is in percent The range of n is 0 to 100 00 96 Linear sweep n 1 In fixed point mode n is the step size in millihertz In floating point mode n is in hertz The range of n is 0 to 10 kHz FSTOP n Oscillator frequency sweep stop frequency Sets the stop frequency for a
101. eference frequency period In this mode the output will be much more stable at low frequencies than it would otherwise be Note when set to this mode output time constants shorter than 100 ms cannot be used ASYNC In this setting the normal mode time constants are not related to the reference frequency period 5 9 Chapter 5 FRONT PANEL OPERATION 5 2 05 Miscellaneous Options Menu Left hand LCD Right hand LCD Trigger Lights Contrast 200Hz EXTERNAL OFF 1 Available BURST 2 49 control settings BURST 3 50 BURST 8 BURST 9 Figure 5 6 Miscellaneous Options Setup Menu This menu shown in figure 5 6 has three controls affecting the auxiliary ADC trigger rate and the front panel display as follows Trigger This control selects the trigger which is used to initiate the conversion of voltages applied to the rear panel ADC1 and ADC2 connectors as follows 200Hz In this setting ADC conversions occur at a 200 Hz rate EXTERNAL In this setting ADC conversions are started by an external trigger signal applied to the rear panel TRIG connector The other eight settings of this control BURST 2 to BURST 9 are only meaningful when using the instrument under computer control since they cause data to be stored to the internal curve buffer They will not therefore be discussed further here but their function is described in chapter 6 Lights This control switches the back lighting of the two LCD displays and
102. el Filters 3 8 01 Slope As with most lock in amplifiers the output filter configuration in the model 7220 is controlled by the SLOPE variable This may seem somewhat strange and a few words of explanation may be helpful In traditional audio terminology a first order low pass filter is described as having a slope of 6 dB per octave because in the high frequency limit its gain is inversely proportional to frequency 6 dB is approximately a factor of 2 in amplitude and an octave is a factor of 2 in frequency similarly a second order low pass filter is described as having a slope of 12 dB per octave These terms have become part of the accepted terminology relating to lock in amplifier output filters and are used in the model 7220 to apply to the envelope of the frequency response function of the digital FIR finite impulse response output filters Accordingly the front panel display control which selects the configuration of the output filters is labelled SLOPE and the options are labelled 6 12 18 24 dB octave 3 11 Chapter 3 TECHNICAL DESCRIPTION The 6 dB octave filters are not satisfactory for most purposes because they do not give good rejection of periodic components in the demodulator output including the inevitable component at double the reference frequency However the 6 dB octave filter finds use where the lock in amplifier is incorporated in a feedback control loop and in some situations where the form of the time
103. el LEDs and LCD backlights according to the following table n Selection All lights off 1 Normal operation M Curve acquisition status monitor Causes the lock in amplifier to respond with four values that provide information concerning data acquisition as follows First value Curve Acquisition Status a number with five possible values defined by the following table First Value Significance 0 No curve activity in progress 1 Acquisition via TD command in progress and running 2 Acquisition via TDC command in progress and running 5 Acquisition via TD command in progress but halted by HC command 6 Acquisition via TDC command in progress but halted by HC command Second value Number of Sweeps Acquired This number is incremented each time a TD is completed and each time a full cycle is completed on a TDC acquisition It is zeroed by the NC command and also whenever a CBD or LEN command is applied without parameters Third value Status Byte The same as the response to the ST command The number returned is the decimal equivalent of the status byte and refers to the previously applied command Fourth value Number of Points Acquired This number is incremented each time a point is taken It is zeroed by the NC command and whenever CBD or LEN is applied without parameters MAG Magnitude In fixed point mode causes the lock in amplifier to respond with the magnitude value in the range 0 to 30000 full scale being 10000 In f
104. en an acquisition is initiated The memory is useful for overcoming speed limitations in the interfaces allowing outputs to be recorded faster than would be possible by transferring them to the computer It also finds use in experiments where data is recorded over a long period of time since it frees the computer from the need to measure time intervals and send requests for output to the instrument On completion of the acquisition the stored curves are transferred to the computer for processing 6 2 03 Burst Mode Acquisition A special use of the curve storage memory is as a transient recorder when the voltage at the ADC1 or ADC input is sampled and stored at rates up to 40 KHz Again stored curves are transferred to the computer for processing 6 1 Chapter 6 COMPUTER OPERATION 6 2 04 Internal Oscillator Frequency Sweep Generator The instrument s internal oscillator may be swept in frequency both linearly and logarithmically over a specified range This facility allows the instrument to function as a simple swept frequency oscillator or in conjunction with the curve storage capability allows frequency response curves to be recorded 6 3 RS232 and GPIB Operation 6 2 6 3 01 Introduction Control of the lock in amplifier from a computer is accomplished by means of communications over the RS232 or GPIB interfaces The communication activity consists of the computer sending commands to the lock in amplifier and the lock in ampli
105. ence phase the display would read 50 00 Noise fs Right hand LCD N fs 300 00 Output 14299 99 display 4 0 01 range 0 00 Figure 5 30 Noise Output as Full Scale Shows the noise accompanying the signal in a bandwidth defined by the setting of the output filter time constant where it is assumed that the noise is Gaussian The value is given as a percentage of the instrument s selected full scale sensitivity setting Note that the displayed value will change as the time constant control is adjusted The instrument offers a second Noise output discussed later in this section expressed directly in terms of volts or amps per root Hertz which takes this effect into account Chapter 5 FRONT PANEL OPERATION Phase in Degrees Right hand LCD PHA deg 180 00 Output 1 179 99 display f 179 99 180 00 Figure 5 31 Phase Output in Degrees Shows the relative phase where phase tan Y output X output in degrees Reference Frequency Right hand LCD FRQ kHz f gt 3kHz 120 00 value 119 99 ne f 0 002 0 001 Figure 5 32 Reference Frequency Display Shows the reference frequency at which the lock in amplifier is operating Note that the display shows values in kHz only when the frequency is greater than 3 kHz At all other values the units are Hz 5 29 Chapter 5 FRONT PANEL OPERATION 5 30 X Volts or Amps Right hand LCD X V LA Output 3x full s
106. er rear panel TRIG input ADC1 only 9 Burst mode variable rate External trigger rear panel TRIG input ADC1 and ADC2 NADNPWNFY COS In the burst modes data is stored in the curve buffer Use the LEN command to set the number of points required Note that it may be necessary to enter CBD 32 before setting the length if the curve buffer has previously been used for more than one data type The data is read out from the buffer using DC 5 for ADC1 and DC 6 for ADC72 If the length is set to more than 16384 and a burst mode which stores both ADC1 and ADC2 is specified then the curve length will automatically be reduced to 16384 points Note also that setting the TADC parameter to any value other than 0 or 1 may affect the CBD parameter as follows 6 21 Chapter 6 COMPUTER OPERATION 6 22 TADC parameter Effect on CBD parameter 0 none 1 none 2 automatically set to 32 3 automatically set to 96 4 automatically set to 32 5 automatically set to 96 6 automatically set to 32 7 automatically set to 96 8 automatically set to 32 9 automatically set to 96 The maximum sampling rate depends on the number of ADC inputs used and whether the sampling is timed or simply runs as fast as possible In the modes above described as Fixed Rate sampling runs at the maximum possible rate nominally 20 kHz when sampling both ADC1 and ADC2 or 40 kHz when sampling ADC1 only In the Variable Rate modes the sampling speed is set by the BURSTRAT
107. er command LINESO which allows this to be done see section 6 4 3 3 Chapter 3 TECHNICAL DESCRIPTION 3 4 3 2 04 AC Gain The signal channel contains a number of analog filters and amplifiers the gain of which are defined by the AC Gain parameter which is specified in terms of decibels dB For each value of AC Gain there is a corresponding value of the INPUT LIMIT parameter which is the maximum instantaneous peak voltage or current that can be applied to the input without input overload as shown in table 3 1 below It is a basic property of the DSP lock in amplifier that the best demodulator performance is obtained by presenting as large a signal as possible to the main analog to digital converter Therefore in principle the AC Gain value should be made as large as possible without causing amplifier or converter overload This constraint is not too critical however and the use of a value 10 or 20 dB below the optimum value makes little difference Note that when signal overload occurs the only action required is to reduce the AC Gain value AC Gain dB INPUT LIMIT mV 0 3000 10 1000 20 300 30 10 40 30 50 10 60 3 70 1 80 0 3 90 0 1 Table 3 1 Input Limit vs AC Gain Further information on the control of AC Gain is given in section 3 4 3 2 05 Anti Aliasing Filter Prior to the main analog to digital converter ADC the signal passes through an anti aliasing filter to remove unwanted frequencies which
108. eration from the front panel should you then turn to chapter 6 for information on how to use the instrument remotely Once you are familiar with the structure of the computer commands appendix E will prove to be convenient as it provides a complete alphabetical listing of these commands in a single easy to use section 1 2 What is a Lock in Amplifier In its most basic form the lock in amplifier is an instrument with dual capability On the one hand it can recover signals in the presence of an overwhelming noise background or alternatively it can provide high resolution measurements of relatively clean signals over several orders of magnitude and frequency Modern instruments such as the model 7220 offer far more than these two basic characteristics and it is this increased capability which has led to their acceptance in many fields of scientific research such as optics electrochemistry materials science fundamental physics and electrical engineering as units which can provide the optimum solution to a large range of measurement problems The model 7220 lock in amplifier can function as a B AC Signal Recovery Instrument B Transient Recorder E Vector Voltmeter E Precision Oscillator Bi Phase Meter E Frequency Meter E Spectrum Analyzer E Noise Measurement Unit These characteristics all available in a single compact unit make it an invaluable addition to any laboratory Chapter 1 INTRODUCTION 1 3 Key Specifications and Be
109. erefore be 1 4 and 5 DCT 1 and DCT 4 would only transfer one curve at a time but DCT 5 would transfer the X output curve and the Magnitude curve simultaneously A typical output data sequence would be Chapter 6 COMPUTER OPERATION X output value gt lt delim gt lt Magnitude value gt lt term gt X output value gt lt delim gt lt Magnitude value gt lt term gt lt X output value gt lt delim gt lt Magnitude value gt lt term gt X output value gt lt delim gt lt Magnitude value gt lt term gt X output value gt lt delim gt lt Magnitude value gt lt term gt etc where delim and term are the delimiter and terminator characters respectively The computer program s subroutine which reads the responses to the DCT command needs to run a FOR NEXT loop of length equal to the value set by the LEN curve length command and must be able to separate the responses on each line for storage Or processing Note that when using this command with the GPIB interface the serial poll must be used After sending the DCT command perform repeated serial polls until bit 7 is set indicating that the instrument has an output waiting to be read Then perform repeated reads in a loop waiting each time until bit 7 is set indicating that a new value is available The loop should continue until bit 1 is set indicating that the transfer is completed 6 4 10 Computer Interfaces RS232 and GPIB RS n n Set read RS232 inte
110. ey Specifications and Benefits srci cca cee or ee breed hme e e aca eR ed 1 3 Chapter Two Installation amp Initial Checks 2 1 LristallatiOti 1 ues OR ee E o Lr c Rog a n cep e ed pc e E D EREK 2 1 2 1 01 Introduction uut deae A on Oe Rod nde o mue hos eoa Rc os o Re Red 2 1 2 1 02 Rack Mounting REST et A tua d sow ERE ieu ose 2 1 2 1 03 n8D ClIOB i cuum wdc acne oi dod cade uA d ux qu dau UR ERU NR eda adus da eque she 2 1 2 104 Line Cord PIS cursores rre Re Oper ia deae ae adus amp had aa 2 1 2 1 05 Line Voltage Selection and Line Fuses crs rcre crecira ciee rerutes s 2 1 22 Minit al CHECKS ui A dha es See pes a Ti A eed 2 3 2 2 0T IntfOdUCUOh i euer ksi als a AOE Rare RR cR aah o Ce algae dai al el riada a ke 2 3 2 2 02 Procedu ona kt nha A Matha Gh Maha ed a ed eed b EO Raub ad KER REA 2 3 Chapter Three Technical Description 3 1 Introduction e wba ea ate oed qc une baw Rex RU RR Ron oe wl wee 3 1 3 2 Principles of Operation i usse ee de sea Ha a EuT EEE dace eed b abe e OR as 3 1 3 2 01 Block Diagram masio rira idR ERO ERE DOE A wba 3 1 3 2 02 Signal Channel Inputs s rire cei cere crr riroces m e nen enn eae 3 2 3 2 03 Line Frequency Rejection Filter 2 0 2 ccc I eae 3 3 3 2 04 AC Galt sii nee iat e oe a RH REP AU Hd ee aede eres a eed CER Eus 3 4 3 2 05 Anti Aliasing Filter ssc cen cena coe ee ee OE RE Ree a EONS REY DOES ER EE y Owe Eg 3 4 3 2 06 Main Analog to Digital Converter 2 0 cece cette nee ae 3 5
111. f chopped light 2 Execute an Auto Offset operation which will reduce the X and Y outputs to zero 3 Re establish the source of input signal The X and Y channel outputs will now indicate the true level of the input signal at the present reference phase setting 4 Execute an Auto Phase operation This will set the reference phase shifter to the phase angle of the input signal However because the offset levels which were applied in step 2 were calculated at the original reference phase setting they will not now be correct and the instrument will in general display a non zero Y output value 5 Remove the source of input signal again 6 Execute a second Auto Offset operation which will reduce the X and Y outputs to zero at the new reference phase setting 7 Re establish the source of input signal This technique although apparently complex is the only way of effectively removing crosstalk which is not generally in the same phase as the required signal AUTO OFFSET In an Auto Offset operation the X offset and Y offset functions are turned on and are automatically set to the levels required to give zero values at both the X and the Y outputs Any small residual values remaining after the initial Auto Offset operation can normally be removed by calling the function for a second time after a suitable delay to allow the outputs to settle The primary use of the Auto Offset is to cancel out zero errors which are usually caused by
112. f the signal These considerations lead to the conclusion that when the magnitude output is being used the time constants of the demodulator should be set to give the required signal noise ratio at the X and Y demodulator outputs improving the signal noise ratio by averaging the magnitude output itself is not to be recommended For analogous reasons the magnitude function also shows signal dependent errors when zero offsets are present in the demodulator For this reason it is essential to reduce zero offsets to an insignificant level usually by the use of the Auto Offset function when the magnitude output is to be used Note that the majority of signal recovery applications are scalar measurements where the phase between the required signal and the reference voltage is constant apart from possible phase reversals corresponding to changes in the sign of the quantity being measured In this situation the lock in amplifier is used in the normal X Y mode with the phase shifter adjusted to maximize the X output and to bring the mean Y output to zero Refer to section 3 12 03 for further information on the correct use of the Auto Phase function for this purpose 3 13 Chapter 3 TECHNICAL DESCRIPTION 3 10 Noise Measurements The noise measurement facility is available only in the baseband mode i e at reference frequencies less than 60 kHz and uses the output processor to perform a noise computation on the Y output where it is assume
113. ference frequency in millihertz The calculation needed to translate these two 16 bit values to one 32 bit value is Reference Frequency 65536 x value in Curve 15 value in Curve 14 Note that the CBD command directly determines the allowable parameters for the DC 6 23 Chapter 6 COMPUTER OPERATION 6 24 and HC commands It also interacts with the LEN command and affects the values reported by the M command LEN n Curve length control NC The value of n sets the curve buffer length in effect for data acquisition The maximum allowed value depends on the number of curves requested using the CBD command and a parameter error results if the value given is too large For this reason if the number of points is to be increased and the number of curves to be stored is to be reduced using the CBD command then the CBD command should be issued first New curve Initializes the curve storage memory and status variables All record of previously taken curves is removed STR n Storage interval control TD TDC Sets the time interval between successive points being acquired under the TD or TDC commands n specifies the time interval in ms with a resolution of 5 ms input values being rounded up to a multiple of 5 The longest interval that can be specified is 1000000 s corresponding to one point in about 12 days In addition n may be set to O which sets the rate of data storage to the curve buffer to 800 Hz However this
114. fier responding either by sending back some data or by changing the setting of one of its controls The commands and responses are encoded in standard 7 bit ASCII format with one or more additional bits as required by the interface see below The two ports cannot be used simultaneously but when a command has been completed the lock in amplifier will accept a command at either port Also when the test echo facility has been activated all output from the computer to the GPIB can be monitored by a terminal attached to the RS232 connector Although the interface is primarily intended to enable the lock in amplifier to be operated by a computer program specially written for an application it can also be used in the direct or terminal mode In this mode the user enters commands on a keyboard and reads the results on a video screen The simplest way to establish the terminal mode is to connect a standard terminal or a terminal emulator to the RS232 port A terminal emulator is a computer running special purpose software that makes it act as a terminal In the default power up state of the port the lock in amplifier sends a convenient prompt character when it is ready to receive a command and echoes each character that is received Microsoft Windows versions 3 1 and 3 11 include a program called Terminal usually in the Accessories group which may be used as a terminal emulator On the other hand a simple terminal program with minimal facili
115. from the date of delivery to the original purchaser Obligations under this Warranty shall be limited to replacing repairing or giving credit for the purchase at our option of any instruments returned shipment prepaid to our Service Department for that purpose provided prior authorization for such return has been given by an authorized representative of EG amp G Instruments Corporation This Warranty shall not apply to any instrument which our inspection shall disclose to our satisfaction to have become defective or unusable due to abuse mishandling misuse accident alteration negligence improper installation or other causes beyond our control This Warranty shall not apply to any instrument or component not manufactured by EG amp G Instruments Corporation When products manufactured by others are included in EG amp G Instruments Corporation equipment the original manufacturers Warranty is extended to EG amp G Instruments customers EG amp G Instruments Corporation reserves the right to make changes in design at any time without incurring any obligation to install same on units previously purchased THERE ARE NO WARRANTIES WHICH EXTEND BEYOND THE DESCRIPTION ON THE FACE HEREOF THIS WARRANTY IS IN LIEU OF AND EXCLUDES ANY AND ALL OTHER WARRANTIES OR REPRESENTATIONS EXPRESSED IMPLIED OR STATUTORY INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE AS WELL AS ANY AND ALL OTHER OBLIGATIONS OR LIABILITIES OF EG amp G INSTRUMENTS C
116. ger input is now provided by an internal link from the output of the direct digital synthesizer Chapter 3 TECHNICAL DESCRIPTION 3 2 08 Internal Oscillator The model 7220 in common with many other lock in amplifiers incorporates an internal oscillator which may be used to drive an experiment However unlike most other instruments the oscillator in the model 7220 is digitally synthesized with the result that the output frequency is extremely accurate and stable The oscillator operates over the same frequency range as the lock in amplifier 1 mHz to 120 kHz The source of the oscillator depends on whether the instrument is operating on internal or external reference mode and on the selected frequency In internal reference baseband mode 60 kHz the oscillator is derived from the reference channel DSP This outputs a series of digital values corresponding to a sinusoid at the required frequency to a 16 bit DAC which in turn feeds a variable attenuator The output of the attenuator is the internal oscillator output In internal reference highband mode gt 60 kHz and external reference mode the oscillator is derived from a dedicated direct digital synthesizer DDS A further choice of output at the OSC OUT connector is offered when the unit is operating in external reference mode In this situation if the synchronous oscillator also called the demodulator monitor control is turned on then the OSC OUT signal becomes a direct an
117. hase Angle Noise DAXA in Harmonic Noise Displays Appendix The unit can simultaneously present any two of these as outputs 2F or 3F Measures noise in a given bandwidth centered on frequency F Two LED backlit two line 16 character alphanumeric dot matrix LCDs giving digital indication of current instrument set up and output readings Edge indicating analog panel meter Menu system with dynamic key function allocation Signal Channel Voltage Inputs Modes Full scale Sensitivity Dynamic Reserve Impedance FET Device Bipolar Device Voltage Noise FET Device Bipolar Device CMRR Frequency Response Gain Accuracy Distortion Line Filter Grounding A only or Differential A B 20 nV to 1 V ina 1 2 5 sequence 100 dB 10 MQ 30 pF 10 kQ 30 pF 5 nVAHz at 1 kHz 2 nVNHz at 1 kHz gt 100 dB at 1 KHz degrading by 6 dB octave 0 001 Hz to 120 kHz 0 5 96 typ full bandwidth 90 dB THD 60 dB AC Gain 1 kHz attenuates 50 60 100 120 Hz BNC shields can be grounded or floated via 1 kQ to ground Appendix A SPECIFICATIONS Current Input Mode Full scale Sensitivity Low Noise Wide Bandwidth Dynamic Reserve Frequency Response Low Noise Wide Bandwidth Impedance Low Noise Wide Bandwidth Noise Low Noise Wide Bandwidth Gain Accuracy midband Low Noise Wide Bandwidth Line Filter Grounding Reference Channel TTL Input rear panel Frequency Range Analog Input front
118. he barrel selector is inserted and the door closed E SCREWDRIVER 5 SLOT mmi Figure 2 1 Line Input Assembly VISIBLE VOLTAGE RANGE 100 90 110 V 120 110 130 V 220 200 240 V 240 220 260 V Table 2 1 Range vs Barrel Position Next check the fuse rating For operation from a nominal line voltage of 100 V or 120 V use a 20 mm slow blow fuse rated at 1 0 A 250 V For operation from a nominal line voltage of 220 V or 240 V use a 20 mm slow blow fuse rated at 0 5 A 250 V To change the fuse first remove the fuse holder by pulling the plastic tab marked with an arrow Remove the fuse and replace with a slow blow fuse of the correct voltage and current rating Install the fuse holder by sliding it into place making sure the arrow on the plastic tab is pointing downwards When the proper fuse has been Chapter 2 INSTALLATION AND INITIAL CHECKS installed close the plastic door firmly The correct selected voltage setting should now be showing through the rectangular window Ensure that only fuses with the required current rating and of the specified type are used for replacement The use of makeshift fuses and the short circuiting of fuse holders is prohibited and potentially dangerous 2 2 Initial Checks 2 2 01 Introduction The following procedure checks the performance of the model 7220 In general this procedure should be carried out after inspecting the instrument for obvious
119. he up ADJUST key to increment the frequency by 1 kHz The cursor will disappear as soon as the frequency is adjusted but its position remains active until changed see figure 4 6 4 3 Chapter 4 FRONT AND REAR PANELS 4 4 Action Display after action OSC 50000 000Hz Press both keys OSC 50000 00QHz where the cursor appears depends on where it was previously left Press both keys repeatedly until OSC 5 j000 000Hz Press A key OSC 51000 000Hz Figure 4 6 Active Cursor Operation The double keypress action can also be performed with one finger by firmly pressing the center of the up and down ADJUST key rocker which will deform to press both keys The active cursor can be used to set any particular digit For example if you only want to adjust the reference phase in 1 degree steps leave the cursor over the first digit to the left of the decimal point of the reference phase value 4 1 05 MENU Key The left hand LCD is also used to access the four auto functions that are built into the instrument To do this from the power up display default press the key marked MENU once Note On early units this key was labelled AUTO The left hand LCD now changes to that shown in figure 4 7 Auto Sensitivity Auto Phase SELECT ADJUST AVY SEN PHASE AJY GI MEASURE OFFSET 4 7 SELECT ADJUST gy EGzG INSTRUMENTS Auto Measure Auto Offset Figure 4 7 Auto Functions Menu Left hand Display To
120. hield switch according to the following table n Selection O Ground 1 Float connected to ground via a 1 KQ resistor 6 10 Chapter 6 COMPUTER OPERATION CP n Input coupling control The value of n sets the input coupling mode according to the following table n Coupling mode 0 AC DC SEN n Full scale sensitivity control The value of n sets the full scale sensitivity according to the following table depending on the setting of the IMODE control n full scale sensitivity IMODE 0 IMODE 1 IMODE 2 4 20nV 20 fA n a 5 50 nV 50 fA n a 6 100 nV 100 fA n a 7 200 nV 200 fA n a 8 500 nV 500 fA n a 9 1 uV 1 pA n a 10 2 uV 2 pA 20 fA 11 5 uV 5pA 50 fA 12 10 pV 10pA 100 fA 13 20 uv 20pA 200 fA 14 50 uV 50pA 500 fA 15 100 uV 100 pA 1 pA 16 200 uV 200 pA 2pA 17 500 uV 500 pA 5pA 18 mV I nA 10 pA 19 2 mV 2 nA 20 pA 20 5mV 5nA 50pA 21 10mV 10nA 100 pA 22 20 mV 20 nA 200 pA 23 50 mV 50 nA 500 pA 24 100 mV 100 nA 1nA 25 200 mV 200 nA 2nA 26 500 mV 500 nA 5nA 27 1V l uA 10 nA Floating point mode can only be used for reading the sensitivity which is reported in volts or amps For example if IMODE 0 and the sensitivity is 1 mV the command SEN would report 18 and the command SEN would report 1 0E 03 If IMODE was changed to 1 SEN would still report 18 but SEN would report 1 0E 09 AS Perform an Auto Sensitivity operation The instrument adjusts its full scale sensitivity so that the magnitude output lies between 3
121. ications however direct manual operation or special purpose control programs may give better results in certain circumstances During application of several of the auto functions decisions are made on the basis of output readings made at a particular moment Where this is the case it is important for the output time constant set by the user to be long enough to reduce the output noise to a sufficiently low level so that valid decisions can be made and sufficient time is allowed for the output to settle The following sections contain brief descriptions of the auto functions Chapter 3 TECHNICAL DESCRIPTION 3 12 02 Auto Sensitivity This function only operates when the reference frequency is above 1 Hz A single Auto Sensitivity operation consists of increasing the full scale sensitivity range if the magnitude output is greater than 90 of full scale or reducing the range if the magnitude output is less than 30 of full scale After the Auto Sensitivity function 1s called Auto Sensitivity operations continue to be made until the required criterion 1s met In the presence of noise or a time varying input signal it may be a long time before the Auto Sensitivity sequence comes to an end and the resulting setting may not be what is really required 3 12 03 Auto Phase In an Auto Phase operation the value of the signal phase is computed and an appropriate phase shift is then introduced into the reference channel so as to bring the value
122. ifferential input mode It is usually given in decibels Hence a specification of gt 100 dB implies that a common mode signal i e a signal simultaneously applied to both A and B inputs of 1 V will give rise to less than 10 uV of signal out of the input amplifier Input Connector Shell Ground Float The input connector shells may be connected either directly to the instrument s chassis ground or they can be floated by being connected via a 1 kQ resistor When in the float mode the presence of this resistor substantially reduces the problems which often occur in low level lock in amplifier measurements due to ground loops Input Device Selection FET Bipolar The voltage preamplifier may be switched between bipolar and FET input devices The bipolar device which has an input impedance of 10 kO shows a relatively high level of added current noise 2 pA N Hz but less than 50 percent of the voltage noise of the FET device As such it is intended for use where the source impedance is resistive or inductive with a resistance of 100 Q or less and there is no input voltage offset WARNING Signal channel overload may occur if the bipolar device is selected and no DC bias path is provided The FET device provides an input impedance of 10 MO AC DC Coupling In normal operation with reference frequencies above a few hertz AC coupled operation is always used Chapter 3 TECHNICAL DESCRIPTION The primary purpose of the DC coupli
123. io output In integer mode the RT command reports a number equivalent to 1000 x X ADCI where X is the value that would be returned by the X command and ADC1 is the value that would be returned by the ADC1 command In floating point mode the RT command reports a number equivalent to X ADC 1 SAMPLE n Main analog to digital converter sample rate control The sampling rate of the main analog to digital converter which is nominally 166 kHz may be adjusted from this value to avoid problems caused by the aliasing of interfering signals into the output passband n may be set to 0 1 2 or 3 corresponding to four different sampling rates not specified near 166 kHz E 14 Appendix E ALPHABETICAL LISTING OF COMMANDS SEN n Full scale sensitivity control The value of n sets the full scale sensitivity according to the following table depending on the setting of the IMODE control n full scale sensitivity IMODE 0 IMODE 1 IMODE 2 4 20 nV 20 fA n a 5 50 nV 50 fA n a 6 100 nV 100 fA n a 7 200 nV 200 fA n a 8 500 nV 500 fA n a 9 1 uV 1pA n a 10 2 uV 2pA 20 fA 11 5 uV 5 pA 50 fA 12 10 uV 10 pA 100 fA 13 20 uV 20 pA 200 fA 14 50 uV 50 pA 500 fA 15 100 uV 100 pA 1 pA 16 200 uV 200 pA 2 pA 17 500 uV 500 pA 5 pA 18 mV 1 nA 10 pA 19 2mV 2nA 20pA 20 5mV 5nA 50pA 21 10mV 10nA 100 pA 22 20 mV 20 nA 200 pA 23 50 mV 50 nA 500 pA 24 100 mV 100 nA I nA 25 200 mV 200 nA 2 nA 26 500 mV 500 nA 5nA 27 1V l uA 10 nA Floating point
124. isplay so that for example if it were O the eight output lines would all be low whereas if set to 255 then they would all be high The centre of the right hand display shows the number in binary format with the value of each bit shown below its bit number Each of the keys around this display will toggle one bit as indicated by the numbers adjacent to them providing a second way of controlling the port Any changes made to the digital output in decimal format will be reflected in the binary display and vice versa Chapter 5 FRONT PANEL OPERATION 5 2 12 Control Setup Menu Left hand LCD Right hand LCD CONTROL Default Sample rate SETUP Setting Available control settings Co Ne O Figure 5 13 Control Setup Menu The final setup menu shown in figure 5 13 is used to set the instrument to a known state and to adjust the sampling rate of the main analog to digital converter Default Setting Pressing a key adjacent to this label will set all of the instrument s controls to a known state This can be useful when performing the initial checks procedure or after taking the instrument over from another user Note that on completion the instrument will leave the setup menu mode and revert to the main display mode A listing of the settings invoked by the use of this function can be found in appendix E Caution The default setting includes controls affecting the RS232 and GPIB interfaces so these will need to be readj
125. ith an instrument of the design of the model 7220 where there are many controls of which only a few are regularly adjusted it is very easy to overlook the setting of one of them Consequently a Default Setting function is provided which sets all the controls to a defined state This is most often used as a rescue operation to bring the instrument into a known condition when it is giving unexpected results A listing of the settings which are invoked by the use of this function can be found in appendix F Front and Rear Panels Chapter 4 4 1 Front Panel A f es SELECT ADJUST MENU SELECT SELECT ose toO rma Os La 1 SS HE AI TI DIG Du v Lo lv BH o SELECT ADJUST ADJUST ADJUST o REF y EGsG INSTRUMENTS ae 7220 DSP LOCK IN AMPLIFIER uN Figure 4 1 Model 7220 Front Panel Layout As shown in figure 4 1 there are four BNC connectors with associated LED indicators two LCD display panels an edge indicating analog meter eight double and three single keys mounted on the model 7220 s front panel The following sections describe the function and location of these items 4 1 01 A and B I Signal Input Connectors The A connector is the signal input connector for use in single ended and differential voltage mode The B I connector is the signal input connector for use in differential voltage mode A B and is also the signal input connector when current input mode is
126. l scale offset to be applied to the X Y or both displays Note however that the rear panel analog outputs limit at 120 96 full scale The output expand facility allows a x10 expansion to be applied to the X Y both or neither outputs and hence to the analog meter indication and the CH1 and CH2 analog outputs if these are set to output X or Y values 3 9 Use of Magnitude and Signal Phase Outputs 3 12 If the input signal V t is a reference frequency sinusoid of constant amplitude and the output filters are set to a sufficiently long time constant the demodulator outputs are constant levels V and Vy The function y V VA is dependent only on the amplitude of the required signal V t i e itis not dependent on the phase of V t Chapter 3 TECHNICAL DESCRIPTION with respect to the reference input and is computed by the output processor in the lock in amplifier and made available as the magnitude output The phase angle between V t and the X demodulation function is called the signal phase this is equal to the angle of the complex quantity V IV where j is the square root of 1 and is also computed by the processor by means of a fast arctan algorithm The magnitude and signal phase outputs are used in cases where phase is to be measured or alternatively where the magnitude is to be measured under conditions of uncertain or varying phase One case of varying phase is that in which the reference input is not deri
127. loating point mode causes the lock in amplifier to respond with the magnitude value in the range 3 000E0 to 0 001E 9 volts or 3 000E 6 to 0 001E 15 amps MP Magnitude phase Equivalent to the compound command MAG PHA MSK n Set read service request mask byte The value of n sets the SRQ mask byte in the range 0 to 255 E 11 Appendix E ALPHABETICAL LISTING OF COMMANDS E 12 Nn NC NHZ NN Address command When the 7220 is daisy chained with other compatible instruments this command will change which instrument is addressed All daisy chained instruments receive commands but only the currently addressed instrument will implement or respond to the commands The exception is the NN n command If n matches the address set from the front panel the instrument will switch into addressed mode If n does not match the address set from the front panel the instrument will switch into unaddressed mode Note The YN n command does not change the address of an instrument but which instrument is addressed Warning All instruments must have a unique address Report overload byte Causes the lock in amplifier to respond with the overload byte Bit 0 not used Bit 1 CHI output overload gt 120 FS Bit 2 CH2 output overload gt 120 FS Bit 3 Y output overload gt 300 FS Bit 4 X output overload gt 300 FS Bit 5 not used Bit 6 input overload Bit 7 reference unlock New curve Initializes the cur
128. lting typical accuracy is 0 5 percent of the full scale sensitivity and 0 5 degree When the higher values of AC Gain are in use the errors tend to increase in the upper part of the frequency range above 25 kHz Chapter 3 TECHNICAL DESCRIPTION 3 3 02 Relative Accuracy The majority of lock in amplifier measurements are concerned with the variation of the input signal with time temperature etc or with the comparison of two different specimens In these cases the absolute accuracy is of less importance than the accuracy with which readings can be transferred from range to range A new feature of the model 7220 is the introduction of a separate control function AC Gain for the gain of the signal channel Where appropriate this can be set to accommodate the existing noise level and subsequent changes in the instrument s full scale sensitivity do not cause any of the errors which might arise from a change in the analog gain 3 4 Full Scale Sensitivity and AC Gain Control The full scale sensitivity is indicated as SEN on the left hand LCD and is adjusted by the use of the adjacent keys The analog outputs and analog meter limit at a level a few percent above the full scale sensitivity value but the digital displays do not limit until a level of 300 percent full scale has been reached As stated in section 3 2 04 the best performance is obtained by making the AC Gain value as large as possible without causing amplifier overload
129. lying the above formula an alias signal of 60 kHz i e 1100 kHz 160 kHz If the signal frequency were now increased towards 60 kHz then the output of the lock in amplifier would increasingly be affected by the presence of the alias signal and the accuracy of the measurement would deteriorate To overcome this problem the signal is fed through the anti aliasing filter which restricts the signal bandwidth When operating at reference frequencies below 60 kHz the reference frequency is less than half the sampling frequency and a conventional elliptic type low pass anti alias filter is used This enables the system to provide the lowest possible noise bandwidth At frequencies above 60 kHz an adaptive bandpass anti alias filter is used The noise bandwidth of this filter is dependent on the reference frequency and is higher than that of the conventional type of filter but typically the noise penalty is negligible It should be noted that the dynamic range of a lock in amplifier is normally so high that practical anti alias filters are not capable of completely removing the effect of a full scale alias For instance even if the filter gives 100 dB attenuation an alias at the input limit and at the reference frequency will give a one percent output error when the dynamic reserve is set to 60 dB or a full scale error when the dynamic reserve is set to 100 dB In a typical low level signal recovery situation many unwanted inputs need to be deal
130. n female 9 pin male connector to connector to computer port lock in amplifier Figure D 2 Interconnecting RS232 Cable Wiring Diagram Alphabetical Listing of Commands Appendix E ACGAIN n AC Gain control Sets the gain of the signal channel amplifier Values of n from 0 to 9 can be entered corresponding to the range O dB to 90 dB in 10 dB steps ADC n Read auxiliary analog to digital inputs AQN AS ASM Reads the voltage appearing at the rear panel ADC1 n 1 and ADC2 n 2 inputs In fixed point mode the response is an integer in the range 12000 to 12000 corresponding to voltages from 12 000 V to 12 000 V In floating point mode it is in volts Default setting command This command will automatically set all the instrument controls and displays to the factory set default values However if the command is used when the interface parameters are at values other than their default settings then communication will be lost Auto Phase auto quadrature null The instrument adjusts the reference phase to maximize the X output and minimize the Y output signals Perform an Auto Sensitivity operation The instrument adjusts its full scale sensitivity so that the X output lies between 30 and 90 46 of full scale Perform an Auto Measure operation The instrument adjusts its full scale sensitivity so that the magnitude output lies between 30 and 90 of full scale and then performs an Auto Phase operation t
131. n is running the M command reports the status of the curve acquisition Once this indicates the acquisition is complete i e parameter 1 0 parameter 2 1 the acquired data may be transferred to the computer using DC 0 Transfers X output values in floating point mode DC 1 Transfers Y output values in floating point mode The input routine of the program must be prepared to read and store 1000 responses to each of these commands 6 5 05 Transient Recorder In this example the signal recovery capabilities of the lock in amplifier are not used but the auxiliary inputs are The voltage applied to the rear panel ADC1 input is sampled and digitized at a rate of approximately 40 KHz with the values being stored to the curve buffer Sampling is required to start on receipt of a trigger at the rear panel TRIG IN connector and must last for 500 ms The sequence of commands is therefore as follows Chapter 6 COMPUTER OPERATION NC Clear and reset curve buffer LEN 20000 500 ms recording time at 40 kHz 20 000 points TADC 6 Set ADCI sampling to burst mode fixed rate 40 kHz external trigger and arm trigger As soon as a trigger occurs the acquisition starts Once it completes the acquired data may be transferred to the computer using DC 5 Transfers ADCI values in floating point mode The input routine of the program must be prepared to read and store 20 000 responses to this command 6 5 06 Frequency Response Measureme
132. n syntax or by a command parameter being out of range or alternatively if an overload or reference unlock is currently being reported by the front panel indicators the prompt is ASCII 63 Otherwise the prompt is ASCII 42 These error conditions correspond to the assertion of bits 1 2 3 or 4 in the status byte When the prompt is received by the computer the ST command may be issued in order to discover which type of fault exists and to take appropriate action The prompts are a rapid way of checking on the instrument status and enable a convenient keyboard control system to be set up simply by attaching a standard terminal or a simple computer based terminal emulator to the RS232 port Where the prompt is not required it can be suppressed by setting the noprompt bit bit 4 in the RS232 parameter byte The default power up state of this bit is zero Because of the limited number of bits in the status byte it can indicate that an overload exists but cannot give more detail An auxiliary byte the overload byte returned by the N command gives details of the location of the overload A summary of the bit assignments in the status byte and the overload byte is given below Status Byte Overload Byte bit 0 command complete not used bit 1 invalid command CH1 output overload bit 2 command parameter error CA2 output overload bit 3 reference unlock Y output overload bit 4 overload X output overload bit 5 new ADC values available af
133. nd GPIB IEEE 488 computer interfaces This processor also drives the instrument s 8 bit digital programmable output port which may be used for controlling auxiliary apparatus The microprocessor has access to memory which may be used for storage of the instrument s outputs as curves prior to transferring them to a computer via the computer interfaces In addition to using this function for the normal outputs such as the X and Y output signals it may also be used with the auxiliary ADC inputs to allow the instrument to operate as a transient recorder The internal oscillator frequency sweep function is also controlled by the microprocessor A particularly useful feature of the design is that only part of the controlling firmware program code which the microprocessor runs is permanently resident in the instrument The remainder is held in flash EEPROM and can be updated via the RS232 computer interface It is therefore possible to change the functionality of the instrument perhaps to include a new feature or update the computer command set simply by connecting it to a computer and running an Update program 3 3 01 Absolute Accuracy Specifications 3 8 When the demodulator is operating under correct conditions the absolute gain accuracy of the instrument is limited by the analog components in the signal channel and the absolute phase accuracy is limited by the analog components in both the signal channel and the reference channel The resu
134. nefits The EG amp G Instruments Model 7220 represents the latest in DSP Lock in Amplifier technology at an affordable price and offers Frequency range 0 001 Hz to 120 kHz Voltage sensitivity 20 nV to 1 V full scale Current input mode sensitivities 20 fA to 1 pA full scale 20 fA to 10 nA full scale Line frequency rejection filter Dual phase demodulator with X Y and R 0 outputs Very low phase noise of lt 0 0001 rms 5 digit output readings Direct Digital Synthesizer DDS oscillator with variable output amplitude and frequency Oscillator frequency sweep generator Output time constant 10 us to 5 ks 8 bit programmable digital output port for system control Two external ADCs two external DACs Full range of auto modes Standard IEEE 488 and RS232 interfaces with RS232 daisy chain capability Dual back lit liquid crystal display LCD with variable contrast control 32768 point curve storage buffer Chapter 1 INTRODUCTION 1 4 Installation Initial Checks Chapter 2 2 1 Installation 2 1 01 Introduction Installation of the model 7220 in the laboratory or on the production line is very simple Because of its low power consumption the model 7220 does not incorporate forced air ventilation It can be operated on almost any laboratory bench or be rack mounted using the optional accessory kit at the user s convenience With an ambient operating temperature range of 0 C to 35 C it is highly tolerant to e
135. ng facility is to enable the use of the instrument at reference frequencies below 0 5 Hz It may also be used to reduce the effect of phase and magnitude errors introduced by the AC coupling circuitry below a few hertz However the use of DC coupling introduces serious problems where the source has a DC offset or is of such high impedance that bias currents cause significant offsets In these cases it may be necessary to include some form of signal conditioning between the signal source and the lock in amplifier The instrument always reverts to the AC coupled mode on power up to protect the input circuitry Input Signal Selection V I Although the voltage mode input is most commonly used a current to voltage converter may be switched into use to provide current mode input capability in which case the signal is connected to the B I connector High impedance sources gt 100 kQ are inherently current sources and need to be measured with a low impedance current mode input Even when dealing with a voltage source in series with a high impedance the use of the current mode input may provide advantages in terms of improved bandwidth and immunity from the effects of cable capacitance The converter may be set to low noise or wide bandwidth conversion settings but it should be noted that even at the wide bandwidth setting the 3 dB point is at 50 KHz Better performance may be achieved using a separate current preamplifier such as the EG amp
136. nitude Output 10000 FS 3 8 Phase 218000 180 4 16 Sensitivity setting 4 to 27 IMODE 0 1 2 0 32 64 5 32 ADCI 210000 10 0 V 6 64 ADC2 10000 10 0 V 7 128 Not used 8 256 DACI 210000 10 0 V 9 512 DAC2 10000 10 0 V 10 1024 Noise 10000 FS 11 2048 Ratio 10000 FS 12 4096 Log ratio 3000 to 2000 13 8192 EVENT variable 0 to 32767 14 16384 Reference frequency bits 0 to 15 mHz 15 32768 Reference frequency bits 16 to 32 mHz The values of the selected curves at the same sample point are transferred as a group in the order of the above table separated by the chosen delimiter character and terminated with the selected terminator This continues until all the points have been transferred As an example suppose CBD 5 had been sent equivalent to asserting bits 0 and 2 then the X and Magnitude outputs would be stored The permitted values of n would therefore be 1 4 and 5 DCT 1 and DCT 4 would only transfer one curve at a time but DCT 5 would transfer the X output curve and the Magnitude curve simultaneously A typical output data sequence would be E 5 Appendix E ALPHABETICAL LISTING OF COMMANDS E 6 X output value gt lt delim gt lt Magnitude value gt lt term gt X output value gt lt delim gt lt Magnitude value gt lt term gt lt X output value gt lt delim gt lt Magnitude value gt lt term gt X output value gt lt delim gt lt Magnitude value gt lt term gt X out
137. nt using Curve Storage and Frequency Sweep In this example a more sophisticated version of that given in section 6 5 03 the internal oscillator frequency sweep generator is used in conjunction with curve storage allowing the acquisition of a frequency response without the need for the computer to perform the frequency setting function for each point As before the lock in amplifier s internal oscillator output signal is fed via the filter stage under test to the signal input The oscillator frequency is stepped between a lower and an upper frequency and the signal magnitude and phase are recorded The required sequence of commands is therefore as follows IE 0 Set reference mode to internal VMODE 1 Single ended voltage input mode FET 1 10 MQ input impedance using FET stage AUTOMATIC 1 AC Gain control automatic FLOAT 1 Float input connector shell using 1 kQ to ground LF 0 Turn off line frequency rejection filter OA 1 0 Set oscillator amplitude to 1 0 V rms OF 100 0 Set initial oscillator frequency to 100 Hz so that AQN runs correctly SEN 27 Set sensitivity to 1 V full scale TC 8 Set time constant to 10 ms AQN Auto Phase The next group of commands set up the frequency sweep FSTART 100 0 Set initial oscillator frequency to 100 Hz FSTOP 1000 0 Set final oscillator frequency to 1000 Hz FSTEP 10 1 Step size 10 Hz linear law SRATE 0 1 100 ms per step There will therefore be 100 steps 100 Hz to 1000 Hz inclusive in
138. nvironmental variables needing only to be protected from exposure to corrosive agents and liquids 2 1 02 Rack Mounting An optional accessory kit part number K02002 is available from EG amp G Instruments to allow the model 7220 to be mounted in a standard 19 inch rack 2 1 03 Inspection Upon receipt the model 7220 Lock in Amplifier should be inspected for shipping damage If any is noted EG amp G INSTRUMENTS should be notified immediately and a claim filed with the carrier The shipping container should be saved for inspection by the carrier 2 1 04 Line Cord Plug A standard IEC 320 socket is mounted on the rear panel of the instrument and a suitable line cord is supplied 2 1 05 Line Voltage Selection and Line Fuses Before plugging in the line cord ensure that the model 7220 is set to the voltage of the AC power supply to be used A detailed discussion of how to check and if necessary change the line voltage setting follows CAUTION The model 7220 may be damaged if the line voltage is set for 110 V AC operation and it is turned on with 220 V AC applied to the power input connector The model 7220 can operate from any one of four different line voltage ranges 90 110 V 110 130 V 200 240 V and 220 260 V at 50 60 Hz The change from one range to another is made by repositioning a plug in barrel selector internal to the Line Input Assembly on the rear panel of the unit 2 1 Chapter 2 INSTALLATION AND INITIAL CHECKS
139. o maximize the X output and minimize the Y output AUTOMATIC n AC Gain automatic control The value of n sets the status of the AC Gain control according to the following table n Status 0 AC Gain is under manual control either using the front panel or the ACGAIN command 1 Automatic AC Gain control is activated with the gain being adjusted according to the full scale sensitivity setting E 1 Appendix E ALPHABETICAL LISTING OF COMMANDS AXO Auto Offset The X and Y output offsets are turned on and set to levels giving zero X and Y outputs Any changes in the input signal then appear as changes about zero in the outputs BURSTRATE n Sets the burst mode sampling rate for ADC1 and ADC2 n sets the sample rate for the Variable Rate burst modes according to the following equations When storing only to ADCI i e TADC 2 TADC 4 TADC 6 and TADC 8 16 000 000 Q5 x n 157 Sample Rate When storing to ADCI and ADC 2 i e TADC 3 TADC 5 TADC 7 and TADC 9 16 000 000 Sample Rate 05 x n 1031 Note that these equations apply only to units manufactured after December 1995 Earlier instruments used a 16 384 MHz instead of a 16 0 MHz crystal so the above equations should be modified accordingly by replacing the 16 000 000 figure with 16 384 000 For example when n 20 the sample rate will be 24 353 Hz for ADCI for an instrument with a 16 0 MHz crystal and 24 937 Hz for a unit with a 16 384
140. ock in amplifier operation is set to unity Throughout the remainder of this text the reference harmonic number will be assumed to be unity unless specifically stated to have a non unity value The outputs from the X channel and Y channel multipliers feed the first stage of the X 3 7 Chapter 3 TECHNICAL DESCRIPTION and Y channel output filters The outputs of these in turn drive two 16 bit digital to analog converters DACs which generate the instrument s FAST X and FAST Y analog outputs In addition the signals are fed to further low pass filters before subsequent processing by the instrument s host microprocessor 3 2 10 Output Processor Although shown on the block diagram as a separate entity the output processor is in fact part of the instrument s main microprocessor It provides more digital filtering of the X and Y channel signals if required calculates the vector magnitude R where R VX Y and phase angle 0 where 0 tan Y X and routes any of these signals via two further 16 bit DACS to the unit s CH1 and CH2 output connectors It also allows one of the two auxiliary analog inputs ADC1 and ADC2 which are digitized by a 16 bit analog to digital converter to be used in ratio calculations 3 2 11 Main Microprocessor 3 3 Accuracy All functions of the instrument are under the control of a microprocessor which in addition drives the front panel displays processes front panel key operations and supports the RS232 a
141. of thumb is to wait for a period of five time constants after the input signal has changed before recording a new value Hence in a scanning type experiment the program should issue the commands to whatever equipment causes the input signal to the lock in amplifier to change wait for five time constants and then record the required output 6 5 03 Frequency Response Measurement In this example the lock in amplifier s internal oscillator output signal is fed via the filter stage under test back to the instrument s signal input The oscillator frequency is stepped between a lower and an upper frequency and the signal magnitude and phase recorded At sampling rates of up to a few points per second there is no need to use the internal curve buffer or oscillator frequency sweep generator The commands to achieve this would therefore be similar to the following sequence IE 0 Set reference mode to internal VMODE 1 Single ended voltage input mode FET 1 10 MQ input impedance using FET stage AUTOMATIC 1 AC Gain control automatic FLOAT 1 Float input connector shell using 1 k to ground LF O Turn off line frequency rejection filter OA 1 0 Set oscillator amplitude to 1 0 V rms OF 100 0 Set oscillator frequency to 100 Hz starting frequency SEN 27 Set sensitivity to 1 V full scale TC 10 Set time constant to 50 ms AQN Auto Phase The frequency sweep would be performed and the outputs recorded by sending the following commands from a FOR NE
142. onnected B 2 Preamplifier Power Connector Pinout Figure B 2 Preamplifier Power Connector PIN FUNCTION 1 15V GROUND 3 15 V All other pins are unused Shell is shield ground B 1 Appendix B PINOUTS B 3 Digital Output Port Connector Figure B 3 Digital Output Port Connector 8 bit TTL compatible output set from the front panel or via the computer interfaces each line can drive 3 LSTTL loads This connector mates with a 20 pin IDC Header Plug The pinout is as follows PIN FUNCTION 1 GROUND 2 GROUND 3 DO 4 GROUND 5 D1 6 GROUND 7 D2 8 GROUND 9 D3 10 GROUND 11 D4 12 GROUND 13 D5 14 GROUND 15 D6 16 GROUND 17 D7 18 GROUND 19 5 V 20 5 V DO Least Significant Bit D7 Most Significant Bit B 2 Demonstration Programs Appendix C C 1 Simple Terminal Emulator This is a short terminal emulator with minimal facilities which will run on a PC compatible computer in a Microsoft GW BASIC or QuickBASIC environment or can be compiled with a suitable compiler 10 MINITERM 9 Feb 96 20 CLS PRINT Lockin RS232 parameters must be set to 9600 baud 7 data bits 1 stop bit and even parity 30 PRINT Hit lt ESC gt key to exit 40 OPEN COM1 9600 E 7 1 CS DS FOR RANDOM AS 1 100 WHILE 1 110 B INKEY 120 IF B CHR 27 THEN CLOSE 1 ON ERROR GOTO 0 END 130 IF B lt gt THEN PRINT 1 B 140 LL95 LOC 1 150 IF LL gt 0 THEN A INPUT LL
143. ons menu 5 10 MP command 6 17 E 11 MSK n command 6 9 6 29 E 11 N command 6 8 6 29 E 12 NC command 6 24 E 12 nF reference mode 3 7 5 4 NHZ command 6 17 E 12 NN command 6 18 E 12 Noise fs display 5 28 Noise in volts amps per root hertz display 5 31 Noise measurements 3 14 NOISE output 5 7 Null modem 6 2 6 5 OA n command 6 19 E 13 OF n command 6 19 E 13 Offset status control 5 25 Operating environment 2 1 OSC OUT connector 3 7 4 1 Oscillator amplitude control 5 24 frequency control 5 23 Output channel filters 3 7 3 11 expand 3 12 expand control 5 5 offset 3 12 offset indicators 5 34 processor 3 8 setup menu 5 5 Overload byte 6 7 PHA command 6 17 E 13 Phase 3 8 Phase in degrees display 5 29 Phase noise 3 6 Phase sensitive detector 3 7 Phase shifter 3 10 Phase values 3 6 PHASE output 5 7 PHASE2 output 5 7 Power consumption 2 1 Power input assembly 2 1 Power up defaults 3 14 PREAMP POWER connector 4 7 Programming examples 6 30 Prompt character 6 7 Rack mounting 2 1 Ratio calculations 3 8 Index 3 INDEX Ratio display 5 32 RATIO output 5 7 Rear panel layout 4 6 REF IN connector 3 10 4 2 5 4 REF MON connector 4 7 REF TTL connector 4 7 5 4 Reference channel DSP 3 6 Reference frequency changes 3 10 Reference frequency display 5 29 Reference harmonic 5 4 Reference harmonic number 3 7 Reference mode external 3 6 internal 3 6 Reference phase 3 10 Refe
144. or byte in input buffer 250 A INPUT 1 1 read input buffer 260 IF A lt gt C THEN PRINT handshake error input byte should be echo 270 NEXT J196 next byte to be sent or 280 RETURN return if no more bytes 290 300 310 read response 320 A Z 330 WHILE A lt gt AND A lt gt read until prompt received 340 Z Z A append next byte to string 350 WHILE LOC 1 0 WEND wait for byte in input buffer 360 A INPUT 1 1 read byte from buffer 370 WEND next byte to be read 380 RETURN return if it is a prompt 390 400 410 fetch status byte 420 B ST CR ST is the status command 430 GOSUB 180 output the command 440 GOSUB 310 read response into Z 450 STATUS VAL Z convert to integer 460 RETURN 470 instrument error message 480 PRINT Error prompt status byte STATUS bits are defined in manual 490 PRINT 500 RETURN 510 1 O error routine 520 RESUME C 2 Appendix C DEMONSTRATION PROGRAMS C 3 GPIB User Interface Program GPCOM BAS is a user interface program which illustrates the principles of the use of the serial poll status byte to coordinate the command and data transfer The program runs under Microsoft GW BASIC or QuickBASIC on a PC compatible computer fitted with a National Instruments IEEE 488 interface card and the GPIB COM software installed in the CONFIG SYS file The program B
145. ould be achieved by repeated manual keypresses Active Cursor The ADJUST keys can be used initially to place a cursor over a given digit in the displayed control setting prior to changing that digit This is done by using the procedure discussed below Step 1 Press both the up and down ADJUST keys simultaneously A cursor appears over one of the displayed digits see figures 4 5 and 4 6 Step 2 With the cursor visible repeating step 1 causes the cursor to move to the left When the cursor reaches the most significant digit available left end of control setting the next keypress returns the cursor to the least significant digit right end of control setting Continue this action until the cursor covers the required digit Step3 Press the up or down ADJUST key to change the digit to the required value Press both keys simultaneously to activate cursor J paci 40 100 V C J osc 50000 000Hz Al y ADJUST Y EGzG INSTRUMENTS jZ Press both keys simultaneously to activate cursor Figure 4 5 Active Cursor Activation As an example of this operation suppose that the oscillator frequency is 50 kHz and it is required to change it to 51 kHz Simultaneously press both the up and down ADJUST keys adjacent to the oscillator frequency display Move the cursor by repeated double keypresses until it is over the required digit in this case the zero to the right of the leading 5 Then press t
146. ould therefore be 0 and 2 so that DC 0 would transfer the X output curve and DC 2 the Magnitude curve Appendix E ALPHABETICAL LISTING OF COMMANDS The computer program s subroutine which reads the responses to the DC command needs to run a FOR NEXT loop of length equal to the value set by the LEN curve length command Note that when using this command with the GPIB interface the serial poll must be used After sending the DC command perform repeated serial polls until bit 7 is set indicating that the instrument has an output waiting to be read Then perform repeated reads in a loop waiting each time until bit 7 is set indicating that a new value is available The loop should continue until bit 1 is set indicating that the transfer is completed DCTn Dump acquired curves to computer in table format This command is similar to the DC command described above but allows transfer of several curves at a time and only operates in fixed point mode Stored curve s are transferred via the computer interface in decimal format The DCT parameter is an integer between 1 and 65 535 being the decimal equivalent of a 16 bit binary number When a given bit in the number is asserted the corresponding curve is selected for transfer When a bit is negated the curve is not transferred The bit corresponding to each curve is shown in the table below Bit Decimal value Curve and output range 0 1 X Output 10000 FS 1 2 Y Output 10000 FS 2 4 Mag
147. panel Impedance Sinusoidal Input Level Frequency Range Squarewave Input Level Frequency Range Phase Set Resolution Accuracy Low Noise or Wide Bandwidth 20 fA to 10 nA in a 1 2 5 sequence 20 fA to 1 uA in a 1 2 5 sequence gt 100 dB with no signal filters 3 dB at 500 Hz 3 dB at 50 kHz lt 2 5 kQ at 100 Hz lt 250 Q at 1 kHz 13 fANHz at 500 Hz 130 fA NHz at 1 kHz lt 0 6 typ 0 6 typ attenuates 50 60 100 120 Hz BNC shields can be grounded or floated via 1 KQ to ground mHz to 120 kHz 1 MQ 30 pF 1 0 V rms 1 Hz to 120 kHz 100 mV rms 300 mHz to 120 kHz Note Lower levels can be used with the analog input at the expense of increased phase errors 0 01 increments 0 5 typ Noise at 100 ms TC 12 dB octave Internal Reference External Reference A 2 lt 0 0001 rms lt 0 01 rms at 1 kHz Orthogonality Drift Acquisition Time Internal Reference External Reference Appendix A SPECIFICATIONS 90 0 0001 lt 0 01 C below 10 kHz lt 0 1 C above 10 kHz instantaneous acquisition 2 cycles 50 ms Reference Frequency Meter Accuracy 120 kHz gt F gt 40 kHz 40 kHz gt F gt 400 Hz 400 Hz gt F gt 1 mHz Demodulator and Output Processing Description Output Zero Stability Digital Outputs Displays Analog Outputs Harmonic Rejection Time Constants Digital Outputs Fast Outputs Roll off Synchronous Filter Operation Offset
148. plement or respond to the commands The exception is the IN n command If n matches the address set from the front panel the instrument will switch into addressed mode If n does not match the address set from the front panel the instrument will switch into unaddressed mode Note The YN n command does not change the address of an instrument but which instrument is addressed Warning All instruments must have a unique address DD n Define delimiter control ST The value of n which can be set to 13 or 32 to 125 determines the ASCII value of the character sent by the lock in amplifier to separate two numeric values in a two value response such as that generated by the MP magnitude and phase command Report status byte Causes the lock in amplifier to respond with the status byte Note this command is not normally used in GPIB communications where the status Chapter 6 COMPUTER OPERATION byte is accessed by performing a serial poll Bit 0 Command complete Bit 1 Invalid command Bit 2 Command parameter error Bit 3 Reference unlock Bit 4 Overload Bit 5 New ADC values available after external trigger Bit 6 Asserted SRQ Bit 7 Data available N Report overload byte Causes the lock in amplifier to respond with the overload byte Bit 0 not used Bit 1 CHI output overload gt 120 FS Bit2 CA2 output overload gt 120 FS Bit 3 Y output overload gt 300 96FS Bit4 X output overload gt 300 96FS Bit 5 not u
149. put value gt lt delim gt lt Magnitude value gt lt term gt etc where lt delim gt and lt term gt are the delimiter and terminator characters respectively The computer program s subroutine which reads the responses to the DCT command needs to run a FOR NEXT loop of length equal to the value set by the LEN curve length command and must be able to separate the responses on each line for storage Or processing Note that when using this command with the GPIB interface the serial poll must be used After sending the DCT command perform repeated serial polls until bit 7 is set indicating that the instrument has an output waiting to be read Then perform repeated reads in a loop waiting each time until bit 7 is set indicating that a new value is available The loop should continue until bit 1 is set indicating that the transfer is completed DD n Define delimiter control The value of n which can be set to 13 or 32 to 125 determines the ASCII value of the character sent by the lock in amplifier to separate two numeric values in a two value response such as that generated by the MP magnitude and phase command ENBWT Equivalent noise bandwidth In fixed point mode reports the equivalent noise bandwidth of the output low pass filters at the current time constant setting in microhertz In floating point mode reports the equivalent noise bandwidth of the output low pass filters at the current time constant setting in hert
150. quired and choose the outputs you wish to display In particular if you want to use the analog outputs of the instrument use the Output Setup menu to specify what the output signals at the CH1 and CH2 connectors should represent 5 35 Chapter 5 FRONT PANEL OPERATION 5 36 Computer Operation Chapter 6 6 1 Introduction The model 7220 includes both RS232 and IEEE 488 also known as GPIB for General Purpose Interface Bus interface ports designed to allow the lock in amplifier to be completely controlled from a remote computer All the instrument s controls may be operated and all the outputs read via these interfaces In addition there are some functions such as curve storage and oscillator frequency sweeps which may only be accessed remotely This chapter describes the capabilities of the instrument when operated remotely and discusses how this is done 6 2 Capabilities 6 2 01 General All instrument controls which may be set using the front panel may also be set remotely In addition the current setting of each control may be determined by the computer All instrument outputs which may be displayed on the front panel may also be read remotely When operated via the interfaces the following features are also available 6 2 02 Curve Storage A 327768 point memory is included in the instrument This may be used as a single curve or split into a number of curves each of which will record chosen instrument outputs wh
151. ral curves at a time and only operates in fixed point mode Stored curve s are transferred via the computer interface in decimal format The DCT parameter is an integer between 1 and 65 535 being the decimal equivalent of a 16 bit binary number When a given bit in the number is asserted the corresponding curve is selected for transfer When a bit is negated the curve is not transferred The bit corresponding to each curve is shown in the table below Bit Decimal value Curve and output range 0 1 X Output 10000 FS 1 2 Y Output 10000 FS 2 4 Magnitude Output 10000 FS 3 8 Phase 18000 180 4 16 Sensitivity setting 4 to 27 IMODE 0 1 2 0 32 64 5 32 ADCI 10000 10 0 V 6 64 ADC2 10000 10 0 V 7 128 Not used 8 256 DACI 10000 10 0 V 9 512 DAC2 10000 10 0 V 10 1024 Noise 10000 FS 11 2048 Ratio 10000 FS 12 4096 Log ratio 3000 to 2000 13 8192 EVENT variable 0 to 32767 14 16384 Reference frequency bits 0 to 15 mHz 15 32768 Reference frequency bits 16 to 32 mHz The values of the selected curves at the same sample point are transferred as a group in the order of the above table separated by the chosen delimiter character and terminated with the selected terminator This continues until all the points have been transferred As an example suppose CBD 5 had been sent equivalent to asserting bits 0 and 2 then the X and Magnitude outputs would be stored The permitted values of n would th
152. rence phase control 5 26 Reference setup menu 5 4 Reference source control 5 4 Reference unlock indicator 4 2 REFP n command 3 11 6 13 E 13 Relative accuracy 3 8 REMOTE n command 6 29 E 13 REV command 6 29 E 13 Right hand LCD display panel 4 5 RS n1 n2 command 6 27 E 14 RS232 interface address 6 4 address control 5 13 character echo control 5 12 choice of baud rate 6 3 choice of number of data bits 6 3 choice of parity check option 6 3 connector 4 7 data delimiter control 5 12 data format control 5 11 data prompt character control 5 12 general features 6 2 handshaking 6 5 input terminator 6 6 microprocessor support of 3 8 output terminator 6 6 setup 1 menu 5 11 setup 2 menu 5 12 setup 3 menu 5 13 RT command 6 17 E 14 SAMPLE n command 6 13 E 14 Sample rate control 5 17 SELECT keys 4 2 5 1 SEN n command 6 11 E 15 Sensitivity control 5 20 Index 4 SET key 4 5 Setup menu mode 4 4 5 1 SIG MON connector 3 4 4 7 Signal channel inputs 3 2 Signal channel overload 3 9 Signal input connectors 4 1 Single ended voltage input 5 2 Single ended voltage input mode 4 1 Slope 3 11 SLOPE n command 6 14 E 15 Slope control 5 23 Specifications detailed listing of A 1 A 6 for Auxiliary Inputs A 4 for Data Storage A 5 for Demodulator and Output Processing A 3 for Measurement Modes A 1 for Oscillator A 3 for Outputs A 4 for Reference Channel A 2 for Signal Channel A 1 SRATE n command 6
153. restores it to the default F mode according to the following table Mode selected The lock in amplifier measures signals at the reference frequency F The lock in amplifier measures at 2F The lock in amplifier measures at 3F UND Rep REFP n Reference phase control In fixed point mode n sets the phase in millidegrees in the range 360000 In floating point mode n sets the phase in degrees Auto Phase auto quadrature null The instrument adjusts the reference phase to maximize the X output and minimize the Y output signals 6 13 Chapter 6 COMPUTER OPERATION FRQ Reference frequency meter If the lock in amplifier is in the EXT or EXT LOGIC reference source modes the FRQ command causes the lock in amplifier to respond with 0 if the reference channel is unlocked or with the reference input frequency if it is locked If the lock in amplifier is in the INT reference source mode it responds with the frequency of the internal oscillator In fixed point mode the frequency is in mHz In floating point mode the frequency is in Hz LOCK System lock control Updates all frequency dependent gain and phase correction parameters 6 4 03 Signal Channel Output Filters SLOPE n Output low pass filter slope roll off control The value of n sets the slope of the output filters according to the following table Slope 6 dB octave 12 dB octave 18 dB octave 24 dB octave WNr os TC n TC Filter time constant control
154. rface parameters The values of n set the baud rate of the RS232 interface according to the following table n Baud rate bits per second 0 75 1 110 2 134 5 3 150 4 300 5 600 6 1200 7 1800 8 2000 9 2400 10 4800 11 9600 12 19200 The lowest five bits in n control the other RS232 parameters according to the following table 6 27 Chapter 6 COMPUTER OPERATION 6 28 bit number bit negated bit asserted 0 data parity 8 bits data parity 9 bits 1 no parity bit 1 parity bit 2 even parity odd parity 3 echo disabled echo enabled 4 prompt disabled prompt enabled GP n n Set read GPIB parameters Nn n sets the GPIB address in the range 0 to 31 n sets the GPIB terminator and the test echo function according to the following table Terminator CR test echo disabled CR test echo enabled CR LF test echo disabled CR LF test echo enabled no terminator test echo disabled no terminator test echo enabled Nh YN OF When the test echo is on every character transmitted or received via the GPIB port is echoed to the RS232 port This is provided solely as an aid to program development and should not be enabled during normal operation of the instrument Address command When the model 7220 is daisy chained with other compatible instruments this command will change which instrument is addressed All daisy chained instruments receive commands but only the currently addressed instrument will im
155. s set up using the STAR command to specify the output s required and invoked by sending an asterisk ASCII 42 to request the data The data returned is specified by the value of n as follows Data returned by command X Y MAG PHA ADCI XY MP ADC1 ADC2 NAYANDNHPWNFY COS Transfer command This command establishes the high speed transfer mode Use the STAR command to set up the desired response to the command and then send an ASCII 42 without terminator to the instrument The instrument will reply with the selected output as quickly as possible and then wait for another If the computer processes the reply quickly and responds immediately with another then very rapid controlled data transfer is possible The first transfer takes a little longer than subsequent ones because some overhead time is required for the model 7220 to get into the high speed transfer mode When in this mode the front panel controls are inactive and display is frozen The mode is terminated by sending any command other than an when the instrument will exit the mode and process the new command or after a period of 10 seconds following the last command Chapter 6 COMPUTER OPERATION Caution Check that the computer program does not automatically add a carriage return or carriage return line feed terminator to the command since these characters will slow down communications 6 4 06 Internal Oscillator OA n Oscillator ampli
156. sed Bit 6 input overload Bit 7 reference unlock MSK n Set read service request mask byte The value of n sets the SRQ mask byte in the range 0 to 255 REMOTE n Remote only front panel lock out control Allowed values of n are 0 and 1 When n is equal to 1 the lock in amplifier enters remote only mode in which the front panel control functions are inoperative and the instrument can only be controlled with the RS232 or the GPIB interfaces When n is equal to 0 the front panel control functions normally 6 4 11 Instrument Identification ID Identification Causes the lock in amplifier to respond with the number 7220 REV Report firmware revision Causes the lock in amplifier to respond with the firmware revision number This gives a four line response which the controlling program must be able to accept VER Report firmware version Causes the lock in amplifier to respond with the firmware version number The firmware version number is the number displayed on the front panel RS232 SETUP 3 screen 6 29 Chapter 6 COMPUTER OPERATION 6 4 12 Front Panel LTS n Lights on off control The value of n controls the front panel LEDs and LCD backlights according to the following table n Selection O Alllights off 1 Normal operation 6 4 13 Default Setting ADF Default Setting command This command will automatically set all the instrument controls and displays to the factory set default values However 1f the comm
157. selected LEDs adjacent to the connectors light to indicate which of them is active when a particular input mode is selected see figure 4 2 One or both of these LEDs will flash to indicate that the input is in overload A input indicator LED B l input indicator LED Figure 4 2 Signal Inputs 4 1 02 OSC OUT Connector This is the output connector for the internal oscillator When internal reference mode is selected the LED adjacent to the connector will be lit see figure 4 3 4 1 Chapter 4 FRONT AND REAR PANELS 4 1 03 REF IN Connector This is the input connector for a general purpose external reference signal When external reference mode is selected the LED adjacent to the connector will be lit see figure 4 3 Under unlock conditions the LED will flash oe OUT indicator LED SELECT oR 4 v ADJUST o Nal AMPLIFIER i i REF IN indicator LED Figure 4 3 OSC OUT and REF IN Connectors 4 1 04 Left hand LCD Display Panel This panel and the two pairs of keys on each side of it are normally used to select and adjust the instrument s controls such as the full scale sensitivity time constant filter slope oscillator frequency and voltage etc In this mode the display shows two of the possible range of controls and their present settings one on the upper menu line and one on the lower see figure 4 4 Upper Menu Control Control
158. shipping damage NOTE any damage must be reported to the carrier and to EG amp G INSTRUMENTS immediately take care to save the shipping container for inspection by the carrier Note that this procedure is intended to demonstrate that the instrument has arrived in good working order not that it meets specifications Each instrument receives a careful and thorough checkout before leaving the factory and normally if no shipping damage has occurred will perform within the limits of the quoted specifications If any problems are encountered in carrying out these checks contact EG amp G INSTRUMENTS or the nearest authorized representative for assistance 2 2 02 Procedure 1 Ensure that the model 7220 is set to the line voltage of the power source to be used as described in section 2 1 05 2 With the rear panel mounted power switch located at the extreme left hand side of the instrument when viewed from the rear set to O off plug in the line cord to an appropriate line source 3 Turn the model 7220 power switch to the I on position 4 The instrument s front panel displays will now briefly display the following message Left hand LCD Right hand LCD EG amp G Model 7220 Instruments Lockin Amplifier Figure 2 2 Opening Display 5 Press the key marked MENU twice to enter the setup menu screens N B on early units this key was marked AUTO 6 Press one of the keys on the left hand side of the left hand display repea
159. signment error END 130 send INTERFACE CLEAR 140 CALL IBSIC GPIBO 150 set bus address assign access code to device 160 SUCCESS 0 170 WHILE SUCCESS 0 180 INPUT BUS ADDRESS A 190 DEVNAME DEV RIGHT STR A LEN STR A 1 200 CALL IBFIND DEVNAME DEV assign access code 210 IF DEV96 0 THEN PRINT device assignment error END 220 A CHR 13 GOSUB 480 test write CR to bus 230 IF IBSTA gt 0 THEN SUCCESS 1 240 IF IBSTA lt 0 AND IBERR 2 THEN BEEP PRINT NO DEVICE AT THAT ADDRESS 250 WEND 260 send SELECTED DEVICE CLEAR 270 CALL IBCLR DEV 280 set timeout to 1 second C 3 Appendix C DEMONSTRATION PROGRAMS C 4 290 V 11 CALL IBTMO DEV V 300 set status print flag 310 INPUT Display status byte y n R 320 IF R Y OR R y THEN DS 1 ELSE DS 0 330 Mal OP ones 340 WHILE 1 350 INPUT command 00 to exit A 360 IF A 00 THEN END 370 A A CHR 13 380 GOSUB 480 390 S 0 400 WHILE S AND 1 0 410 GOSUB 530 420 IF DS THEN PRINT S S 430 IF S AND 128 THEN GOSUB 500 PRINT B 440 WEND 445 IF S AND 4 THEN PRINT parameter error 450 IF S AND 2 THEN PRINT invalid command 460 WEND 470 end of main Oseas 480 write string to DUS 490 CALL IBWRT DEV A RETURN 500 read string from Dus 510 B
160. subsequent sweep of the internal oscillator frequency In fixed point mode n is in millihertz In floating point mode n is in hertz GP n n Set read GPIB parameters n sets the GPIB address in the range 0 to 31 n sets the GPIB terminator and the test echo function according to the following table Terminator CR test echo disabled CR test echo enabled CR LF test echo disabled CR LF test echo enabled no terminator test echo disabled no terminator test echo enabled nNBWNrF OS When the test echo is on every character transmitted or received via the GPIB port is echoed to the RS232 port This is provided solely as an aid to program development and should not be enabled during normal operation of the instrument E 8 HC IE n Appendix E ALPHABETICAL LISTING OF COMMANDS Halt curve acquisition Halts curve acquisition in progress It is effective during both single data acquisition initiated by TD command and continuous data acquisition initiated by TDC command curve acquisitions The curve may be restarted by means of the TD or TDC command as appropriate Identification Causes the lock in amplifier to respond with the number 7220 Reference channel source control Internal External The value of n sets the reference input mode according to the following table Selection INT internal EXT LOGIC external rear panel TTL input EXT external front panel analog input N O0Op IMO
161. t modern software there is no need to assert any RS232 handshake lines and a simple three wire connection can be used The actual handshake function is performed by means of bytes transmitted over the interface The more critical handshake is the one controlling the transfer of a command from the computer to the lock in amplifier because the computer typically operates much faster than the lock in amplifier and bytes can easily be lost if the command is sent from a program Note that because of the limited speed of human typing there is no problem in the terminal mode Therefore an echo handshake is used which works in the following way after receiving each byte the lock in amplifier sends back an echo that is a byte which is a copy of the one that it has just received to indicate that it is ready to receive the next byte Correspondingly the computer does not send the next byte until it has read the echo of the previous one Usually the computer makes a comparison of each byte with its echo and this constitutes a useful check on the validity of the communications Where the echo is not required it can be suppressed by negating bit 3 in the RS232 6 5 Chapter 6 COMPUTER OPERATION 6 6 parameter byte The default power up state of this bit is for it to be asserted The program RSCOM2 BAS in section C 2 illustrates the use of the echo handshake 6 3 09 Terminators In order for communications to be successfully established betw
162. t with and it is normal practice to make small adjustments to the reference frequency until a clear point on the frequency spectrum is reached In this context an unwanted alias is treated as just another interfering signal and its frequency is avoided when setting the reference frequency A buffered version of the analog signal just prior to the main ADC is available at the rear panel signal monitor SIG MON connector it may be viewed on an oscilloscope to monitor the effect of the signal channel filters and amplifiers 3 2 06 Main Analog to Digital Converter Following the anti alias filter the signal passes to the main 18 bit analog to digital converter running at a sampling rate of 166 KHz This rate is not fixed but is adjusted automatically by up to 1 46 as a function of the reference frequency to ensure that the sampling process does not generate a beat frequency close to zero hertz For example if the reference frequency were 82 95 kHz and the sampling frequency were not adjusted a beat frequency of 50 Hz 182 95 kHz 166 kHz 2 l would be generated and would appear at the output if the time constant were not set to a large enough value 3 5 Chapter 3 TECHNICAL DESCRIPTION 3 6 There is one situation where this automatic correction might not be sufficient to give good performance Consider the case where the signal being measured is at 73 kHz which is 10 KHz away from half the sampling frequency If there were also a s
163. tedly until 2 3 Chapter 2 INSTALLATION AND INITIAL CHECKS 2 4 7 8 9 the CONTROL SETUP menu is displayed which will look similar to the following Left hand LCD Right hand LCD CONTROL Default ud rate SETUP Setting Figure 2 3 Control Setup Menu Press one of the keys on the right hand side of the left hand display once This will set all the instrument s controls and displays to a known state The displays will revert to the normal mode with the left hand panel showing the AC Gain and Full Scale Sensitivity controls and the right hand one the instrument s outputs in the form of magnitude as a percentage of full scale and phase in degrees The right hand display should now look as follows Mag 4fs PHA deg 0 00 0 0 Figure 2 4 Right hand LCD Main Display Connect a BNC cable between the OSC OUT and A input connectors on the front panel 10 The right hand display should now indicate a magnitude close to 100 of full scale i e the sinusoidal oscillator output which was set to 1 kHz and a signal level of 0 5 V rms by the Default Setting key is being measured with a full scale sensitivity of 500 mV rms and a phase of near zero degrees if a short cable is used This completes the initial checks Even though the procedure leaves many functions untested if the indicated results were obtained the user can be reasonably sure that the unit incurred no hidden damage in shipment and is in good
164. ter external trigger not used bit 6 asserted SRQ input overload bit 7 data available reference unlock Chapter 6 COMPUTER OPERATION 6 3 14 Service Requests The interface defined by the IEEE 488 standard includes a line pin 10 on the connector called the SRQ service request line which is used by the instrument to signal to the controller that urgent attention is required At the same time that the instrument asserts the SRQ line it also asserts bit 6 in the status byte The controller responds by executing a serial poll of all the instruments on the bus in turn and testing bit 6 of the status byte in order to discover which instrument was responsible for asserting the SRQ line The status byte of that instrument is then further tested in order to discover the reason for the service request and to take appropriate action In the model 7220 the assertion of the SRQ line is under the control of a byte called the SRQ mask byte which can be set by the user with the MSK command If any bit in the status byte becomes asserted and the corresponding bit in the mask byte has a non zero value the SRQ line is automatically asserted If the value of the mask byte 1s zero the SRQ line is never asserted Hence for example if the SRQ mask byte is set to 16 a service request would be generated as soon as an overload occurred if the SRQ mask byte were set to 0 then service requests would never be generated 6 9 Chapter 6 COMPUTER OPERATION
165. the frequency meter to give a fully accurate value With internal reference operation regardless of the frequency mode the frequency dependent parameters are updated on any change of reference frequency without the need to press the front panel SET key or to issue the LOCK command 3 7 Reference Phase and Phase Shifter 3 10 If the reference input is a sinusoid applied to the REF IN socket the reference phase is defined as the phase of the X demodulation function with respect to the reference input This means that when the reference phase is zero and the signal input to the demodulator is a full scale sinusoid in phase with the reference input sinusoid the X output of the demodulator is a full scale positive value and the Y output is zero Chapter 3 TECHNICAL DESCRIPTION The circuits connected to the REF IN socket actually detect a positive going crossing of the mean value of the applied reference voltage Therefore when the reference input is not sinusoidal its effective phase is the phase of a sinusoid with positive going zero crossing at the same point in time and accordingly the reference phase is defined with respect to this waveform Similarly the effective phase of a reference input to the TTL REF IN socket is that of a sinusoid with positive going zero crossing at the same point in time The reference phase is adjusted to its required value by the use of a digital phase shifter which is accessed from the front panel by
166. the input configuration according to the following table n Input configuration 0 Both inputs grounded test mode 1 A input only 3 A B differential mode Note that the IMODE command takes precedence over the VMODE command X output In fixed point mode causes the lock in amplifier to respond with the X demodulator output in the range 30000 full scale being 10000 In floating point mode causes the lock in amplifier to respond with the X demodulator output in volts or amps XOF n n X output offset control XY YL The value of n sets the status of the X offset facility according to the following table n Selection 0 Disables offset facility 1 Enables offset facility The range of n is 30000 corresponding to 300 full scale X Y outputs Equivalent to the compound command X Y Y output In fixed point mode causes the lock in amplifier to respond with the Y demodulator output in the range 30000 full scale being 10000 In floating point mode causes the lock in amplifier to respond with the Y demodulator output in volts or amps Appendix E ALPHABETICAL LISTING OF COMMANDS YOF n n Y output offset control The value of n sets the status of the Y offset facility according to the following table n Selection 0 Disables offset facility 1 Enables offset facility The range of n is 230000 corresponding to 300 full scale E 21 Appendix E ALPHABETICAL LISTING OF COMMANDS E
167. ties can be written in a few lines of BASIC code see appendix C 1 6 3 02 RS232 Interface General Features The RS232 interface in the model 7220 is implemented with three wires one carries digital transmissions from the computer to the lock in amplifier the second carries digital transmissions from the lock in amplifier to the computer and the third is the Logic Ground to which both signals are referred The logic levels are 12 V referred to Logic Ground and the connection may be a standard RS232 cable in conjunction with a null modem or alternatively may be made up from low cost general purpose cable The pinout of the RS232 connectors are shown in appendix B and cable diagrams suitable for coupling the instrument to a computer are shown in appendix D Chapter 6 COMPUTER OPERATION The main advantages of the RS232 interface are 1 It communicates via a serial port which is present as standard equipment on nearly all computers using leads and connectors which are available from suppliers of computer accessories or can be constructed at minimal cost in the user s workshop 2 Itrequires no more software support than is normally supplied with the computer for example Microsoft s GWBASIC QBASIC or Windows Terminal mode A single RS232 transmission consists of a start bit followed by 7 or 8 data bits an optional parity bit and 1 stop bit The rate of data transfer depends on the number of bits per second sent over the interface
168. tors can be set either from the front panel or by the use of remote computer commands The output range is 10 0 V and the resolution is 1 mV 4 2 15 FAST X FAST Y Connectors The signals at these two connectors are the X channel and Y channel output signals derived from a point after the first stage of output low pass filtering The maximum time constant that can be used is 640 us with a fixed slope of 6 dB octave Visual interpretation of the waveforms at these connectors as displayed on an oscilloscope when the instrument is operating in the highband mode i e above 60 KHz is difficult Front Panel Operation Chapter 5 5 1 Introduction This chapter describes how to operate the model 7220 using the front panel controls and describes its capabilities when used in this way Chapter 6 provides similar information in the situation where the unit is operated remotely using one of the computer interfaces Readers should refer to chapter 4 for a detailed description of the use of the SELECT and ADJUST keys and the functions of the left and right hand display panels However for ease of use some of this information is repeated here The model 7220 uses a flexible menu based control structure which allows many instrument controls to be adjusted from the front panel with only a few keys Furthermore this design makes it very easy to introduce new features or improve existing ones without the limitation resulting from a fixed front panel
169. trong interfering signal at 93 KHz i e 166 kHz 2 10 kHz then an alias of this would give rise to a spurious output Note that under these circumstances the reference frequency is not sufficiently close to half the sampling frequency to cause the latter to be automatically adjusted The problem is overcome by providing the Sample Rate control which allows the user to adjust the main ADC sampling rate in steps of about 1 A 1 change moves the alias by about 1 KHz which is normally sufficient to ensure rejection by the output low pass filters and thereby remove any error The output from the converter feeds the first of the digital signal processors which implements the digital multiplier and the first stage of the output low pass filtering for each of the X and Y channels 3 2 07 Reference Channel DSP The second DSP in the instrument is responsible for implementing the reference trigger phase locked loop digital phase shifter and internal oscillator look up table functional blocks on the block diagram The processor generates two main outputs the first being a series of phase values which are used to drive the other DSP s reference channel input and the second being a sinusoidal signal which may be used as the instrument s internal oscillator output The normal operating mode of the instrument incorporates two reference frequency ranges namely the baseband from 1 mHz to 60 kHz and the highband from 60 kHz to 120 kHz Different hardwar
170. ts in the status byte is instrument dependent apart from bit 6 the request service bit whose functions are defined by the standard In the model 7220 bits 0 and 7 signify command complete and data available 6 7 Chapter 6 COMPUTER OPERATION 6 8 respectively In GPIB communications the use of these bits can lead to a useful simplification of the control program by providing a response subroutine which is the same for all commands whether or not they send a response over the bus The principle is that after any command is sent serial poll operations are repeatedly executed After each operation bit 0 is tested if this is found to be negated then bit 7 is tested and if this is asserted then a read operation is performed Serial poll operations continue until the lock in amplifier asserts bit O to indicate that the command response sequence is complete This method deals successfully with compound commands In RS232 communications comparatively rapid access to the status byte is provided by the prompt character which is sent by the lock in amplifier at the same time as bit O becomes asserted in the status byte This character is sent out by the lock in amplifier after each command response whether or not the response includes a transmission over the interface to indicate that the response is finished and the instrument is ready for a new command The prompt takes one of two forms If the command contained an error either i
171. tude control In fixed point mode n sets the oscillator amplitude in mV The range of n is 0 to 5000 representing 0 to 5 V rms In floating point mode n sets the amplitude in volts OFT n Oscillator frequency control In fixed point mode n sets the oscillator frequency in mHz The range of n is O to 120 000 000 representing 0 to 120 kHz In floating point mode n sets the oscillator frequency in Hz The range of n is 0 to 1 2E5 SYNCOSC n Synchronous oscillator demodulator monitor control This control operates only in external reference mode The parameter n has the following significance n Effect 0 Synchronous Oscillator Demodulator Monitor disabled 1 Synchronous Oscillator Demodulator Monitor enabled When enabled and in external reference mode the instrument s OSC OUT connector functions as a demodulator monitor of the X channel demodulation function FSTARTT n Oscillator frequency sweep start frequency Sets the start frequency for a subsequent sweep of the internal oscillator frequency In fixed point mode n is in millihertz In floating point mode n is in hertz FSTOP n Oscillator frequency sweep stop frequency Sets the stop frequency for a subsequent sweep of the internal oscillator frequency In fixed point mode n is in millihertz In floating point mode n is in hertz 6 19 Chapter 6 COMPUTER OPERATION FSTEP n n Oscillator frequency sweep step size and type The frequency ma
172. unwanted coupling or crosstalk between the signal channel and the reference channel either in the external connections or possibly under some conditions in the instrument itself Note that if a zero error is present the Auto Offset function should be executed before any execution of Auto Phase AUTO MEASURE This function only operates when the reference frequency is greater than 1 Hz It performs the following operations 5 19 Chapter 5 FRONT PANEL OPERATION The line filter is disabled AC coupling is established the voltage measurement mode is entered using the single ended A input the FET input devices are enabled the FLOAT mode is set If the reference frequency is more than 10 Hz the output time constant is set to 100 ms otherwise it is set to the lowest synchronous value the filter slope is set to 12 dB octave output expand is switched off the reference harmonic mode is set to 1 the X offset and Y offset functions are switched off Auto Phase and Auto Sensitivity functions are called The Auto Sensitivity function also adjusts the AC Gain if required The Auto Measure function is intended to provide a means of setting the instrument s controls quickly to conditions which will be approximately correct for typical simple single ended measurement situations For optimum results in any given situation it may be convenient to start with an Auto Measure operation and subsequently fine tune the setup conditions manually The
173. usted following use of this function unless you have used their default settings Sample rate This control adjusts the sampling rate of the instrument s main ADC offering four possible settings It should only be necessary to use it if you suspect that an interfering signal is being aliased into the instrument s output 5 17 Chapter 5 FRONT PANEL OPERATION 5 3 Auto Functions Menu 5 18 When in the Main Display mode one press of the MENU key accesses the AUTO MENU showing four auto functions that are built into the instrument The left hand LCD changes to that shown in figure 5 14 Auto Sensitivity Auto Phase SELECT ADJUST AJLY SEN PHASE A LY GI MEASURE OFFSET 4 7 SELECT ADJUST gy EGzG INSTRUMENTS Auto Measure Auto Offset Figure 5 14 Auto Functions Menu Left hand Display To activate one of the auto functions press one of the keys adjacent to it as shown in figure 5 14 The display will immediately change to a message indicating that the selected function is in progress and will revert to the Main Display mode when the function is completed The four functions operate as follows AUTO SEN This function only operates when the reference frequency is greater than 1 Hz A single Auto Sensitivity operation consists of increasing the full scale sensitivity range if the magnitude output is greater than 90 of full scale or reducing the range if the magnitude output is less than 30
174. usually called the baud rate In the model 7220 the baud rate can be set to a range of different values up to 19 200 corresponding to a minimum time of less than 0 5 ms for a single character Mostly for historical reasons there are a very large number of different ways in which RS232 communications can be implemented Apart from the baud rate options there are choices of data word length 7 or 8 bits parity check operation even odd or none and number of stop bits 1 or 2 With the exception of the number of stop bits which is fixed at 1 these settings may be adjusted using the three front panel RS232 SETUP menus discussed in chapter 4 They may also be adjusted by means of the RS command In order to achieve satisfactory operation the RS232 settings must be set to exactly the same values in the terminal or computer as in the lock in amplifier 6 3 03 Choice of Baud Rate Where the lock in amplifier is connected to a terminal or to a computer implementing an echo handshake the highest available baud rate of 19 200 is normally used if as is usually the case this rate is supported by the terminal or computer Lower baud rates may be used in order to achieve compatibility with older equipment or where there is some special reason for reducing the communication rate 6 3 04 Choice of Number of Data Bits The model 7220 lock in amplifier uses the standard ASCII character set containing 127 characters represented by 7 bit binary words
175. ve storage memory and status variables All record of previously taken curves is removed Causes the lock in amplifier to respond with the square root of the noise spectral density measured at the Y output expressed in volt VHz or amp V Hz referred to the input This measurement assumes that the Y output is Gaussian with zero mean Section 3 10 The command is only available in floating point mode This command is not available when the reference frequency exceeds 60 kHz Noise output In fixed point mode causes the lock in amplifier to respond with the mean absolute value of the Y output in the range 0 to 12000 full scale being 10000 If the mean value of the Y output is zero this is a measure of the output noise In floating point mode causes the lock in amplifier to respond in volts Appendix E ALPHABETICAL LISTING OF COMMANDS OA n Oscillator amplitude control In fixed point mode n sets the oscillator amplitude in mV The range of n is 0 to 5000 representing 0 to 5 V rms In floating point mode n sets the amplitude in volts OF n Oscillator frequency control In fixed point mode n sets the oscillator frequency in mHz The range of n is O to 120 000 000 representing 0 to 120 kHz In floating point mode n sets the oscillator frequency in Hz The range of n is 0 to 2E5 PHA Signal phase In fixed point mode causes the lock in amplifier to respond with the signal phase in centidegrees in the range 18000
176. ved from the same source as that which generates the signal and is therefore not exactly at the same frequency In this case if the input signal is a sinusoid of constant amplitude the X and Y demodulator outputs show slow sinusoidal variations at the difference frequency and the magnitude output remains steady However the magnitude output has disadvantages where significant noise is present at the outputs of the demodulator When the required signal outputs i e the mean values of the demodulator outputs are less than the noise the outputs take both positive and negative values but the magnitude algorithm gives only positive values this effect sometimes called noise rectification gives rise to a zero error which in the case of a Gaussian process has a mean value equal to 0 798 times the combined root mean square rms value of the X and Y demodulator noise Note that unlike other forms of zero error this is not a constant quantity which can be subtracted from all readings because when the square root of the sum of the squares of the required outputs becomes greater than the total rms noise the error due to this mechanism disappears A second type of signal dependent error in the mean of the magnitude output occurs as a result of the inherent non linearity of the magnitude formula this error is always positive and its value expressed as a fraction of the signal level is half the ratio of the mean square value of the noise to the square o
177. wanted coupling or crosstalk between the signal channel and the reference channel either in the external connections or possibly under some conditions in the instrument itself Note that if a zero error is present the Auto Offset function should be executed before any execution of Auto Phase 3 15 Chapter 3 TECHNICAL DESCRIPTION 3 16 3 12 05 Auto Measure This function only operates when the reference frequency is greater than 1 Hz It performs the following operations The line filter is disabled AC coupling is established the voltage measurement mode is entered with the single ended A input mode the FET input device is selected and the FLOAT mode is set If the reference frequency is more than 10 Hz the output time constant is set to 100 ms otherwise it is set to the lowest synchronous value the filter slope is setto 12 dB octave output expand is switched off the reference harmonic mode is set to 1 the X offset and Y offset functions are switched off and the Auto Phase and Auto Sensitivity functions are called The Auto Sensitivity function also adjusts the AC Gain if required The Auto Measure function is intended to give a quick setting of the instrument which will be approximately correct in typical simple single ended measurement situations For optimum results in any given situation it may be convenient to start with Auto Measure and to make subsequent modifications to individual controls 3 12 06 Default Setting W
178. y be swept either linearly or logarithmically as specified by parameter n The step size is specified by parameter n Log sweep n 0 In fixed point mode n is the step size in thousandths of a percent In floating point mode n is in percent The range of n is O to 100 00 Linear sweep n 1 In fixed point mode n is the step size in millihertz In floating point mode n is in hertz The range of n is 0 to 10 kHz SRATE n Oscillator frequency sweep step rate Sets the sweep rate in time per step in the range 50 ms to 1000 s in 5 ms increments SWEEP n Oscillator frequency sweep start stop Starts stops the internal oscillator frequency sweep depending on the value of n according to the following table n Frequency sweep status O Stop Pause Run When a frequency sweep has been defined applying SWEEP 1 will start it The sweep will continue until the stop frequency is reached If during the sweep SWEEP 0 is applied the sweep will stop at the current frequency If SWEEP 1 is then applied the sweep will restart from this point Once the sweep reaches the stop frequency and stops the next SWEEP 1 command will reset the frequency to the start frequency and restart the sweep 6 4 07 Auxiliary Outputs DAC n n Auxiliary DAC output controls Sets the voltage appearing at the rear panel DACI and DAC2 outputs The first parameter n which specifies the DAC is compulsory and is either 1 or 2 The value of n sp
179. y means of the GP command A most important aspect of the GPIB is that its operation is defined in minute detail by the IEEE 488 standard usually implemented by highly complicated special purpose semiconductor devices that are present in each instrument and communicate with the instrument s microprocessor The existence of this standard greatly simplifies the problem of programming the bus controller i e the computer to implement complex measurement and test systems involving the interaction of numerous instruments There are fewer interface parameters to be set than with RS232 communications The operation of the GPIB requires the computer to be equipped with special purpose hardware usually in the form of a plug in card and associated software which enable it to act as a bus controller The control program is written in a high level language usually BASIC or C containing additional subroutines implemented by software supplied by the manufacturer of the interface card Chapter 6 COMPUTER OPERATION Because of the parallel nature of the GPIB and its very effective use of the control lines including the implementation of a three wire handshake see below comparatively high data rates are possible up to a few hundred thousand bytes per second In typical setups the data rate of the GPIB itself is not the factor that limits the rate of operation of the control program 6 3 08 Handshaking and Echoes A handshake is a method of ensuring
180. z This command is not available when the reference frequency exceeds 60 kHz EVENT n Event marker control During a curve acquisition if bit 13 in the CBD command has been asserted the lock in amplifier stores the value of the Event variable at each sample point This can be used as a marker indicating the point at which an experimental parameter was changed The EVENT command is used to set this variable to any value between O and 32767 Appendix E ALPHABETICAL LISTING OF COMMANDS EX n Output expansion control Expands X and or Y outputs by a factor of 10 Changes meter CH1 and CH2 outputs full scale to 10 if X or Y selected The value of n has the following significance n Expand mode 0 Off 1 Expand X 2 Expand Y 3 Expand X and Y FET n Voltage mode input device control The value of n selects the input device according to the following table n Selection O0 Bipolar device 10 KQ input impedance 2 nV VHz voltage noise 1 FET 10 MQ input impedance 5 nV VHz voltage noise FLOAT n Input connector shield float ground control The value of n sets the input shield switch according to the following table n Selection O Ground Float connected to ground via a 1 kQ resistor ENF n Reference harmonic mode control The value of n sets the reference channel to one of the NF modes or restores it to the default F mode according to the following table n Mode selected 1 The lock in amplifier measures signals at the r

IBM Car Amplifier 7220 User's Manual

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