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Metrolab NMR Teslameter Manual 2003 (See Mod P. 5 for 2010

1. N A Not Applicable bits 7 to 4 N A Always set to 0 bit 3 NMR Signal Seen This bit is set to 1 if the instrument detects the appearance of the NMR signal since this register was last read This bit is only RESET by the reading of this register bit 2 NMR Signal State This bit gives the state of the NMR signal at the time this register is read A 1 indicates the presence of the NMR signal and A 0 the absence of the NMR signal bit 1 TOO HI 1 indicates that at the time of reading this register one or several of the TOO HI LEDs were lit bit 0 TOO LO 1 indicates that at the time of reading this register one or several of the TOO LO LEDs were lit Note Bits O and 1 are useful to center the 5 scan window when the PT 2025 is in AUTO mode METROLAB Instruments SA 33 NMR Teslameter PT 2025 User s Manual 5 3 3 Status Register 3 Instrument Functions 1 byte Bits 7 6 5 4 3 2 1 0 Fast SEARCH Field AUTO Display Reading Mode MAN Tesla Display MUEIELERER Mode MHz bit 7 Fast reading display 1 indicates that the PT 2025 is operated in the Fast reading display mode bits 6 5 4 Multiplexer channel These three bits indicate which channel of the multiplexer is selected Bits 654 Channel 000 A 001 B 010 C 011 D 100 E 101 F 110 G 111 H bit 3 SEARCH mode 1 indicates that the SEARCH mode is acti
2. indicates an occurrence of the NMR signal in the last measurement cycle it may also indicate that the signal is present but that the PT 2025 is not in AUTO mode The character W for Wrong indicates that the data given has no significance and should be ignored e g after a TRIGGER d The displayed value is composed of an ASCII character between 0 and 9 inclusive Leading O suppression is performed Note that the decimal point is represented by the ASCII character and is included in every message 42 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual F Represents the ASCII character F to indicate the value is in MHz T Represents the ASCII character T to indicate that the value is converted into Tesla lt CR gt lt LF gt Represents the ASCII characters carriage return and line feed respectively These characters are always transmitted at the end of a RS 232 C message Note in the Fast reading display mode the last digit of the display is not visible 6 4 1 2 REMOTE Message format R This message disables the front panel of the PT 2025 with the exception of the LOCAL push button and puts the instrument into the RS 232 C REMOTE mode The instrument will now respond to the messages described in this chapter Messages issued prior to the REMOTE message will be ignored with the exception of the reading of the displayed value and the four status regist
3. 0 8 Amp for 220 VACs 1 6 Amp for 110 VAC Slow Blow OPTIONAL HIGH STABILITY COUNTER HS OVEN CONTROLLED QUARTZ OSCILLATOR Warm up Stability Ageing 5 minutes at 25 C lt 5 ppb C within 10 to 40 C ambient temperature lt 2 ppb day after 30 working days METROLAB Instruments SA 9 NMR Teslameter PT 2025 User s Manual METROLAB Instruments SA 10 3 NMR Teslameter PT 2025 User s Manual USER S GUIDE Ensure that the line voltage which is indicated on the back panel of the PT 2025 corresponds to that of your location A selector situated just below the line voltage input enables you to change the voltage accordingly With the power cable removed use a screwdriver to extract the fuse holder and replace it in the desired position The main power switch situated on the back panel should be set to OFF Either the 220 VAC or the 115 VAC power cord whichever is required should then be connected The approximate magnitude of the field should be known or measured by external means eg Hall Probe value of magnet coil current etc Choose the relevant probe according to the magnitude of the field Where the probe ranges overlap it is recommended that the lower field range probe to use which will produce a larger NMR signal If an amplifier 1030 is used set correctly the Probe Select Switch one position for probes 2 to 8 and the other position for probe 1 only It is possible
4. It is detected with two Schottky diodes and transmitted by means of an emitter follower through the coaxial cable and a low pass filter to the Amplifier The a c portion of the detected voltage is amplified by a factor of 3000 while unity gain is provided for the d c component The upper frequency limit is approximately 20 kHz i e higher than the 10 kHz frequency used for automatic tuning After this Amplifier the amplitude of the NMR signal may vary from about 100 mV which is near the lower limit for locking the radio frequency to the field to several Volts A slightly smoothed output RC integration with 10 kQ and 10 nF is available at the front panel of the PT 2025 for scope inspection of the NMR signal Its d c component indicates the radio frequency voltage amplitude at the LC resonance circuit in the probe which should be about 0 1 to 1 V depending on frequency For example this checks very quickly whether the connected probe corresponds to the selected frequency range and whether the automatic tuning works properly Because of the weakness of the NMR signal the radio frequency voltage must be extremely clean with respect to any spurious amplitude or frequency modulation and noise otherwise the signal to noise ratio becomes poor The waveform shape of the radio frequency signal however is not important since the LC resonant circuit and the NMR sample in the probe are insensitive to any harmonics In the Amplif
5. User s Manual 6 4 THE TWO OPERATIONAL MODES OF THE RS 232 C INTERFACE The interface of the PT 2025 can function in two different ways The first is called conversational and allows the user to send messages to the PT 2025 as well as read the last measured field value or the internal instrument status registers The second method is called autonomous the PT 2025 is sending the last measured value to the RS 232 C interface at regular intervals user defined with the micro switches In this mode all incoming messages are ignored 6 4 1 Conversational Mode To select this mode micro switch 7 must be set to 1 The PT 2025 now awaits messages coming from the external device via the RS 232 C interface These messages are now described in detail 6 4 1 1 Reading the Displayed Value lt ENQ gt Message format lt ENQ gt Code ASCII Hex 05 After the reception of the ASCII character lt ENQ gt the displayed value is transmitted to the user in the following message format vdd ddddddF lt CR gt lt LF gt vdd dddddddT lt CR gt lt LF gt V The ASCII character L for Locked indicates that the Teslameter was locked during the measurement cycle and therefore the displayed value is valid The character N for Not Locked indicates that the Teslameter has not seen an NMR signal during the measurement cycle and therefore the displayed value is invalid The character S for Signal
6. 1 5 Tesla METROLAB Instruments SA 57 NMR Teslameter PT 2025 User s Manual 58 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 8 PRINCIPLE OF OPERATION The measuring head comprises a small glass tube which contains either H or H around which a flat radio frequency coil for modulating the field width is wound The applied field modulation Bmog is a symmetric 30 Hz to 70 Hz triangular waveform with an amplitude of a few hundred ppm of the measured field Bo The radio frequency coil a tuning diode and the coaxial cable from the measuring head to the signal detection circuit form a parallel LC resonant circuit This resonant circuit is weakly coupled by means of a resistor to the output of a radio frequency Amplifier with a stabilized output amplitude and is automatically tuned to the applied frequency If the chosen frequency is close enough to the nuclear resonance frequency corresponding to the main field Bo an absorption signal i e amplitude variation appears in the LC resonant circuit every time the resonance is crossed due to field modulation This signal is amplified in the Amplifier and transmitted to the PT 2025 A sample and hold circuit produces an error voltage which is proportional to the modulating field at the instant when nuclear resonance occurs With this error voltage the frequency of the radio frequency oscillator in the PT 2025 is regulated in such a way that nuclear reso
7. Mode Message format Xn Where n is a number from 1 to 8 inclusive X1 is selected on power up of RESET This message tells the PT 2025 the number of multiplexer channels that are to be scanned in the SEARCH mode When the PT 2025 enters the SEARCH mode it starts its search on the channel that was last selected either by the MUX switch when the instrument was put into REMOTE mode or the last value received in the Pc message If the signal is not found on this channel the PT 2025 will follow on to the next channel and so on up to the number of channels given by the Xn message When the last channel has been searched and no signal is detected the search restarts on the first selected channel and the cycle is repeated Example Consider that probes 3 4 and 5 are connected respectively to channels B C and D of the multiplexer Send following messages PB starts search on channel B X3 searches over 3 channels H amp CHR 13 amp CHR 10 starts searching The search will in this case start on channel B probe 3 and if no NMR signal is found it will continue on channel C probe 4 and then on to channel D probe 5 If the search fails to locate a field the PT 2025 will recommence the search on channel B If an NMR signal is detected the PT 2025 will lock on to it The SEARCH mode has been designed to follow the NMR signal even if it goes out of one probe s ra
8. This interface supports the following two modes Listener Talker addressed mode Talker only The PT 2025 can perform following functions SH1 AH1 T5 L4 SR1 RL1 PPO DC1 DT1 C0E1 The micro switch configuration on rear panel to select one of the above two modes is now given Micro switch N Comment 1to5 Device address in Listener Talker mode or repetition rate when in Talker only mode refer to section 5 4 Must be 0 Listener Talker addressed mode Talker Only section 5 4 suppresses the transmission of lt CR gt lt LF gt as message terminator PT 2025 sends lt CR gt lt LF gt to terminate messages 0 selects RS 232 C interface selects IEEE 488 interface In this document the IEEE functions such as REMOTE and LOCAL are referred to as Commands and ASCII data sent to the PT 2025 when it is addressed as a Listener are referred as Messages METROLAB Instruments SA 19 NMR Teslameter PT 2025 User s Manual 5 1 LISTENER TALKER Reading the Display refer to section 5 1 3 Switch No Setting 1to5 Device address 6 0 7 0 8 1 0 9 1 Refer to note in section 5 4 In this mode the PT 2025 can receive specific messages which give the user full control of the instrument To receive these messages the PT 2025 must be addressed as a Listener The Teslameter also reacts to the standard IEEE 488 functions such as REMOTE or LOCAL etc It is also p
9. the user is advised to set the FINE potentiometer to 5 0 The microprocessor sweeps the radio frequency from the low end of the probe and waits to see if the NMR signal is seen Once it has detected a signal it must decide whether this is really the NMR signal or if it is caused by some external interference The algorithm rapidly rescans this zone to check for the reoccurrence of the NMR signal and if this is seen puts the PT 2025 into the AUTO mode to try to lock on to the NMR signal If the second occurrence is not seen the algorithm presumes that the signal was due to noise Once in the AUTO mode the algorithm waits for five seconds and if the locked condition is not reached changes the polarity of the field After another period of five seconds the PT 2025 is either locked or the scan is continued as no signal was found Once in the locked state the PT 2025 s search algorithm uses a servo control to avoid losing a signal from a drifting field A variation of 1 sec over the probe s range increasing or decreasing can be tolerated Should the signal become unlocked the algorithm will restart the search from the present field value A typical search may last for about 12 seconds depending on field value and polarity To quit the SEARCH mode the user must push the LOCAL SEARCH button at which point the PT 2025 will return to front panel control During the search for the NMR signal noise signals which pass the de
10. 27 NMR Teslameter PT 2025 User s Manual Example OUTPUT 708 S3 request status reg no 3 ENTER 7 8 R addresses PT 2025 as a Talker DISP R R S35 If switch 8 1 then the status will be followed by a lt CR gt lt LF gt 5 1 3 The PT 2025 Addressed as a Talker Reading the Displayed Value When the instrument is addressed as a Talker it sends the displayed value according to the format described below The EOI signal is set true before the last character is transmitted in order to indicate the end of the message The displayed value can be transmitted to the user in the following message format vdd dddddddT lt CR gt lt LF gt vdd ddddddF lt CR gt lt LF gt v The ASCII character L for Locked indicates that the Teslameter was locked during the measurement cycle and therefore the displayed value is valid The character N for Not Locked indicates that the Teslameter has not seen an NMR signal during the measurement cycle and therefore the displayed value is invalid The character S for Signal indicates an occurrence of the NMR signal in the last measurement cycle it may also indicate that the signal is present but that the PT 2025 is not in AUTO mode The character W for Wrong indicates that the data given has no significance and should be ignored e g after a TRIGGER d The displayed value is composed of an ASCII character between O and
11. 4 Multiplexer channel These three bits indicate which channel of the multiplexer is selected Bits 654 Channel 000 A 001 B 010 C 011 D 100 E 101 F 110 G 111 H bit 3 SEARCH mode 1 indicates that the SEARCH mode is active bit 2 Field This bit indicates if the field is positive or negative with respect to the probe orientation 1 indicates a positive field 0 indicates a negative field bit 1 MANUAL AUTO mode state This bit indicates whether the PT 2025 is in MANUAL or AUTO mode 0 indicates MANUAL Mode 1 indicates AUTO Mode bit 0 DISPLAY mode data format mode 1 indicates that the displayed value is given in Tesla 0 indicates that the displayed value is given in MHZ METROLAB Instruments SA 51 NMR Teslameter PT 2025 User s Manual 6 4 2 4 Status Register 4 DAC Status 2 bytes These two bytes give the contents of the DAC 6 5 AUTONOMOUS MODE This mode is used to connect the PT 2025 to a printer without needing an external controller To select this mode micro switch 7 must be set to 0 In this mode the Teslameter transmits the measured value at regular intervals according to the message format described in section 6 4 1 1 all incoming messages are ignored The intervals between each value can be defined by the micro switches 1 to 3 as shown in the following table Micro switches Interval 321 000 no message transmission 001 sends every measured value 010
12. Teslameter PT 2025 User s Manual bit 7 Power ON or RESET This bit is set to 1 on power up or on return of current after a power failure or after having pressed the RESET button located on the rear panel bit 6 Unused Always set to 0 bit 5 NMR Lock This bit is set when the instrument becomes Locked It is cleared by reading the Internal Instrument Status Register or when the PT 2025 is no longer Locked bit 4 LOCAL push button This bit is set by pushing the LOCAL button situated on the front panel bit 3 Hardware Error This bit is set to 1 if the control program cannot read the display correctly i e following a failure in the counter circuitry bit 2 Syntax Error This bit is set to 1 if an incoming message does not conform to the formats described in this document The command generating a syntax error is ignored bit 1 NMR Signal Seen Indicates that there has been an occurrence of an NMR signal since this status register was last read This bit is the same as bit 3 in the internal status register 2 bit 0 Data Ready This bit is set to 1 after each measurement cycle and can be used to inform the controller of the end of a measurement cycle 32 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 5 3 2 Status Register 2 NMR signal status 1 byte Bit 7 6 5 4 3 2 1 0 N A N A N A N A NMR INMR ITOO TOO Signal Signal HI LO Seen State
13. lowest and 1 6 mV uT 0 16 V G for the highest field range This error voltage is integrated and then sent to the frequency control input of the radio frequency oscillator By choosing the gain and the integration time constant appropriately the error can be corrected entirely within the time between two consecutive NMR signals For this optimum loop gain setting the time lag of frequency tracking is equal to the spacing of the NMR signals which is roughly 17 ms The loop gain at d c is of the order of 10 The size and width of the NMR signals depend strongly on the field strength and homogeneity of Bo During field mapping for example the amplitude may vary by a factor of 10 and the width by a factor of 4 Therefore the trigger level and the timing of the sample and hold circuit are adjusted automatically in the Teslameter in order to maximise its range of operation The trigger level is set automatically to about half the signal amplitude the latter being measured with a special peak detector circuit which is insensitive to possible occasional large single parasitic signals The trigger point may be early or late with respect to proper proton resonance In order to correct this both the strobe pulse as well as the triangular wave voltage proportional to the modulating field are delayed appropriately before being fed to the sample and hold circuit which produces the error voltage Wrong timing does not change the mean value of t
14. must generate interrupts In the following example the bits corresponding to the NMR signal bit 1 and the LOCAL push button bit 4 will be active Next determine the octal code containing two digits for these conditions Finally output the M message followed by the two octal digits see also section 5 1 2 12 5 2 3 The SRQ Status Register A Service Request is generated by the PT 2025 when bit 6 RQS is set to 1 This bit must be tested by the controller when it executes a serial poll The remaining bits allow the controller to determine the nature of the Service Request a 1 representing the active state The status register is RESET to 0 after the controller has finished the serial poll The bits have the following significance Bit 7 6 5 4 3 2 1 0 Power RQS NMR Front Hard Syntax NMR Data ON Lock Panel Error Error Signal Ready or LOCAL Seen RESET Button PecknaN 426 64 32 16 8 4 2 1 Value bit 7 power ON or RESET This bit is set to 1 on power up or on return of electric current after a power failure or after having pressed the RESET button located on the rear panel bit 6 RQS Require Service Set to 1 to generate a Service Request bit 5 NMR Lock This bit is set when the instrument becomes Locked It is cleared after the serial poll or when the PT 2025 is no longer Locked bit 4 Front panel LOCAL button Pushing the LO
15. ppm STABILITY Over fifty display readings in a 1 5 T superconducting coil in stable laboratory conditions the standard deviation is less than 5 0 x 10 Note The specified value holds for a signal to noise ratio safely above the limit for automatic frequency tracking METROLAB Instruments SA 5 NMR Teslameter PT 2025 User s Manual SIGNAL TO NOISE RATIO IN A HIGHLY HOMOGENEOUS FIELD H probes at min of field range approximately 10 at max of field range approximately 100 7H probes at min of field range approximately 5 at max of field range not measured FREQUENCY TRACKING SPEED Af f up to 1 sec Time lag min 17 ms Both depend on the loop gain and the maximum tracking speed Af f max as well as on the setting of the frequency and the amplitude of the modulation Therefore the frequency tracking speed and the time lag may be of an order of magnitude poorer than the optimum values given above LOOP GAIN AT D C Greater than 10 worst case for H probes but typically greater than 10 Front panel screwdriver adjustment for a maximum of 10 times attenuation of loop gain MANUAL FREQUENCY ADJUSTMENT COARSE 10 turns precision potentiometer FINE 10 turns precision potentiometer see Field Tracking Range MAGNETIC ENVIRONMENT The PT 2025 should not be used in magnetic environments over 0 1 Tesla However for the Amplifier 1030 and the Multiplexer Amplifier 2031 the operational li
16. sends every 4 measured value 011 sends every 9 measured value 100 sends every 20 measured value 101 sends every 150 measured value 110 sends every 720 measured value 111 sends every 3600 measured value 52 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 6 6 SUMMARY OF THE RS 232 C MESSAGES Message Parameter An Bnn Cnnnn lt CR gt lt LF gt Dn Fn Hnnnn lt CR gt lt LF gt K METROLAB Instruments SA Function Select MANUAL 0 or AUTO 1 mode section 6 4 1 8 Binary load DAC 12 bits section 6 4 1 7 ASCII load DAC 12 bits section 6 4 1 6 Display mode Tesla 1 MHz 0 section 6 4 1 10 Field sense 1 0 section 6 4 1 9 Select SEARCH mode section 6 4 1 12 Set LOCAL lockout and disable front panel LOCAL button section 6 4 1 4 LOCAL Mode puts PT 2025 under front panel control section 6 4 1 3 Select SEARCH time section 6 4 1 15 Select multiplexer channel section 6 4 1 11 Quit SEARCH mode section 6 4 1 13 REMOTE mode section 6 4 1 2 53 NMR Teslameter PT 2025 User s Manual Sn Xn lt ENQ gt Request status register section 6 4 1 18 RESET NMR time base section 6 4 1 16 Fast reading display section 6 4 1 17 Select multiplexer range section 6 4 1 14 Read measured field value section 6 4 1 1 Messages sent by the PT 2025 vdd dddddddT vdd ddddddF Saa 54 Display format Tesla Display format MHz Statu
17. the MANUAL mode refer to the previous section Note For the 1H probes the sweep range is typically 5 but can be reduced to 3 at the upper end of the probe s range For the 2H probes the sweep range is reduced by a factor of 3 in comparison to the 1H probes 16 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 4 3 THE SEARCH MODE 4 3 1 Introduction In this mode the microprocessor takes control of all the front panel commands rendering them inoperative Simultaneously an automatic field search is activated The entire range of the probe is scanned from bottom to top until the NMR signal is seen at which point the PT 2025 locks on to the signal Noise and interference signals are detected and by passed by the search algorithm Once the PT 2025 has locked on to a signal the search algorithm can follow the field over the whole probe range When connected to a computer and with the aid of the probe multiplexer the PT 2025 can be programmed to search for and track a field over several probes 4 3 2 Operation of the SEARCH Mode The SEARCH mode is activated by pressing the LOCAL SEARCH push button when the PT 2025 is under front panel control The LED REMOTE SEARCH will indicate the start of the field search The front panel controls the sense of the field FIELD The multiplexer setting and the display format TESLA MHz are taken into account at the start of the search all other settings are ignored
18. then on to channel D probe 5 If the search fails to locate a field the PT 2025 will start again the search on channel B If an NMR signal is detected the PT 2025 will lock on to it The SEARCH mode has been designed to follow the NMR signal even if it goes out of one probe s range by changing to the next channel Suppose that the signal has been found on channel C probe 4 if the field drifts to a value of less than 0 35 T then the PT 2025 will select channel B and continue with probe 3 the algorithm functions in both positive and negative drift conditions In this example the PT 2025 can therefore search and follow an NMR signal in a field ranging from 0 175 T low end of probe 3 up to 2 10 T high end of probe 5 In order to operate this scan correctly it is necessary to ensure that the probes are connected in an ascending order vis a vis the multiplexer channels Note If the first channel scanned is selected as G and the PT 2025 is programmed to search over 4 channels then the sequence for scanning is G H A and B 5 1 2 9 Select the SEARCH Time Message format On where n is a number from 1 to 6 n 3 on power up of RESET METROLAB Instruments SA 25 NMR Teslameter PT 2025 User s Manual 26 This message allows the speed of the search to be changed The number 1 corresponds to the most rapid i e 9 seconds to scan the field range of a probe Ea
19. to place the probe detector circuitry within the magnetic field as this results in less than 1 ppm on the field value measured by the probe Whereas the 1030 Amplifier may be used in high fields the PT 2025 should only be used in fields of below 100 mT The front panel outputs FIELD MODULATION and NMR 5IGNAL may be connected to the high impedance inputs of an oscilloscope 2 0V div 0 2V div respectively and an internal time base of 5 ms div Set the MH2 TESLA toggle switch to the TESLA position Set the MANUAL AUTO switch to the MANUAL position On powering up the Teslameter a yellow LED situated on the front panel indicates which probe is in fact connected to the PT 2025 If a scope is connected then verify the d c value of the NMR signal output on the display Depending on the frequency and the probe used it should be between 0 1 to 1V Should no positive d c voltage be registered at the NMR SIGNAL output turn the COARSE control to maximum and then back to the required value Note that the radio frequency oscillator may not start when set to a low frequency if the power has been turned off and on within a delay of approximately 15 seconds Set the Modulation amplitude to its maximum value by using a screwdriver to turn the MODULATION AMPLITUDE trimpot fully clockwise In certain cases where high magnetic fields are to be measured the stability of the measurement can be improved by reducing the Modulation amplitude METROLAB
20. 2025 User s Manual 5 1 2 5 Select Multiplexer Channel Message format Pc Where c is either A B C D E F G or H and represents the appropriate multiplexer channel Example OUTPUT 708 PE selects channel E of the multiplexer 5 1 2 6 Activate SEARCH Mode Message format Hnnnn lt CR gt lt LF gt Where nnnn is optional and can be any value between O and 4095 inclusive This message H for Hunt activates the automatic field searching algorithm see section 4 3 An optional start frequency can be supplied if the approximate field value is known This has the advantage of reducing the search time If no DAC value is specified i e H lt CR gt lt LF gt then searching starts at the minimum frequency DAC 0 When the PT 2025 is in the SEARCH mode it can still receive and interpret interface messages with the exception of An Bnn Cdddd and Fn which would interfere with the search They are therefore ignored The SEARCH mode can be made to work over several channels of the multiplexer to allow searching over larger field ranges See the message X Example OUTPUT 708 H amp CHR 13 amp CHR 10 activates searching at the lowest frequency for the selected probe or OUTPUT 708 H934 amp CHR 13 amp CHR 1 activates searching at the frequency corresponding to the DAC value of 934 5 1 2 7 Quit SEARCH Mode Message format Q When this message is received and the PT 2025 is in SEA
21. 5 4 3 2 1 0 N A Power NMR Front Hard Syntax NMR Data ON Lock Panel Error Error Signal Ready or SRQ Seen RESET N A Not Applicable bit 7 Unused Always set to 0 bit 6 Power ON or RESET This bit is set to 1 on power up or on return of current after a power failure or after having pressed the RESET button located on the rear panel bit 5 NMR Lock This bit is set when the instrument becomes Locked It is cleared by reading the Internal Instrument Status Register or when the PT 2025 is no longer Locked bit 4 LOCAL push button Pushing the LOCAL button situated on the front panel sets this bit bit 3 Hardware Error This bit is set to 1 if the control program cannot read the display correctly i e following a failure in the counter circuitry bit 2 Syntax Error This bit is set to 1 if an incoming message does not conform to the formats described in this document The command generating a syntax error is ignored METROLAB Instruments SA 49 NMR Teslameter PT 2025 User s Manual 50 bit 1 NMR Signal Seen Indicates that there has been an occurrence of an NMR signal since this status register was last read This bit is the same as bit 3 in the Internal status register 2 bit 0 Data Ready This bit is set to 1 after each measurement cycle and can be used to inform the external computer of the end of a measurement cycle without the need for the control
22. 7 inclusive relate to the mask pattern in octal for bits 0 1 and 2 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual This message allows the user to set up a mask for the SRQ register in order to prevent the PT 2025 from interrupting the controller when a particular event occurs On power up or RESET the default value for the mask is MOO SRQ Mask register M Masked A Active Z rezerezre gt PrPrr z zze PEPr EZzrzrpreEe NO oOfP WBN CO gt PrPrr z zze rPrZzEzarreze 0 1 2 3 4 5 6 7 gt Note It is not possible to mask bits 6 and 7 On power up and RESET both bits are active Example OUTPUT 708 M00 gives all bits masked OUTPUT 708 M77 gives all bits active Note See section 5 2 for more details on the SRQ register 5 1 2 13 Request Instrument Status Message format Sn Where n is 1 2 3 or 4 and indicates the status register to be read The status registers are described in section 5 3 Following reception of this message when the PT 2025 is subsequently addressed as a Talker it sends back to the controller the value of the status register as requested instead of the last measured field value The message sent by the PT 2025 after a request for status has the following format Message format Saa where S is the ASCII character S meaning status where a is the ASCII character forming a hexadecimal pair METROLAB Instruments SA
23. 9 inclusive leading O suppression is performed Note that the decimal point is represented by the ASCII character and is included in every message F Represents the ASCII character F to indicate that the value pertains to an NMR frequency in MHZ T Represents the ASCII character T to indicate that the value is given in Tesla lt CR gt lt LF gt Represents the ASCII characters carriage return and line feed respectively These characters are only transmitted if micro switch 8 is set to 1 In all transfers the EOI End Or Identify is set true just before the last character of the message is sent 28 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual Note in the Fast reading display mode the last digit of the display is not visible Reading of the Status Registers When the PT 2025 is addressed as a Talker after having received a demand for a return of status see section 5 1 2 13 it sends back the contents of the requested status register refer to section 5 3 The following examples demonstrate the sequences in which the measured value should be read and how to access status register 2 Example 100 ENTER 708 F addresses as Talker 110 DISP F F L82 125867F 180 OUTPUT 708 S2 request status reg 2 190 ENTER 708 F 200 DISP F F S45 5 2 SERVICE REQUEST SRQ An important feature of the PT 2025 is its ability to interrupt the cont
24. CAL button situated on the front panel sets this bit METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual bit 3 Hardware Error This bit is set to 1 if the control program cannot read the display correctly i e following a failure in the counter circuitry bit 2 Syntax Error This bit is set to 1 if an incoming message does not conform to the formats described in this document bit 1 NMR Signal Seen This bit indicates that there has been an occurrence of an NMR signal bit 0 Data Ready masked on power up and on RESET Bit O is set to 1 after each measurement cycle This bit can be used to inform the controller of the end of a measurement cycle without the need for the controller to continually scan the PT 2025 After power up this bit is masked To use this function the user must program the mask accordingly 5 3 INSTRUMENT STATUS REGISTERS The PT 2025 has four internal status registers which can be accessed by the user see section 5 1 2 13 The format of these four registers is now given 5 3 1 Status Register 1 Internal Instrument Status 1 byte This register is cleared after reading Together with the SRQ Mask it is used to form the SRQ Status Register Bit 7 6 5 4 3 2 1 0 Power N A INMR Front Hard Syntax NMR Data ON Lock Panel Error Error Signal Ready or LOCAL Seen RESET Button N A Not Applicable METROLAB Instruments SA 31 NMR
25. ER Symptom An incorrect reading was registered by the external frequency meter The cable was unterminated from the NMR FREQUENCY output to the frequency meter Use a cable with a 50 termination at the frequency meter METROLAB Instruments SA 75
26. Instruments SA 11 NMR Teslameter PT 2025 User s Manual 12 Set the Modulation frequency to 30 Hz Again this can be done by using a screwdriver to turn the MODULATION FREQ trimpot fully anti clockwise For better stability the frequency of modulation may be increased up to 70 Hz in high uniformity fields The amplitude will decrease however when the frequency is increasing since the slope of the modulation signal is constant The COARSE control must be adjusted until the Teslameter s field reading roughly corresponds to the magnitude of the field then gradually turned until the NMR Lock LED flashes or remains lit If an oscilloscope is connected to the front panel outputs see above the NMR signal should now be displayed on the screen The FINE control can also be used if necessary Set the MANUAL AUTO toggle switch to the AUTO position The Teslameter will scan the full range of the FINE control and lock on to the field Should the Teslameter not lock then the modulation may be in the wrong polarity with respect to the measured field In this case reverse the FIELD toggle switch polarity For optimum results the probe should be fixed in a position of high homogeneity this being indicated by the wiggles and maximum amplitude of the NMR signal refer to Fig 3 The GAIN potentiometer of the frequency control loop should normally be turned fully clockwise maximum gain for the highest precision measurement N
27. METROLAB Instruments SA PI 2029 NMR TESLAMETER User s Manual Version 2 0 Revision 1 0 September 2003 CONTENTS N o GENERAL DESCRIPTION iic ccsttesecssitececesteccecestecececssuteceresteceereseveceesevsceeeres 1 SPECIFICATIONS oanet N OT vate ceed ede ieeeeas 5 ZW SPIODCS 224 sist inet eaaare fe ssbeds eclane TA T 5 2 2 PT 2029 TESLAME FER creert anae RaRa AR A sneha acetates es AAA OARA REETA RASTA 5 USER S GUIDE aA aeaa Aea NE TANCAR ALAE OAA TAE EETA YAA AERAN SARSAN 11 PT 2025 OPERATING MODES cccssseeeeeeeeeeeeeseeeeeeeeeeeeeeeseeeeeeneesseeneeessenens 15 4 1 The MANUAL Mode cc2 cccsatecietieeeee tance eetiendeveteedeveteeedevehiengereiieeeeeeteas 15 42 The AUTO Mode oiire aiai i del taea iini ile ia anA Ea ATA 16 ko the SEARCH Mode aere aaen EE E A AORA 17 Aai ANMODUCHON eorr aki E E E 17 4 3 2 Operation of the SEARCH Mode ccceceeeeeeeeeeeeeeteeeeteteees 17 4 4 Use of the Three Modes via the Interface eeeeeeeeeeeeeeeeeeneeeeteeaees 18 PT 2025 IEEE 486 INTERFACE serina sects ctecceteecuscetunessanctectssseuecsvessttecds 19 S1 IStGMGr TAIKGN eia ee etd sheewbacadeeet A ieewsatestieead 20 Sl IEBE 488 CommanidSsis csscc teecscvehictechhaeteieieatashevsapdaceieiiedisteaetitieadia 20 5 1 2 The PT 2025 Addressed as a LIStOnel ee eeceeeeeeeeeeeeeteeeeeeneeeees 22 5 1 2 1 Preselection of the Radio Frequency cccececceeeieeeeeetteeeees 22 5 1 2 2 Select MANUAL or AUTO
28. Mode cceeeeeeeeeeeeeeeeseetteeeeeneeeees 23 3 1 2 3 Select Field SENSE iss cond eea aumentdaeday 23 5 1 2 4 Select Display 23 5 1 2 5 Select Multiplexer Channel ccccccccecceceeeeeeeeeeeeeseeeeeeeees 24 5 1 2 6 Activate SEARCH Mode eecccceecceeeeeeneeeeeeeeneeeseenaeeeeeenaeeees 24 5 1 2 7 Quit SEARCH Mode rreri ienei aian 24 5 1 2 8 Select Number of MUX channels Used in SEARCH Mode 25 5 1 2 9 Select the SEARCH TiMe sesessecersrrrserrrerrirresrrenerrrrrrserrsdsena 25 5 1 2 10 Enable Disable TrigQer ccccccceecceeeeeceeeeeseeeeeeseeneeeseenaeeees 26 5 1 2 11 Fast reading display ccccceceeeeeeeeeeeeeeeeeeeeeeeeeeeteeaeeeeeenaneees 26 5 1 2 12SRQ Mask 26 5 1 2 13 Request Instrument Status 1 0 2 0 cceceeeeceeeeeceeeeeeeseeseceeeeeeeeees 27 5 1 3 The PT 2025 Addressed as a TalKel cccesececeeceeeeeeenteeeeeeneeees 28 5 2 Service Request SR aieri eiiean teadegsssnadefaaceadgesateeadecesinade feaceadereste 29 S21 HON foUse fhe SRO oeir raar A a A A A 29 5 2 2 Setting the SRQ MaSK 0 0 ecceeeeeeeeeeeeeeeeeeeeeseeeeeeeeseeeeeeeeeaeees 29 5 2 3 The SRQ Status Register ce cceeeeeeeeeeeeeeeeeeneeeeeeneeeeeeaeeeeee 30 5 3 Instrument Status RegisterS sssssrssessrresrirnesrernssninnnnsinnasnnnnnnenennennnnnat 31 5 3 1 Status Register 1 Internal Instrument Status 1 byte 31 5 3 2 Status Register 2 NMR signal status 1 byte 33 5 3 3 Status Register 3 Instru
29. RCH mode it inactivates the search in progress and leaves the configuration as it was at the instant the Q message was received 24 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 5 1 2 8 Select Number of MUX channels Used in SEARCH Mode Message format Xn where n is a number from 1 to 8 inclusive X1 is selected on power up or RESET This message tells the PT 2025 the number of multiplexer channels that are to be scanned in the SEARCH mode When the PT 2025 enters the SEARCH mode it starts its search on the channel that was last selected either by the MUX switch when the instrument was put into REMOTE mode or the last value received in the Pc message If the signal is not found on this channel the PT 2025 will follow on to the next channel and so on up to the number of channels given by the Xn message When the last channel has been searched and no signal is detected the search restarts on the first selected channel and the cycle is then repeated Example Consider that probes 3 4 and 5 are respectively connected to channels B C and D of the multiplexer Execute the following messages OUTPUT 708 PB selects channel B OUTPUT 708 X3 selects search over 3 channels OUTPUT 708 H amp CHR 13 amp CHR 10 starts SEARCH mode The search will in this case start on channel B probe 3 and if no NMR signal is found it will continue on channel C probe 4 and
30. Radio Frequency Amplifiers cccceseeeteeeeenees 65 9 4 Automatic Trigger Threshold and Delay Circuits 2 0 0 0 eeeeeeeeeneeeeeeeeee 67 9 5 Frequency Control and Loop Gain c cccccecceeeeeeeeneeceeeeeeesecstaeeeeeeeeees 68 9 6 Frequency Counters iickiicesneiesecuiiteeedviiasde leu eteeniishesdescieeuia eae RERE ERETA 70 10 FAULT FINDING ornearre ei ci anrea aN aden cl A S 73 10 1 About auto mode iis edie ttc ai tained dei atin dat 73 10 2 about field tracking zearren cece bacedecderebetdense A AES 74 10 3 Unstable value s 0 s ehiehekesakiin when ein at ells RR Actes 74 10 4 external frequency Meter sseseeesseeireserrestirrssttrrssttrrnssttrnsstennstttnnsstnn nae 75 NMR Teslameter PT 2025 User s Manual 1 GENERAL DESCRIPTION The PT 2025 NMR Teslameter System Fig 1 comprises the following Bench top main unit with RS 232C and as an option IEEE 488 interfaces e At least one NMR probe A multiplexer if several probes are required Each Probe consists of a measuring head and a detection circuit Probes model 1060 need an external amplifier model 1030 the probe cable to the Amplifier is 7m long and the cable of the Amplifier to the PT 2025 may be up to 100 meters long Probes model 1062 have the amplifier integrated They are connected directly to the PT2025 The cable of the probes model 1062 may be up to 100 meters long A block diagram of the Teslameter including Probe
31. TERFACE The PT 2025 is a DCE device the connector for serial communications with the instrument is a sub D 25 way female type We recommend the following pin to pin connections to the computer 6 3 Shield Ground 1 cable shield 1 Transmit Data 2 2 TX 3 Receive Data RX 3 3 RX 2 Request To Send RTS 4 4 RTS 7 Clear To Send CTS 5 5 CTS 8 Data Set Ready DSR 6 6 DSR 6 Data Carrier Detect DCD 8 8 DCD 1 Data Terminal Ready DTR 20 20 DTR 4 Signal Ground GRD 7 GND 5 PT 2025 connect pin no__ Computer 25 pin connector DCE PC AT 9 pin connector Fig 5 RS 232 C connector pin outs SETTING THE SPEED FOR THE RS 232 C INTERFACE One of the following baud rates may be selected Note 300 600 1200 2400 4800 9600 19200 The PT 2025 is delivered with a baud rate of 2400 To change the baud rate follow the procedure outlined below 40 Turn off the Teslameter Unscrew completely the four retaining screws on the I O module situated on the rear panel Remove the I O module by using the knurled screws as leverage Chose the required baud rate by moving the jumper to the desired position see Fig 6 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual Reinstall the module in the Teslameter following the inverse procedure P e j eoo E E Fig 6 Baud rate selection 1 HANDSHAKE 2 NO HANDSHAKE METROLAB Instruments SA 41 NMR Teslameter PT 2025
32. UT LLO The IEEE 488 command LLO LOCAL LOCKOUT disables the LOCAL push button on the front panel of the PT 2025 Only the GTL LOCAL command or a RESET can hence return the instrument to LOCAL front panel mode TRIGGER GET The IEEE 488 command GET Group Execute Trigger forces a RESET of the PT 2025 time base thus starting a new measurement cycle Note that if the PT 2025 is addressed as a Talker it will return the field value with the letter W until a valid cycle has been completed see section 5 1 3 Before to its execution a trigger command must be enabled with the T1 command see section 5 1 2 10 INTERFACE CLEAR IFC This command IFC aborts the present communication and frees the bus The PT 2025 is unaddressed Example ABORTIO 7 7 is the address of the IEEE 488 bus in the HP 85 DEVICE CLEAR DCL This command DCL puts the PT 2025 into its initial state as for a RESET All communications are aborted and the SRQ mask is also set to its default setting Example CLEAR 708 7 is the address of the IEEE 488 bus in the HP 85 and 08 is the address of the PT 2025 on the IEEE 488 bus METROLAB Instruments SA 21 NMR Teslameter PT 2025 User s Manual 22 5 1 2 The PT 2025 Addressed as a Listener When the PT 2025 is addressed as a Listener it can receive specific messages which permit the control of the instrument 5 1 2 1 Preselection of the Radio Frequency It is possible
33. and Amplifier is shown in Fig 2 The radio frequency oscillator in the PT 2025 has a frequency range of 30 to 90 MHZ which corresponds to the highest proton resonance field range of 0 7 to 2 1 Tesla The other field ranges are obtained by dividing the radio frequency f by 2 4 8 or 16 f 4 f 2 or f being used for the three H probes This results in a very comfortable overlap of the eight field ranges An internal frequency counter measures the NMR frequency which is sent to the probe the result is displayed in Tesla with a resolution of 0 1 uT 1 mGauss or in frequency with a resolution of 1 Hz METROLAB Instruments SA 1 NMR Teslameter PT 2025 User s Manual Fig 1 Model PT 2025 NMR Teslameter 2 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual COARSE frequency adjustment is done manually with a 10 turns potentiometer or via the interfaces FINE adjustment of the frequency is done with a second 10 turns potentiometer that allows precise adjustment of 1 to 5 of full scale depending on the COARSE frequency setting and the type of Probe H or H used In the automatic mode the unit sweeps the frequency up and down through the FINE adjustment range until an NMR signal is detected Then it locks automatically to this signal a feedback control adjusts the frequency such that it equals the NMR frequency of the connected probe The resulting frequency tracking with any changes of t
34. anges between 0 045 and 2 1 Tesla by using an appropriate number of turns of the modulating coil of the different probes see below for a further explanation of the H probes The number of turns is chosen such that at a given current in the modulating coil the ratio Of Bmoa to Bo is the same for all H probes at the same VCO setting Hence the number of turns decreases roughly linearly with the decreasing field range of the probe while a AVe ABmog increases inversely METROLAB Instruments SA 69 NMR Teslameter PT 2025 User s Manual R the resistor of the modulation current sense being constant Therefore switching the frequency range and changing the corresponding H probes does not change the loop gain For the H probes the resistor R for limiting and measuring the modulating current is switched to a three times lower value 30 Q instead of 90 Q in order to keep the necessary number of turns of the modulating coil below impracticable limits The ratio of Bmoa to Bo at a given voltage drop over the resistor R is the same for both the H and H probes and the resulting factor a compensates for the frequency division and the lower gyromagnetic ratio of the deuterons To understand this point the following argument may be helpful the sensitivity of the error voltage to a frequency error in relative terms e g ppm is the same for all 1H probes and H probes at a given VCO setting and the relative change of the probe freque
35. be modulation field must be situated along the measured field direction transversal or axial despite the fact that the NMR probe gives a precise field or frequency reading independent of orientation Verify that the arrow on the probe is in the direction of the measured field Note that the field polarity is unimportant as it can be corrected by the FIELD polarity switch Incorrect probe modulation Verify the triangular modulation waveform approx 30 Hz to 70 Hz at the FIELD MODULATION OUTPUT with the mode switch turned to MANUAL If necessary use a screwdriver to regulate the MODULATION AMP adjustment on the front panel until the correct signal amplitude is registered H20 probes approx 16 V peak to peak D20 probes 8 V peak to peak Amplifier box switch not in correct position Set the switch to the correct position one position for probe 1 and one position for probes 2 to 8 METROLAB Instruments SA 73 NMR Teslameter PT 2025 User s Manual Probe not tuned In some conditions the auto tuning of the probe can take a few seconds Turn the coarse field potentiometer to the top of the range for 2 3 seconds before returning to the actual field value Inadequate signal to noise level Compare the signal at the NMR SIGNAL output with that shown in Fig 1 A minimum signal of about 100 mV is required for the system to lock Should this signal level not be reached it is possible tha
36. ble screened coaxial cable which transmits both the radio frequency and the NMR signals plus any detected 10 kHz signal which is used for automatic tuning The measuring head of the probe contains a radio frequency coil wound around the NMR sample active volume The NMR sample is made of a solid material containing a large number of protons H for probes 1 to 5 or in the case of probes 6 to 8 a sealed glass tube containing D20 H The number of turns of the radio frequency coil is defined for each probe by its highest operating frequency and the lowest attainable value of the capacitance of the LC resonance circuit This capacitance is essentially the sum of the capacitance of the coaxial cable and of the tuning diode With the type of tuning diode used a frequency range of a factor of three can be covered with a maximum cable capacitance of 17 pF i e a maximum length of 17 cm The number of turns of the modulating coil depends on the field range of the probe and is chosen such that a field modulation of 100 ppm is produced by a current of a few tens of mA This number of turns is also important for the loop gain of the frequency control loop The modulating field in the sample is not homogeneous This does not harm the accuracy as the resonance occurs when the modulating field goes through zero but it has the welcome effect of dampening the wiggles 9 2 AUTOMATIC PROBE TUNING Although for space reasons the circuits for g
37. boe Poe be pepe pe Pe wb z6 peL Baas me ane eee GOSANA SOONG SOS nanam ni apes pi pe EEs e eE a neba ee ae fies a Ett oe es eee nee ees Se T peas b ze on Pb pe sb 1 2 s bis pis i i Peu p p up ET PRTIRID EIN E pide po iO Lop Anu PR te tere t09 past o f op os F 16 ay aS i Pe pb bes pref OP eee fA 07 e 14 DES DESSE DOENE E f pp a pa bas wi s HE 06 F 22 vines aes ae i 7 t Fig 7 NMR frequency versus DAC setting 6 4 1 8 Select MANUAL or AUTO Mode Message format An Where n 0 for MANUAL mode n 1 for AUTO mode This message selects the MANUAL or AUTO mode When the PT 2025 is in AUTO mode it can lock on to the NMR signal and follows the field if it drifts within 5 of the value preselected by the DAC or the COARSE potentiometer In the AUTO mode the FINE potentiometer is disconnected for more details see section 4 2 AUTO mode 6 4 1 9 Select Field Sense Message format Fn Where n 0 or for negative fields n 1 or for positive fields METROLAB Instruments SA 45 NMR Teslameter PT 2025 User s Manual 46 This message determines the sense of the probe with respect to the orientation of the field to be measured If the FIELD is in the wrong sense the PT 2025 does not lock on to the NMR signal 6 4 1 10 Select Display Message format Dn Where n 0 for MHz n 1 for Tesla This message determines whether the displayed value given in Tes
38. ch increase in n slows up the search by 3 seconds It may be necessary to slow the scan slope used in the PT 2025 under certain critical field conditions for example if the NMR signal is diminished due to a non homogeneous field It is possible to change the scan slope even when the search is in progress Example OUTPUT 708 O2 5 1 2 10 Enable Disable Trigger Message format Tn Where n 0 disables the TRIGGER message default value Or n 1 enables the TRIGGER message When n 0 the PT 2025 does not respond to the TRIGGER message In order that the PT 2025 can accept the TRIGGER message the user must send a T1 message Example OUTPUT 708 T1 the PT 2025 will respond to the TRIGGER message 5 1 2 11 Fast reading display Message format Vn Where n OorN for Normal reading display rate n 1 or F for Fast reading display rate This message determines the display reading rate i e Normal 1 per second or Fast 10 per second The last digit of the display is not visible and is not transferred to the computer Therefore the PT 2025 resolution is reduced by a factor 10 5 1 2 12 SRQ Mask Message format Mno M the ASCII character M indicates that the two following characters constitute the byte mask for the internal status of the PT 2025 n signifies the ASCII characters from 0 to 7 inclusive relating to the mask pattern in octal for bits 3 4 and 5 o the ASCII characters from O to
39. described by the spin lattice relaxation time T1 This is the reason why protons continuously absorb energy from the alternating field if the coil is driven at the proton resonance frequency thereby reducing slightly the quality factor Q of the coil A practical way of detecting this effect is to tune a parallel LC resonant circuit to the proton resonance frequency using the water filled coil as the inductor and to apply to this tank circuit a stable sine wave of that frequency via a resistor The resistor value chosen should be high compared to the resonance impedance of the tank circuit in order to avoid damping If the proton resonance frequency is now modulated by superimposing a modulating magnetic field parallel to the static field Bo the reduction of the Q factor due the proton resonance can be detected as a small amplitude variation of the radio frequency voltage across the tank circuit Adding a paramagnetic salt to the water can enhance the signal This reduces the relaxation time T and therefore increases the steady state energy absorption of the protons at resonance The METROLAB probes contain either water doped with NiSO protons or heavy water doped with GdCls deuterons METROLAB Instruments SA 55 NMR Teslameter PT 2025 User s Manual A small flat coil in the probes produces the modulating field Bmog Its frequency is 30 Hz to 70 Hz and its amplitude 100 to 1000 ppm of Bo The NMR electronics detects and amplifies t
40. e limit the output voltage of A4 to values safely above the base line and below the top of the pulse at the inverting input of C2 The range of automatic delay is from O to 5 ms which is considering the fixed delay of 0 5 ms at the sample and hold input equivalent to 0 5 to 4 5 ms This is quite sufficient for all practical operating conditions of the Teslameter 9 5 FREQUENCY CONTROL AND LOOP GAIN The frequency control loop diagram is shown in Fig 13 As any frequency drifts of the voltage controlled oscillator VCO are corrected by the frequency control loop the problem of long term stability of the VCO is not very critical Any frequency modulation or noise above 1 Hz is however very harmful therefore the following precautions are taken very careful filtering of the varicap bias voltage and of the supply voltage of the oscillator the oscillator is enclosed in a copper box for radio frequency screening and to avoid thermal convection effects The various frequency ranges are obtained by division in steps of two using MECL 10 000 flip flops The selection of these ranges is done with MECL 10 000 gates A long tailed transistor pair produces the NMR frequency output NIM level for external CAMAC or other counters whereas an MECL 10 000 gate is used as an output stage for the probe radio frequency signal Using a well filtered supply voltage for this gate results in the necessary cleanness of the amplitude of the radio frequenc
41. eenea 46 6 4 1 13Quit SEARCH Modes cninciws inners dada 46 6 4 1 14Select Number of MUX Channels Used in SEARCH Mode 47 6 4 1 15Select the SEARCH Time cccccceeeceeeeeeeeeeeeeeteneeeeeeeneeeeeeae 47 6 4 1 16Reset NNR Time base Trigger ccceeeeeeeeeeettteeeeettteeeeeeeee 48 6 4 1 17 Fast Reading Display 0 ccceecceeeeeeeteeeeeeeeneeeeeeeneeeeeeneeeeeeeae 48 6 4 1 18 Request Status48 6 4 2 Instrument Status ReGisSters cece eeeeeeeeteteeeeeteneeeeeteeeeereee 49 6 4 2 1 Status Register 1 Internal Instrument Status 1 Byte 49 6 4 2 2 Status Register 2 NMR signal status 1 byte ee 50 6 4 2 3 Status Register 3 Instrument Functions 1 byte ee 51 6 4 2 4 Status Register 4 DAC Status 2 bytes 0 ee eeeeeeeeeeeeee 52 6 5 Autonomous Mode w 2inideie niet ein idee aeri kar i Eea abies 52 6 6 Summary of the RS 232 C MeSSaGES 0 eeccceeeeeteeeeeenteteeeeeteeeeeentaeeeeeeaaes 53 THEORY OF OPERATION ccccesseceeeseeeeeeseseeeeeseseeeseeesneesesesneeseesesneesenssnenens 55 PRINCIPLE OF OPERATION 6 ccce cecccccecececteecesecctieceseetdeewssccueeescctteeeeseetieewess 59 CIRCUIT DETAIL S arae cece ccccecestacdeccevecticas ccuccetecteesescceuntcss canssatexsieieeexesuseess 63 O17 PIODOS 2 rtesctaicodd epiiceses tated bike hoohd agouncnend cited E EER 63 9 2 Automatic Probe TUNing cccccceeeseeeeeeeeeteeeeeeeneeeeeesneeeeeeiaeeeeeeneeeeneaes 63 9 3 The NMR Signal and
42. enerating the varicap voltage are located in the main unit their operation is discussed in this section as the automatic tuning is a very essential feature of the probes The simplified circuit diagram is shown in Fig 10 The bias voltage of the tuning diode in the probe is composed of the voltage VF2 given by the COARSE potentiometer and the output of an integrator INT in Fig 10 A square wave signal of 0 6 mV amplitude is superimposed on it and modulates very weakly the capacitance of the tuning diode Ac C 100 ppm This results in a 10 kHz amplitude modulation if the resonant circuit is slightly mistuned being in phase or 180 degrees out of phase with respect to the injected square wave depending on whether the capacitance is too small or too large METROLAB Instruments SA 63 NMR Teslameter PT 2025 User s Manual The amplitude modulation is detected and amplified by a factor of 3000 together with the NMR signal The superposition of both is fed to the X input of an analogue multiplier where the NMR portion is reduced by RC differentiation and diode clipping the latter being necessary in order to avoid spurious signals when there are large and slowly decaying wiggles The Y input of the multiplier is connected to the 10 kHz square wave generator which produces a bi polar output signal of 10 V The same signal is used with approximately 90 dB attenuation for modulating the varicap This attenuation is split into two s
43. equally spaced in time if the resonance occurs at the zero crossing of the modulation Fig 8 If the LC resonance circuit in the Probe is slightly mistuned a dispersion signal is mixed with the absorption signal and the NMR signals at upward zero crossing of METROLAB Instruments SA 59 NMR Teslameter PT 2025 User s Manual the modulation look different from those at downward zero crossing Automatic tuning of the probe eliminates this effect It is therefore not necessary to know a priori at which point of the 10 to 100 ppm wide signal the applied frequency is equal to the proton resonance frequency The criterion is simply that the time difference between any point of the NMR signal and the close by zero crossing point of the modulation is equal for the upward going as well as for the downward going modulating field Then the applied frequency is equal to the proton resonance frequency of the field Bo with Bmoa 0 this is the criterion upon which the frequency control loop works The automatic tuning of the Probe combined with the good symmetry of the field modulation form the basis for the high accuracy of the Teslameter To achieve a short response time of the frequency control loop the sample and hold circuit mentioned above is used to produce an error voltage which indicates after each NMR pulse how far away the resonance was from zero modulation The sensitivity of the error voltage is 160 mV uT 16 V G for the
44. equency counter METROLAB Instruments SA 71 NMR Teslameter PT 2025 User s Manual 10 ps 100 Khz Preset I counter One shot 1 20u3 j PAS One shot 4 p 7 4 Transfert Data a Yeto scaler J i Reset scaler I mmm aes and load preset counter Fig 15 G1 G2 ou G3 Pulse sequence diagram of the time base of the frequency counter 72 METROLAB Instruments SA 10 FAULT FINDING NMR Teslameter PT 2025 User s Manual The examples outlined below demonstrate The types of problem that might occur The reason The solution 10 1 ABOUT AUTO MODE Symptom The Teslameter failed to generate a stable field or frequency reading whilst in AUTO mode the NMR lock LED remained off Reason Solution Incorrect COARSE control setting Revert to MANUAL mode and tune the COARSE control to achieve a field or frequency reading of within approximately 1 of the known field Change back to AUTO mode In the event that the field or frequency reading is unknown search the NMR signal output by using an external oscilloscope sensitivity of approx 100 mV cm Gradually tune the COARSE control until the NMR signal is observed then revert back to AUTO mode Incorrect field Ensure that the FIELD switch position is polarity correct according to the measured field polarity and that the probe range corresponds to the field about to be measured Incorrect probe type The pro
45. ers When the PT 2025 is put into the REMOTE state the front panel controls are disabled with the exception of the LOCAL push button The PT 2025 retains the same configuration it had before the REMOTE state was selected except for the preselection of the radio frequency Note The DAC has a default value of 2048 which represents the middle of the frequency range 6 4 1 3 LOCAL Message format L This message forces the PT 2025 to the LOCAL state the PT 2025 is configured by the front panel controls which becomes operational By pressing the LOCAL push button once when the instrument is in the REMOTE state has the same effect as the LOCAL message provided that the button has not been disabled by the LOCAL lockout message see below 6 4 1 4 LOCAL Lockout Message format K Since this message disables the LOCAL push button on the front panel of the PT 2025 only the L message or a RESET can return the instrument to LOCAL front panel mode METROLAB Instruments SA 43 NMR Teslameter PT 2025 User s Manual 6 4 1 5 Selection of the Radio Frequency It is possible to select the radio frequency via the RS 232 C interface thus replacing the use of the COARSE potentiometer The selected value can be sent either in binary or in decimal form The value is then sent to a 12 bit digital analog convertor DAC in order to select the radio frequency As the range of the DAC is from 0 to 4095 all values greater t
46. evertheless a reduction of the loop gain may lead to a more stable field reading in the event of a poor signal to noise ratio or if there are rapid fluctuations in the field Should the LEDs TOO HI or TOO LO appear faint this implies that the Teslameter is locked but is not at the center of the field tracking range In this case turn the COARSE potentiometer very gently in the relevant direction until the LED is off If the magnetic field changes slowly the Teslameter will automatically follow the variations within the field tracking range as given in the specifications The position of the COARSE field setting in relation to the actual field is indicated by the LEDs TOO HI or TOO LO In the MANUAL mode these are off only when the FINE control is set to 5 0 The resonance frequency may be read in Hz by setting the MHz TESLA switch accordingly The 1 Hz digit will be relevant only in high uniformity fields To ensure high precision during long term measurements a temperature stabilizer frequency meter of suitable stability may be connected to the NMR FREQ output located on the front panel Verify that the input impedance of the frequency meter is 50Q To use the SEARCH mode of operation see section 4 3 for details push the SEARCH button The REM SEARCH LED is lit and the front panel functions are disabled In this mode of operation the FINE potentiometer must be in position 5 0 In particular field conditions a different positio
47. han this will set the DAC to its maximum setting 6 4 1 6 Selection of the Radio Frequency Decimal Form Message format cnnnn lt CR gt lt LF gt Cc the ASCII character C signifies that the preselected value is expressed in decimal n represents the preselected value given in ASCII characters from 0 to 9 inclusive The user may perform leading 0 suppression on the preselected value lt CR gt lt LF gt the ASCII characters carriage return and line feed respectively must terminate the message Example C12 lt CR gt lt LF gt or C1028 lt CR gt lt LF gt 6 4 1 7 Selection of the Radio Frequency Binary Form The binary format of the selection of the resonant frequency message is as follows Message format Bnn B the ASCII character B signifies that the preselectedvalue is expressed in binary n represents a two byte preselected value 0 to 4095 inclusive The right hand 12 bits of nn are sent to the DAC Example B amp CHR 128 amp CHR 255 44 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual There is not a linear relationship between the preselected value and the NMR frequency Fig 7 shows the typical relationship between the NMR frequency versus the DAC setting roa 3 a s PEPE prope eeen epee bere pea obe
48. he error voltage but produces a 30 Hz rectangular signal superimposed on it and synchronous to the modulating field See Fig 9 It is therefore the speed rather than the accuracy of the field measurement which would deteriorate because a larger integration time constant would be needed However the Teslameter automatically adjusts the delay of the strobe pulse such that the above mentioned 30 Hz component of the error voltage disappears 60 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual MODULATING FIELD ERROR VOLTAGE o RESONANCE POINTS SAMPLING POINTS Bmod Verror Fig 9 A 30 Hz square wave component in the error voltage indicates wrong timing of the sample and hold circuit In the example shown the sampling pulses are assumed to be early with respect to the nuclear resonance METROLAB Instruments SA 61 NMR Teslameter PT 2025 User s Manual 62 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 9 CIRCUIT DETAILS 9 1 PROBES Eight probes are necessary for a field range of 0 043 to 13 7 Tesla Each probe consists of a measuring head and NMR detection circuit interconnected by a short 50 coaxial cable which is part of the LC resonant circuit and also a screened cable with two wires for modulation The probe and Amplifier are interconnected with a 5 wires cable for probe identification modulation a negative supply and tuning diode bias voltage and a 50 Q dou
49. he magnetic field at the Probe is restricted to the FINE frequency adjustment range Various other automatic controls simplify the use of the Teslameter these are automatic trigger threshold and timing of the NMR signal processing circuits NMR SIGNAL DETECTION BOX I I i I 1 l E a ToT l I ans l t e I 1 1 1 1 MEASURING l HEAD CONTAINS AUTOMATIC THARSNOLB 1 maduistion AND DELAY CONTROLS T I l l l 1 I I l l l l l l i l NIM PLUG IN PROBE AMPLIFIER Boxi NMR PROBE l L Fig 2 Block diagram of the PT 2025 Teslameter METROLAB Instruments SA 3 NMR Teslameter PT 2025 User s Manual 4 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 2 SPECIFICATIONS 2 1 PROBES Probe N Field Range Probe Frequency Active Volume Tesla Type Range MHz Diam x L mm 1 0 043 to 0 13 1H 19 to 56 7x 4 5 2 0 09 to 0 26 1H 3 8 to 11 2 5x4 5 3 0 17 to 0 52 1H 7 5 to 22 5 4x4 5 4 0 35 to 1 05 1H 15 0 to 45 0 4x4 5 5 0 70 to 2 1 1H 30 0 to 90 0 4x4 5 6 1 5 to 3 4 H 7 5 to 22 5 4x4 5 7 3 0 to 68 2H 15 0 to 45 0 4x 4 5 8 6 0 to 13 7 H 30 0 to 90 0 4x4 5 For these probes the signal to noise ratio is low at the lower end of their frequency range 2 2 PT 2025 TESLAMETER ABSOLUTE ACCURACY Better than 5 ppm this can be improved by absolute calibration of the probes RELATIVE ACCURACY Approximately 0 1
50. he nuclear resonance signals of the LC circuit and measures the current in the modulating coil at the instant when resonance occurs A voltage controlled oscillator produces the radio frequency voltage Its frequency is controlled by a high gain feedback loop to ensure that resonance occurs at the instant when Bmoa crosses zero Therefore this frequency fo equals the proton resonance frequency of Bo and automatically follows any changes in Bo The LC circuit is automatically tuned to the applied frequency by means of a varicap diode In concluding these general remarks it should be mentioned that the field modulation in the NMR Probes sweeps far too quickly through resonance to obtain adiabatic conditions Therefore the observed signals have neither the form nor the width of a real proton or deuteron resonance curve The width is several times the natural line width and transient effects for example wiggles appear Fig 8 However this fast modulation is convenient for practical reasons and an accuracy better than 1 ppm is nevertheless achievable using a symmetry criterion 56 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 1 Vidiv Time scale 4 ms div 2 Vidiv 1 Vidiv Time scale 400 us div 2 Vidiv 1 Vidiv Time scale 400 us div 2 Vidiv c Misalignement resulting from a 1 ppm frequency error Fig 8 NMR signal and modulation field chopped y t display with B
51. ier the radio frequency signal is amplified to the level needed for the probe which is about 5 V peak to peak The radio frequency Amplifier consists of a fast differential amplifier with voltage controlled gain a common emitter stage and a push pull output stage which is able to drive a 50 load at the required level With a typical input signal of 0 5 V peak to peak amplitude the differential amplifier works in a switching mode rather than linearly Its sensitivity to amplitude variations of the input signal is therefore reduced Moreover the signal output from the radio frequency Amplifier is measured with a diode detector circuit and compared with a clean adjustable reference voltage Any difference is amplified and fed back to the gain control This feedback control of the amplitude in addition to the switching operation of the input transistors smoothes any amplitude modulation of the input signal by a factor of 50 to 100 This helps in particular to reduce the very disturbing interference effects beating when more than one probe operated at slightly different frequencies is used METROLAB Instruments SA 65 NMR Teslameter PT 2025 User s Manual The output signal of the radio frequency Amplifier looks more like a badly shaped square wave than a sine wave As already mentioned this is not a disadvantage since the probe is hardly sensitive to the waveform The output level is roughly the same for all frequencies F
52. la or in MHz It also affects the format the measured value given via the interface 6 4 1 11 Select Multiplexer Channel Message format Pc Where c is one of A B C D E F G or H and represents the appropriate multiplexer channel 6 4 1 12 Activate SEARCH Mode Message format Hnnnn lt CR gt lt LF gt Where nnnn is optional and can have any value between 0 and 4095 This message H for Hunt activates the automatic field searching algorithm see section 4 3 An optional start frequency can be supplied if the approximate field value is known This has the advantage of reducing the search time If no DAC value is specified i e H lt CR gt lt LF gt then searching starts at the minimum frequency DAC 0 When the PT 2025 is in the SEARCH mode it can still receive and interpret interface messages with the exception of An Bnn Cdddd Fn Xn and Pc which would interfere with the search They are therefore ignored The SEARCH mode can be made to work over several channels of the multiplexer to allow searching over larger field ranges refer to section 6 4 1 14 6 4 1 13 Quit SEARCH Mode Message format Q When this message is received and the PT 2025 is in SEARCH mode it inactivates the search in progress and leaves the configuration as it was at the instant the Q message was received METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 6 4 1 14 Select Number of MUX Channels Used in SEARCH
53. ler to continually scan the PT 2025 6 4 2 2 Status Register 2 NMR signal status 1 byte Bit 7 6 5 4 3 2 1 0 N A N A N A N A NMR NMR TOO TOO Signal Signal HI LO Seen State N A Not Applicable bits 7 to 4 N A Always set to 0 bit 3 NMR Signal Seen This bit is set to 1 if the instrument detects the appearance of the NMR signal since this register was last read This bit is only reset by the reading of this register bit 2 NMR Signal State This bit gives the state of the NMR signal during the reading time of this register 1 indicates the presence of the NMR signal 0 indicates the absence of the NMR signal bit 1 TOO HI 1 indicates that during the reading time of this register one or several of the TOO HI LEDs were lit bit 0 TOO LO 1 indicates that during the reading time of this register one or several of the TOO LO LEDs were lit Note Bits O and 1 are useful to center the 5 scan window when the PT 2025 is in AUTO mode see section 4 2 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 6 4 2 3 Status Register 3 Instrument Functions 1 byte Bits 7 e 5 4 3 2 1 0 Fast AUTO Display Reading MULTIPLEXER sa Field maNuaL Tesla Display Mode MHz bit 7 Fast reading display 1 indicates that the PT 2025 is operated in the Fast reading display mode bits 6 5
54. ment Functions 1 byte 34 5 3 4 Status Register 4 DAC Status 2 bytes eeeeeeeeeeeeeeeeees 35 54 Talker Only tnce tance aged diced lad yai eha E eR AAEE ache EEA EAA 35 5 5 Summary of the IEEE Commands and PT 2025 Messages 37 PT 2025 RS 232 C INTERFACE DCE cccccsssseeeeeeseeeeeeeseeeeeeeeseeeeeneneeeenens 39 6 1 Programming of the RS 232 C Interface ecceceeeeeeeeeeenteeeeeeneeeeeeaes 39 6 2 Connecting the RS 232 C Interface ices eeeeeeeeeneeeeeeeteeeeeeneeeeeeaas 40 6 3 Setting the Speed for the RS 232 C Interface s 40 6 4 The Two Operational Modes of the RS 232 C Interface ceee 42 6 4 1 Conversational Mode ccc cccceccccc ce eeeceeeeeesseeeeseesauueeeeaeseeeeneeeeees 42 6 4 1 1 Reading the Displayed Value lt ENQ gt 0 ecceeeeeeetreeeeeeee 42 6 4 1 2 REMOTE 43 6 4 1 3 LOCAL 43 6 4 1 4 LOCAL Lockout43 6 4 1 5 Selection of the Radio Frequency eccceecceeeeeeesteeeeeeteeeeerens 44 6 4 1 6 Selection of the Radio Frequency Decimal Form 44 6 4 1 7 Selection of the Radio Frequency Binary Form s es 44 6 4 1 8 Select MANUAL or AUTO Mode 0 ecceceeeeieeeeettteeeetnieeeeeeene 45 6 4 1 9 Select Field S6nSei ists tees eeciatis ea eetet sist ite Bed inet ie et 45 6 4 1 10Select Display 46 6 4 1 11 Select Multiplexer Channel ecccceceeceeeeeeeeeeeeeeeeeeeeeeeeteees 46 6 4 1 12Activate SEARCH Mode 0 cccececceeeeeeeeeeeeeieeeeeeeeeeetneeee
55. mit can be extended up to 1 Tesla 6 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual REQUIRED HOMOGENEITY OF THE FIELD The following table gives the maximum field gradients in ppm cm for which the resulting signal to noise ratio just allows for automatic frequency tracking Probe N Field Range 1 2 3 4 5 6 7 8 Note The field gradient effect on the NMR signal can in some conditions be compensated for with an appropriate external correcting coil FIELD TRACKING RANGE IN AUTO MODE H probes up to 70 of the frequency range gt 5 at the upper extremity of the frequency range approx 3 7H probes up to 70 of the frequency range gt 1 5 at the upper extremity of the frequency range approx 1 0 Two LEDs indicate the approach of the upper TOO HI or lower TOO LO limit respectively of the frequency tracking range NMR SIGNAL OUTPUT BNC connector located on the front panel for scope inspection of the NMR signal Output impedance 10 KQ 10 nF to ground for noise filtering NMR signal negative pulses of 100 mV to 5 V NMR LOCK INDICATOR The LED is on in the presence of an NMR signal METROLAB Instruments SA 7 NMR Teslameter PT 2025 User s Manual FIELD MODULATION OUTPUT BNC connector located on the front panel for scope inspection of Probe field modulation waveform Output impedance 1 kQ Modulation signal is a 30 Hz to 70 Hz triangular wavef
56. n may cause difficulties to lock the NMR signal To stop the SEARCH mode press again the SEARCH button METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual Fig 3 Typical NMR waveform showing ringing or wiggles METROLAB Instruments SA 13 NMR Teslameter PT 2025 User s Manual 14 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 4 PT 2025 OPERATING MODES The PT 2025 has three operating modes which are explained in this chapter These three modes called MANUAL AUTO and SEARCH respectively allow the user to measure field values under varying field conditions and can be used with or without the computer interface The basic use of these three modes without the interface is now described followed by details of what can be gained by using the interface commands 4 1 THE MANUAL MODE The MANUAL mode is the most basic mode of operation and gives the user full control of all the instrument functions It may be useful in some very non homogeneous fields that the AUTO mode does not lock on to the NMR signal Set the MANUAL AUTO to the MANUAL position i e the button is released The radio frequency can be adjusted with the COARSE and FINE potentiometers The NMR signal can be found by slowly turning the COARSE potentiometer or if necessary the FINE one The presence of the NMR signal will be indicated by the flashing of the NMR LOCK LED located to the left of the display No
57. nance occurs exactly at the zero crossing of the modulation This frequency is therefore equal to the nuclear resonance frequency of the field B as seen by the protons or deuterons in the sample but without modulation It automatically follows all changes of Bo within the range covered by the FINE frequency adjustment This kind of field modulation has been chosen for the following reasons The modulation amplitude of a few hundred ppm facilitates the locking of the radio frequency to the field The modulation frequency of 30 Hz was found to be a reasonable compromise between radio frequency tracking speed and signal line width However the frequency can be increased up to 70 MHZ if the field is very uniform A triangular wave crosses through zero more slowly than a sine wave of the same frequency and amplitude which reduce spreading of the line width Moreover it can easily be generated very symmetrically thus improving the accuracy of the Teslameter The resulting line width is 10 to 100 ppm depending on the measured field and the modulation amplitude setting An accuracy better than 1 ppm can still be reached provided that the LC resonance circuit in the Probe is well tuned to the applied frequency and that the field modulation is symmetric in respect to zero i e Bmoa t T 2 Bmoa t T being the period of the modulation With both conditions fulfilled the NMR signals become identical in form and size and
58. ncy Af f produced by AVvco does not depend on the frequency dividing factor The signal to noise ratio is much smaller for the 7H probes than for the H probes Therefore for the H probes an additional attenuation factor of three is switched in the frequency control loop in order to facilitate the locking of the Teslameter to the field This reduces by the same factor of three the rate of the frequency variation in the SEARCH mode the loop gain and the frequency tracking range The accuracy of the Teslameter is not influenced by the lower loop gain which is still greater than 10 at d c 9 6 FREQUENCY COUNTER A eight and a half digit frequency counter with a special time base measures the NMR frequency and displays it either in Tesla with a resolution of 0 1 uTesla or in MHz with a resolution of 1 Hz When Field display has been selected the counter gate is about 1 s with H probes and about 1 5 s with H probes if Frequency display has been selected the counter gate is 1 s the gate length being defined by the gyromagnetic ratios and the chosen predividing factor of 4 for the H probes There are no predividing factors for the 7H and the frequency display The frequency counter is built up with an ECL circuit for the first stage and high speed C MOS circuits for the others The data transfer signal for its display register and the reset signal are generated by the time base circuit Fig 14 shows a block diagram of the freq
59. nge by changing to the next channel Suppose that the signal has been found on channel C probe 4 if the field drifts to a value of less than 0 35 T then the PT 2025 will select channel B and continue with probe 3 the algorithm functions in both positive and negative drift conditions In this example the PT 2025 can search and follow an NMR signal in a field ranging from 0 175 T low end of probe 3 up to 2 10 T high end of probe 5 In order to operate this scan correctly it is necessary to ensure that the probes are connected in an ascending order vis vis the multiplexer channels Note If the first channel scanned is selected as G and the PT 2025 is programmed to search over 4 channels then the scanning sequence would be G H A and B 6 4 1 15 Select the SEARCH Time Message format On Where n is a number from 1 to 6 n 3 on power up of RESET METROLAB Instruments SA 47 NMR Teslameter PT 2025 User s Manual This message allows the slope of the search to be changed The number 1 corresponds to the most rapid i e 9 seconds to scan the field range of a probe Each increase in n slows down the search by 3 seconds It may be necessary to slow the scan slope used in the PT 2025 under certain critical field conditions for example if the NMR signals were diminished due to a non homogeneous field It is possible to change the scan slope even when the searching is in progre
60. nts SA NMR Teslameter PT 2025 User s Manual 5 5 SUMMARY OF THE IEEE COMMANDS AND PT 2025 MESSAGES IN LISTENER TALKER MODE IEEE COMMANDS LISTENER REFER TO SECTION 5 1 1 REMOTE REN LOCAL GTL LOCAL LOCKOUT LLO DEVICE CLEAR DCL INTERFACE CLEAR IFC TRIGGER MESSAGES TO THE PT 2025 An Bnn Cnnnn lt CR gt lt LF gt Dn Fn Hnnnn lt CR gt lt LF gt Mno On Pc Q Sn Tn METROLAB Instruments SA Allows the PT 2025 to accept messages via the IEEE 488 interface Returns the PT 2025 to front panel control Blocks the function of the local push button Resets the PT 2025 internal counters to start a new measurement cycle LISTENER Select MANUAL 0 or AUTO 1 mode section 5 1 2 2 Binary load DAC 12 bits section 5 1 2 1 Decimal load DAC 12 bits section 5 1 2 1 Display mode Tesla 1 MHz 0 section 5 1 2 4 Field sense 1 0 section 5 1 2 3 Select SEARCH Mode section 5 1 2 6 Program mask register section 5 1 2 12 Select SEARCH time section 5 1 2 9 Select multiplexer channel section 5 1 2 5 Quit SEARCH mode section 5 1 2 7 Request status register section 5 1 2 13 Enable disable TRIGGER message 5 1 2 10 section 37 NMR Teslameter PT 2025 User s Manual Vn Fast reading display section 5 1 2 11 Xn Select multiplexer range section 5 1 2 8 MESSAGES EMITTED BY THE PT 2025 TALKER REFER TO SECTION 5 1 3 vdd DddddddT Display fo
61. o be measured is known approximately the search time can be reduced if the search is started from a value close to that of the field to be measured When using the multiplexer with several probes where the field value could be in one of several probes ranges the user may specify with the Xn message the number n of probes to be used in the search If the PT 2025 does not find an NMR signal in the last probe s range it restarts the search from the bottom of the first probe s range In poor field conditions the rate at which the search is executed may need to be decreased in order to improve the locking This may be achieved with the On message In this case n 1 gives a fast search of 9 seconds per probe Each increment in n causes the scan time to be increased by 3 seconds 18 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 5 PT 2025 IEEE 486 INTERFACE The PT 2025 Teslameter is equipped with a RS 232C and an IEEE 488 interfaces The desired interface can be selected by using micro switch 9 located on the back panel 0 RS 232C and 1 IEEE 488 Via these interfaces the user has control of the front panel functions and can access to the measured field value and the instrument status The SEARCH mode may also be entered via the interfaces Chapters 5 and 6 describe in detail the interfaces and their protocols In order to select the IEEE 488 interface micro switch 9 must be set to 1
62. or obtaining the best signal to noise ratio the optimum radio frequency voltage of the LC resonance circuit in the different probes is set by an appropriate choice of the coupling resistor RC to the radio frequency input KMR SIGNAL NMRA SIGNAL AMPLIFIER Thsv nue SIC WAL AMPLITUOE DETECTOR NMA SIGNAL OCTECTION Gor RF AMPLITUO DETECTOR RF Levee AO MEASURING HEAO NMR AMPLIFIER NMR PROBE Fig 11 Simplified circuit diagram of Amplifier and Probe 66 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 9 4 AUTOMATIC TRIGGER THRESHOLD AND DELAY CIRCUITS The NMR signals are fed to the input of comparator C1 via a filter network which eliminates the d c component and reduces the noise Fig 12 shows the diagram of the corresponding circuits The trigger level is set automatically to about half the signal amplitude the threshold of comparator C1 is 0 6 times the voltage at point A which is produced by the circuit around A1 in a charge pumping mode and which is slightly less than the NMR signal amplitude This kind of amplitude detection has the advantage of not being very sensitive to occasional large single parasitic signals since the voltage at point A can change at most by 0 2 V per input pulse of any size The lowest trigger level which is set without an NMR signal is kept safely above the noise level Since noise increases with the radio frequency the minimum thre
63. orm with an amplitude from 0 to 8 V Amplitude and frequency are adjustable with front panel screwdriver trimpots NMR FREQUENCY BNC connector located on the front panel for scope inspection or external precision frequency counter It s a current square wave of 0 to 16 mA amplitude NIM level 0 to 0 8 V amplitude if the input impedance of the measuring instrument is 50 Q RADIO FREQUENCY OUTPUT FOR NNR AMPLIFIER BNC connector located on the rear panel giving a square wave of 0 8 Vp p amplitude into 50 Q SIGNAL OUTPUT FOR NMR AMPLIFIER LEMO connector located on the rear panel used for NMR signal Modulation and Amplifier power supply INTERNAL FREQUENCY COUNTER 9 digits LED display indicating the field strength in Tesla or NMR frequency in MHz Resolution 0 1 uT or 1 Hz 8 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual TEMPERATURE COMPENSATED QUARTZ OSCILLATOR Stability Ageing 1 ppm within 10 to 40 C ambient temperature Refer also to the following picture which shows the 5 point compensation within the temperature range 20 to 70 C 1 Uncompensated curve 2 Compensation point 3 Compensated curve Af f ppm lt 1 0 x 107 day lt 1 0 x 10 month lt 2 0 x 10 year POWER REQUIREMENT Voltage Power Frequency Fuses 220 VAC or 110 VAC 10 depending on the position of the line voltage selector approx 40 VA 50 or 60 Hz
64. ormat described in section 5 1 3 The intervals between each value can be defined by using micro switches 1 to 5 as shown in the following table Microswitches Interval 5 43 2 1 0000 0 no message transmission 0000 1 sends every measured value 0001 0 sends every 2 measured value 0001 1 sends every 3 measured value 0010 0 sends every 4 measured value 0010 1 sends every 5 measured value 0011 0 sends every 6 measured value 0011 1 sends every 7 measured value 0100 0 sends every 8 measured value METROLAB Instruments SA 35 NMR Teslameter PT 2025 User s Manual O O O O O O O 36 O O O O O O O O gt a O O O sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every sends every 9 measured value 12 measured value 16 measured value 20 measured value 27 measured value 36 measured value 48 measured value 60 measured value 80 measured value 100 measured value 150 measured value 180 measured value 240 measured value 300 measured value 420 measured value 540 measured value 720 measured value 900 measured value 1200 measured value 1600 measured value 2100 measured value 2700 measured value 3600 measured value METROLAB Instrume
65. ossible for the user to access certain data contained in the PT 2025 such as the field value or instrument status in this case the instrument must be addressed as a Talker The following section describes the messages used in the IEEE 488 interface Note The examples of the message sequences shown are for an HP 85 as bus controller 5 1 1 IEEE 488 Commands REMOTE The PT 2025 is put into the REMOTE mode when the REN line of the IEEE 488 bus is set true and the instrument is addressed as a Listener When the PT 2025 is put into the REMOTE state the front panel controls are disabled with the exception of the LOCAL push button The PT 2025 retains the same configuration as it had before the REMOTE state was selected with the exception of the selection of the radio frequency Note The DAC has a default value of 2048 which represents the middle of the frequency range When the PT 2025 is not in the REMOTE state it cannot be addressed as a Listener 20 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual GO TO LOCAL GTL The IEEE 488 command GTL Go to LOCAL forces the PT 2025 into the LOCAL state the PT 2025 is then configured by the front panel controls which become operational By pressing the LOCAL push button once when the instrument is in the REMOTE state has the same effect as the Go To LOCAL command as long as the button has not been disabled by the LOCAL lockout command see below LOCAL LOCKO
66. requency actually selected by the COARSE potentiometer If the NMR signal appears within the range scanned the PT 2025 will automatically lock on to the signal providing that the polarity of the modulation as selected by the FIELD is in accordance with that of the field if the field sense is wrong select the correct sense The LED NMR LOCK should then be permanently lit to indicate that the PT 2025 has found the correct field value The LEDs TOO HI and TOO LO indicate the position of the COARSE potentiometer in relation to the NMR frequency when the signal is locked Example The field to be measured has a value of 1 02 T probe 5 The PT 2025 is in MANUAL mode with the FINE potentiometer set to 5 0 and indicates 1 000 T When the AUTO mode is selected the PT 2025 will scan the field from 0 95 T 5 to 1 05 T 5 When the PT 2025 arrives at the field value of 1 02 T it will lock on to the NMR Signal and the display will show the actual field value The LEDs TOO LO will be lit indicating that the COARSE potentiometer is too low Increasing the COARSE very slowly until the TOO LO LEDs are off will ensure that the NMR signal is centred within the scan range The magnetic field can be tracked within the range of 5 without losing the locked condition providing the variation is not greater than 1 sec If the NMR signal is too weak and the PT 2025 cannot lock on to it the instrument must be returned to
67. rmat Tesla vdd ddddddF Display format MHz Saa Status format 38 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 6 PT 2025 RS 232 C INTERFACE DCE The PT 2025 Teslameter is equipped with an RS 232 C and an IEEE 488 interface The desired interface can be selected by using micro switch situated on the back panel 0 RS 232 C 1 IEEE 488 Via these interfaces the user has control of the front panel functions and can access the measured field value and the instrument status The SEARCH mode may also be entered via the interfaces Chapters 5 and 6 describe in detail the interfaces and their protocols 6 1 PROGRAMMING OF THE RS 232 C INTERFACE To use this interface micro switch 9 must be set to 0 The remaining eight micro switches are used to define the transmission characteristics as follows Micro switch N Comment Interval refer to section 6 5 0 7 bit transmission reception 1 8 bit transmission reception 0 without parity 1 with parity 0 even parity if micro switch 5 1 1 odd parity If micro switch 5 0 then 6 has no meaning 0 1 stop bit 0 RS 232 C Note It is not possible to select a 8 bit transmission reception with parity as well as 2 stop bits All other combinations are acceptable 7 0 autonomous mode 1 conversational mode METROLAB Instruments SA 39 NMR Teslameter PT 2025 User s Manual 6 2 CONNECTING THE RS 232 C IN
68. roller when certain conditions occur 5 2 1 How to Use the SRQ When a service request is sent the controller must firstly determine which instrument instigated the demand This operation is carried out by a serial poll spoll of each device connected to the bus capable of generating a Service Request When the instrument is polled it replies with the contents of its SRQ Status Register which indicates if it is the originator of the Service Request and if so the nature of the request The above sequence supposes that the controller is programmed to receive the SRQ interrupts The Internal Status Register together with the Service Request SRQ Mask determine whether or not the Require Service RQS bit will be set by an interrupt However as soon as the Require Service bit is set the PT 2025 puts the SRQ line of the IEEE bus true which in turn causes an interrupt in the controller 5 2 2 Setting the SRQ Mask The SRQ Mask can only be applied to bits 0 to 5 of the SRQ Register Each 1 in the SRQ Mask can be considered as a hole which allows the information in the Internal Status Register through to generate a Service Request METROLAB Instruments SA 29 NMR Teslameter PT 2025 User s Manual 30 Example bits 7 6 5 4 3 2 1 0 RQS Internal Status Register 1 0 0 1 1 0 0 1 SRQ Mark Register 1 1 0 1 0 0 1 0 SRQ Status register 1 1 0 1 0 0 0 0 To create a mask firstly determine which conditions
69. s format METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 7 THEORY OF OPERATION In the presence of a static magnetic field Bo a nucleus with a magnetic moment u can take 2I 1 distinct energy states I being the spin quantum number The separation of these states is AE uB I Applying an alternating magnetic field perpendicular to the static field induces transitions between levels if its frequency equals the resonant frequency f AE h yBy with y p hel For magnetic fields of the order of 1 Tesla NMR frequencies lie in the radio frequency region For protons and deuterons G is known very precisely Gp H 42 57608 12 MHz Tesla for protons Gd H 6 53569 2 MHz Tesla for deuterons For detecting the proton magnetic resonance a small water filled coil is placed in static field Bo with its axis perpendicular to Bo The magnetic moments of the protons in the water molecules point preferentially in the direction of Bo i e the lower energy magnetic states are more populated than the higher ones Therefore if transitions are induced with an alternating field those from lower to higher energy states are more frequent than the contrary The protons absorb more energy from the alternating field than they supply to it and the difference between the populations of the two energy states is reduced The thermal equilibrium populations are re established due to spin lattice interactions at a rate
70. sed since the DAC has only 12 bits There is not a linear relationship between the selected value and the NMR frequency Fig 7 shows the typical relationship between the NMR frequency versus the DAC setting 5 1 2 2 Select MANUAL or AUTO Mode Message format An Where n 0 for MANUAL mode n 1 for AUTO mode This message selects the MANUAL or AUTO mode When the PT 2025 is in AUTO mode it can lock on to the NMR signal and follow the field if it drifts within 5 of the value preselected by the DAC or the COARSE potentiometer In AUTO mode the FINE potentiometer is disconnected for more details refer to section 4 2 AUTO mode Example OUTPUT 708 A1 selects AUTO mode 5 1 2 3 Select Field Sense Message format Fn Where n 0 or for negative fields n 1 or for positive fields This message determines the sense of the field to be measured with respect to the orientation of the probe If the FIELD is in the wrong sense the PT 2025 does not lock on to the NMR signal Example OUTPUT 708 F selects FIELD or OUTPUT 708 F1 selects FIELD 5 1 2 4 Select Display Message format Dn where n 0 for MHz n 1 for Tesla This message determines whether the displayed value is given in Tesla or in MHz It also affects the format of the measured value given via the interface Example OUTPUT 708 D1 displays the value in Tesla METROLAB Instruments SA 23 NMR Teslameter PT
71. shold is derived from the COARSE frequency adjustment VF2 and varies from 40 mV to 100 mV AMPLIFIERS A1 A4 SN7274IP 7 INEUT FILTER 5V 9 Rr SY COMPARATORS C1 C2 LM311 v as 12y 47u 560 NMR e J 100 mv v 330 ONES SHOTS 1 2 SN74123N SIGNAL ALL DIODES IN914 90 Q FOR H 0 PROBES R a_a 30 gq FOR D PROBES ONE SHOT 2 T 30ys ERROR VOLTAGE Fig 12 Circuits for generating the error voltage including the automatic threshold and timing controls METROLAB Instruments SA 67 NMR Teslameter PT 2025 User s Manual The modulating current Imog is sampled during the 30 us pulses produced by one shot 2 shown in Fig 12 The pulse width is not critical but has to be long enough to allow the sample and hold amplifier to settle The voltage at the output of integrator A4 determines the delay relative to the instant when the NMR signal crosses the threshold at C1 This voltage is regulated so that no 30 Hz to 70 Hz component appears at the error voltage output If there is a signal in phase or 180 degrees phase difference relative to the modulation it is amplified by A2 and integrated by A4 only during the positive half waves of the modulation the MOS FET switch being controlled by A3 This results in a decrease or increase respectively of the output voltage of A4 and therefore in a decrease or increase respectively of the delay until the 30 Hz component disappears The three diodes and the zener diod
72. ss 6 4 1 16 Reset NNR Time base Trigger Message format T This message forces a reset of the NMR time base thus starting a new measurement cycle Note that the NMR value will return the letter W until a valid cycle been completed 6 4 1 17 Fast Reading Display Message format Vn Where n 0 or N for Normal rate n 1 or F for Fast rate This message determines the display reading rate i e Normal 1 per second or Fast 10 per second The last digit of the display is not visible and is not transferred to the computer Therefore the PT 2025 resolution is reduced by a factor 10 6 4 1 18 Request Status Message format Sn where n is 1 2 3 or 4 and indicates the status register to be read The status registers are described in section 6 4 2 Immediately after receipt of this message the PT 2025 sends back the requested status in the format shown below Message format Saa where S is the ASCII character S meaning status where a is the ASCII character forming a hexadecimal pair 48 METROLAB Instruments SA 6 4 2 NMR Teslameter PT 2025 User s Manual Instrument Status Registers The PT 2025 has four internal status registers that can be accessed by the user see section 6 4 1 18 The format of these four registers is now given 6 4 2 1 Status Register 1 Internal Instrument Status 1 Byte This register is cleared after reading Bit 7 6
73. t the field uniformity is inadequate In some instances the next lower range probe may register a higher signal It could also be worthwhile considering the use of Gradient Compensation Coils Faulty cable Verify that the Probe to Amplifier and Amplifier to Teslameter cables are undamaged and securely in place 10 2 ABOUT FIELD TRACKING Symptom Whilst in AUTO mode the field tracking range was restricted FINE control not centred The FINE control should be set to approximately 5 0 and the COARSE control adjusted if necessary When compared to COARSE setting the This is indicated by the LEDs TOO HI or TOO LO being the lit Gradually turn the COARSE control to high or low actual field remained either until both LEDs are extinguished too high or too low Operating at the probe The tracking range is lower at the extremes of the probe range s extremity ranges and so where possible use a different probe 10 3 UNSTABLE VALUE Symptom Unstable field or frequency reading Unstable field Verify the stability of the magnet power supply and magnet and the probe mounting Too low GAIN control setting Turn clockwise the GAIN adjustment on the front panel to maximum unless a poor signal to noise ratio prevents a stable lock at maximum gain 74 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 10 4 EXTERNAL FREQUENCY MET
74. te Noise can also cause the NMR LOCK to flash if it is great enough to pass the threshold level If however the NMR signal is so small due to the non homogeneity of the magnetic field that the detection level is not reached the NMR LOCK will not light In this case it is necessary to use an oscilloscope Connect channel 1 of the oscilloscope to the NMR SIGNAL BNC output 0 1 V Div and channel 2 to the FIELD MODULATION 5 V Div Set the time base to 5ms Div and trigger on channel 2 FIELD MODULATION A precision of the order of 10 can be easily obtained if the NMR signal is made symmetrical in relation to the modulation signal see below 50 50 Modulation field Champ de modulation Resonance point I Point de resonance Fig 4 NMR signal upper Modulation signal lower METROLAB Instruments SA 15 NMR Teslameter PT 2025 User s Manual The LEDs TOO HI and TOO LO have no significance in the MANUAL mode and will only be off if the FINE potentiometer is set to 5 0 If the NMR signal is sufficient to light the NMR LOCK LED then set the AUTO mode to ensure a stable lock condition See the following section 4 2 THE AUTO MODE To select the AUTO mode push the MANUAL AUTO button to AUTO the button is depressed In the AUTO mode the PT 2025 sweeps the radio frequency over the whole range of the FINE potentiometer the latter being disconnected This represents a variation of 5 of the f
75. tection threshold will appear to be the first signs of the NMR signal Providing the noise is of a random nature the search algorithm will ignore it however this adds a delay of about four seconds to the search time If the noise is not random but appears at a precise frequency for example an external radio frequency beating with the internal radio frequency of the PT 2025 the algorithm will take about ten seconds to identify the noise signal during this METROLAB Instruments SA 17 NMR Teslameter PT 2025 User s Manual time the LEDs TOO HI and TOO LO will light up alternatively Once the noise has been identified the search will be continued 4 4 USE OF THE THREE MODES VIA THE INTERFACE The above explanation of the three modes of operation remains valid when using them via the computer interface However more options are available in the SEARCH mode these options are now described The MANUAL AUTO mode is selected with the An message Where n 0 for MANUAL n 1 for AUTO This has exactly the same effect as the front panel button With regard to the SEARCH mode the use of interface gives the user more possibilities than the front panel controls For example Starting the search at a defined radio frequency e Performing a Search over several probes if a probe multiplexer is used Changing the speed at which the search is performed The use of these functions is now outlined If the value of the field t
76. teps approximately 50 dB in the Teslameter and approximately 40 dB in the probe where a low pass filter is placed for reducing the noise pick up in the long interconnecting cables A positive or negative current is produced at the multiplier output whenever the resonance circuit is mistuned This current is fed to the integrator INT which changes the tuning diode bias voltage until the multiplier output current falls to zero i e the 10 kHz signal at the X input disappears The time constant has been chosen such that automatic tuning easily follows the fastest frequency variations in the SEARCH mode An appropriate feedback network of the integrator compensates for the effect of the low pass filter in the probe on the frequency characteristic of the loop gain PROBE MAIN UNIT 10 kHz GENERATOR a zd Je NMR MVv1404 Sample K r 39k 10 nF an J Ct a 10k AAZ15 aga iad i az S 47k 27k 3 3 pF i ra AMPLITUDE COARSE LOWPASS DETECTOR ACTIV FREQUENCY FILTER ADJUSTMENT Vv Renn perce Fig 10 Simplified circuit diagram of automatic tuning circuit 64 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual 9 3 THE NMR SIGNAL AND RADIO FREQUENCY AMPLIFIERS A simplified circuit diagram of the probe and Amplifier is shown in Fig 11 The NMR absorption signal is an amplitude variation of the radio frequency voltage of the LC circuit and is very small typically of the order of 0 1
77. to preselect the radio frequency via the IEEE 488 interface thereby in effect replacing the use of the COARSE potentiometer The selected value can be sent either in binary or in decimal form The value is then sent to a 12 bit digital analog convertor in order to select the radio frequency The range of the DAC is from 0 to 4095 Should a value greater than 4095 be received by the PT 2025 it will be interpreted as 4095 DECIMAL FORMAT Message format Cnnnn lt CR gt lt LF gt Cc the ASCII character C signifies that the selected value is expressed in decimal n represents the preselected value given in ASCII characters from 0 to 9 inclusive The user may perform leading 0 suppression on the selected value lt CR gt lt LF gt the ASCII characters carriage return and linefeed respectively must terminate the message Example OUTPUT 708 C1068 amp CHR 13 amp CHR 10 BINARY FORMAT The binary format of the preselection of the radio frequency message is as follows Message format Bnn B the ASCII character B signifies that the preselected value is expressed in binary n represents a two bits selected value 0 to 4095 inclusive Example OUTPUT 708 Bx1 corresponds to 1329 or OUTPUT 708 B amp CHR 4 amp CHR 12 corresponds to1042 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual Note The 4 most significant bits of the 16 bit pattern given by these two digits are not u
78. uency counter time base A 100 kHz crystal oscillator is used as the clock frequency for generating the required gate lengths This low clock frequency value has been chosen in order to avoid interference with the radio frequency signal of the probe risk of beating The required gate lengths are 0 93949464 s for the H probes and 1 5300599 s for the H probes if field display has been selected or 1 0000000 s in the case of Frequency display The clock period of 10 psec is too long for generating these times sufficiently accurately by simple countdown therefore three one shots for fine adjustment of the gate lengths 40 ppm have been added 70 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual v r 4 Prost Divider by We Ne ow Ns Crislel Reset down T Te wT Ore shot Oscillator ik 4 Tz ISAs ji P 12003 Ths pat is on the RF BOARD FRONT PANEL BOARD Fig 14 Block diagram of the time base of the frequency counter One shot 1 is used for calibrating the Tesla reading of the H probes one shot 2 is used for calibrating the H probes and one shot 3 for calibration in MHz The stability of these one shots typically less than 1 is not critical since they add only a small tens of ppm to the total gate width This interpolation technique has also the advantage that the frequency tolerance of the 100 kHz crystal is relaxed Fig 15 shows a pulse sequence diagram of some lines in the time base of the fr
79. ve bit 2 Field This bit indicates if the field is positive or negative with respect to the probe orientation 1 indicates a positive field 0 indicates a negative field bit 1 MANUAL AUTO mode state This bit indicates whether the PT 2025 is in MANUAL or AUTO mode 0 indicates MANUAL Mode 1 indicates AUTO Mode bit 0 DISPLAY mode data format mode 1 indicates that the displayed value is given in Tesla 0 indicates that the displayed value is given in MHz 34 METROLAB Instruments SA 5 3 4 NMR Teslameter PT 2025 User s Manual Status Register 4 DAC Status 2 bytes These two bytes give the contents of the DAC 5 4 TALKER ONLY This mode is used to connect the PT 2025 to a device for example a printer without the controller functions Micro Switch N Setting 1to5 Repetition rate see table below 6 0 7 1 8 1 0 9 1 Micro switch 8 should be set to 1 if the user wishes the PT 2025 to send carriage return and line feed at the end of every message Setting this micro switch to 0 suppresses the carriage return and line feed characters To indicate the end of the message the EOI signal is made true immediately prior to the transmission of the last character In this mode the Teslameter transmits the measured value at regular intervals each measurement cycle lasting approximately one second according to the message f
80. y signal Its square wave like shape does not cause any disturbance The sample and hold circuit produces an error voltage AV_e proportional to the modulating field at the instant when the nuclear resonance occurs AVE a ABmoa 68 METROLAB Instruments SA NMR Teslameter PT 2025 User s Manual The frequency control voltage of the oscillator is derived from AVe by integration and attenuation AVvco 9 AVE which results in a frequency change of AF a f g AB Error Veltage Acct Olu AV v c 0 FREO DIVIDER Loop gain AF y AB a B g y Fig 13 Block diagram of frequency control loop By choosing an appropriate integration time constant and attenuation AF reaches ABmod y just when the next NMR signal appears i e the frequency error which produced AVe 0 is entirely corrected by this time This is the optimum loop gain setting for fast frequency tracking Owing to the non linearity of the oscillator frequency control curve the optimum loop gain setting at a medium frequency is not valid for the full frequency range The loop gain decreases in the worst case by a factor of five close to the upper and lower limits of the oscillator frequency range For all frequency ranges except one the oscillator frequency is divided by a factor of 2 which results in a reduction of B by the same factor This is compensated for by the factor a i e the product a is made constant for all field r

Metrolab NMR Teslameter Manual 2003 (See Mod P. 5 for 2010

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