Tektronix 5A26 User's Manual
Contents
1. b Connect a second probe for de coupied operation to CH 1 input Apply 100 volt peak to peak square wave to both probes Free run sweep and adjust probe compensation adjustment of second probe for minimum display amplitude c Steps a and b match the probes for use at any sensitivity that employs the particular input attenuator 1X or 100X used in steps a and b When it is necessary to use the other input attenuator steps a and b should be repeated d Repeat steps a b and c for Channel 2 e When examining a small differential signal in the presence of relatively large common mode components adjust probe by temporarily connecting both probes to either of the two signal sources Characteristics Deflection Factor Calibrated Range Accuracy Uncalibrated Range Frequency Response Bandwidth 8 Div Reference DC Direct Coupled 1 MHz 10 kHz AC Capacitive Coupled 1 MHz 10 kHz Step Response Aberrations Input R and C Maximum Safe Input Voltage DC Direct Coupled 50 uV Div to 50 mV Div 100 mV Div to 5 V Div AC Capacitive Coupled 50 uV Div to 50 mV Div 100 mV Div to 5 V Div DC Rejection AC Capacitive Coupled Input Gate Current Common Mode Rejection DC Direct Coupled 50 uV Div to 50 mV Div 100 mV Div to 5 V Div Operating Instructions 5A26 SPECIFICATIONS TABLE 1 1 Electrical Specifications Performance Requirement 50 uV div to 5 V div in a 1 2 5 sequence2. time base unit via pin A4 The triggering signal is of the same polarity as that applied to the gate of Q54B and has an amplitude of about 0 25 volts per displayed division Vertical Switching VOLTS DIV switch 20 contains a series of cam lobes which engage and disengage various contacts at different positions of the switch Cams 1 2 4 and 5 control switch ing of the Input Attenuator and cams 11 through 19 control switching of gain setting network resistors R228 through R236 in the Gain Switching Amplifier stage Those contacts that are engaged at any VOLTS DIV switch position are shown by black dots on the switch logic diagram READOUT D en Factor VOLTS DIV Knob Skirt and CRT Either of two famp bulbs DS417 or DS421 located behind the VOLTS DIV knob skirt illuminate the selected deflec Theory of Operation 5A26 tion factor to provide a direct readout Normally DS421 is lit to indicate a 1X deflection factor Connecting a readout coded 10X probe to either input with the GND buttons out automatically changes the crt readout by a factor of 10 and lights DS417 to indicate a 10X deflection factor With a 10X readout coded probe connected to the input the probe coding resistor is connected in parallel with R410 through the probe coding ring The probe resistor biases 0417 on which turns on indicator DS417 Consequently 0421 turns off and extinguishes indicator DS421 The probe coding resistor in parallel with R410
3. TEKTRONIX INSTRUCTION MANUAL Tektronix inc P O Box 500 Beaverton Oregon 97077 Serial Number 070 1947 00 First Printing JUNE 1975 t may no x n DEAD Zaj eg TE i BEARS AA o A 5A26 DISPLAY Fig 1 1 5A26 Dual Differential Amplifier 1947 1 Section 1 5A26 OPERATING INSTRUCTIONS The 5A26 Dual Differential Amplifier is a dual trace high gain differential amplifier plug in for use with Tektronix 5000 Series Oscilloscopes The unit contains two identical independent amplifier channels that feature high sensitivity and a high common mode rejection ratio Each channel has a bandwidth capability of dc to 1 megahertz and a front panel push button switch allows the reduction of the upper bandwidth limit thus increasing the signal to noise ratio for low frequency applications Either channel may be used for single channel displays or electronically switched to produce dual trace displays An iiluminated knob skirt provides a deflection factor readout which is also displayed on the crt when used in an oscilloscope with readout capabilities PREPARATION FOR USE The 5A26 is calibrated and ready for use when received The unit may be instalied in any plug in compartment Re fer to the oscillosc
4. Within 2 of deflection factor setting Variable VOLTS DIV contro provides continuously variable deflection factor between calibrated settings and extends deflection factor range to 12 5 V div Dc to at feast 1 MHz Dc to 10 kHz within 20 2 Hz or less to at least 1 MHz 2 Hz or less to 10 KHz within 20 2 2 or less Total not to exceed 4 p p 1 MQ within 0 15 paralleled by approximately 47 pF 10 V dc peak ac 350 V dc peak ac 350 V de coupling capacitor pre charged 10 V peak ac 350 V dc peak ac At least 100 000 1 after 20 minute warmup 100 pA or less 100 UV or less depending on external loading at 25 C At least 100 000 1 from dc to 30 kHz with up to 20 V p p sine wave At least 300 1 from de to 30 kHz with up to 100 V p p sine wave 1 7 Operating instructions 5A26 TABLE 1 1 CONT Electrical Specifications Characteristics Performance Requirement Common Mode Rejection continued AC Capacitive Coupled 50 uV Div to 50 mV Div At least 20 000 1 at 5 kHz and above decreasing to not less than 2000 1 at 60 Hz With 2 P6062A Probes 400 1 at 10X probe attenuation Displayed Noise Tangentially Measured 30 UV or less at full bandwidth Channel Isolation at 4 MHz Single Ended At least 100 1 with 8 div sine wave applied to one channel at 2 V div and other channel at 1 mV div with no signal applied Channel Isolation at 30 kHz Common Mode 50 uV or less at any deflectio
5. also changes the bias on 0450 which changes the crt readout When the DISPLAY button is out illuminating voltage is removed from indicators DS417 and DS421 and 0484 is gated off Thus indicators DS417 and DS421 are extinguished and the crt readout Is blanked With a 10X readout coded probe connected to the input R413 functions the same as R410 in the input circuit Thus a 10X readout coded probe connected to either or both inputs will provide a 10X readout on the VOLTS DIV knob skirt and on the ert When pressed in contacts of GND switches S2D input and S2B input open the circuit between the and input probe coding rings and the readout circuitry Conse quentiy with a readout coded probe connected to both inputs and both and GND buttons pressed in the VOLTS DIV knob skirt and crt will indicate a 1X deflection factor If only one of the GND buttons is pressed in the VOLTS DIV knob skirt and crt will indicate a 10X deflection factor If the probe coding ring on the or input connector is grounded DS417 lights to indicate a 10X deflection factor and the ert readout is extinguished The crt readout is determined by the VOLTS DIV contro which establishes the bias on 0458 0466 and 0474 Changing the bias on the base of these transistors also changes the emitter voltage thereby changing the voltage across their associated emitter resistors The current drawn from pin A28 through 0458 0466 or 0474 and
6. of 1 millivolt peak to peak If an attempt is made to display the described signal single ended measurement at 0 2 millivolts division the 60 hertz signal will produce a deflection equivalent to 5000 divisions and the 1 millivolt signal will be lost If the same 1 millivolt signal is measured differentially with the 60 hertz signal common to both inputs no more than one part in 100 000 of the common mode signal will appear in the display The desired signal will produce a display of 5 divisions with not more than 0 1 division of display produced by the common mode signal cmrr not specified when residual display is 0 1 division or less There are a number of factors which can degrade common mode rejection The principle requirement for maximum rejection is that the common mode signal arrives at the input FET gates in precisely the same form A difference of only 0 01 in the attenuation factors of the input attenuators may reduce the rejection ratio to 10 000 1 Likewise any difference in source impedance at the two points in the source under test will degrade the rejection ratio Attenuator probes which do not have adjustable resistance and capaci tance may reduce the rejection ratio to 100 1 or less Outside influences such as magnetic fields can also degrade the performance particularly when low level signals are involved Magnetic interference may be minimized by using identical signal transporting leads to the two inputs and tw
7. signal applied potentiometer R273 Gain 1 2 3 Theory of Operation 5A26 provides emitter degeneration The gain is determined by the tota emitter resistance which allows the overall gain of the 5A26 to be adjusted to match the mainframe require ments Also the Variable VOLTS DIV control R264 provides continuously variable gain to 2 5 times the calibra ted deflection factor setting Transistors 0267 and 0277 are positioning current drivers Position POS control R274 provides an adjustable change in the conduction of the transistors to provide a current that either adds to or substracts from the Q260 0270 currents to alter the quiescent vertical position of the display INPUT CONSTANT CURRENT RETURN Q66A 0668 INPUT 0548 Channel Switching Channel switching is accomplished by 0282 0292 0286 and 0386 The channel switch signal from the mainframe through pin B21 sets the bias on 0386 while the bias on 0286 is determined by the MODE buttons The bias level at the base of 0286 relative to that at the base of 0386 determines which of these transistors is on With the CH 1 MODE button pressed in and the CH 2 MODE button out the bias level on Q286 is lower more negative than that at 0386 which turns 0286 off and Q386 on With 0286 turned off 5 volts de through R286 reverse biases diodes CR283 and CR293 which turns on 0282 and 0292 in the channel switch stage Thus the channel 1 signal is coup
8. volt range of the VOLTS DIV switch the input impedance is paralleled by the resistors in the attenuator When the strap is removed the attenuation ratio is affected causing the deflection factors in this range to be incorrect To determine the de flection factor check the deflection with an input signal of known amplitude The signal source impedance is an important factor since gate current will produce an offset For example a 100 picoampere gate current through 10 megohms produces a 1 millivolt offset which may result in significant error where small voltages are of concern The high frequency response will also depend upon the signal source impedance since various shunt capacitances between the source and the input gate must charge and discharge through that impedance Display Polarity Single ended signals applied to the input connector pro duce a display in phase with the input signal Signals applied to the input connector will be inverted A similar polarity relationship exists for differentially applied signals but pertains to the direction of voltage change at one input with respect to the other rather than with respect to chassis potential Operating Instructions bA26 Deflection Factor The amount of trace deflection produced by a signal is determined by the signal amplitude the attenuation if any of the probe the setting of the VOLTS DIV switch and the setting of the Variabie control The deflec
9. 205A O208A and Q210A on the input side and O205B 0208B and 0210B on the input side Total gain of the stage is determined by R210 R220 and the setting of the VOLTS DIV control which selects the series parallel combination of R203 R213 and R228 through R236 Potentiometer R225 Var Bal 1 located in the collector circuit of transistors 0210A and 02108 is adjusted with no input signal to set the voltage across the Variable VOLTS DIV control R264 in the Output Amplifier stage to zero volts With both sides of the amplifier balanced trace deflection is prevented as the Variable VOLTS DIV control is rotated throughout its range Normally the frequency response of the Gain Switching Amplifier is from de to at least 1 megahertz however for low frequency applications where high input sensitivities are used the overall frequency response can be limited to about 10 kilohertz to reduce noise referred to the input This is achieved by pushing in the BW LIMIT 10 kHz switch 5239 which connects C239 across the output of the Gain Switching Amplifier Isolation Stage The differential signal developed at the collectors of Q210A and 02108 is passed through emitter followers 0242 and 0252 to the Output Amplifier Transistors 0242 and 0252 isolate the Gain Switching Amplifier from the loading of the Output Amplifier and Trigger Amplifier Output Amplifier The Output Amplifier consists of push pull amplifiers 0260 and Q270 With a
10. K DIAGRAM DESCRIPTION When the DISPLAY button is pressed a logic level is applied to the oscilloscope to enable 5A26 operation switch func tion is limited to operation in a vertical deflection compart ment Signals applied to the and input connectors can be passed directly through the input coupling switches to the attenuators dc coupled or they can be capacitively ac coupled to block the dc component of the signal The GND switch provides a ground reference to the Preamplifier input The Input Attenuator consists of frequency compensated voltage dividers 1X attenuation is provided for the 50 4 to 50 m positions of the VOLTS DIV switch and 100X attenuation is provided for the 1 to 5 positions Balance i to a low frequency common mode signal between the attenuators of the two inputs is set by adjustment of the internal LF CMR potentiometer From the Input Attenuator the signal is passed directly to the Preamplifier The inputs to the Preamplifier are fuse and diode protected The Preamplifier consists of two identical operational amplifiers connected in a differential configuration Common mode signals between ground and the two inputs are rejected due to a bootstrapped floating power supply that moves with the common mode signal to maintain constant operating characteristics of the active devices The difference between the two inputs is amplified The constant current return facilitates the de balance of the Prea
11. d unsatisfactory results because of interference resulting from ground loop currents between the 5A26 and the device under test In other cases it may be desirable to eliminate a dc voltage by means other than the use of a de blocking capacitor which could limit the low frequency response These limitations of single ended measurements are effectively eliminated by connecting the input and input to selected points in the test circuit Since the chassis of the BA26 need not be connected in any way to the test circuit there are few limitations to the selection of these test points In any case do not exceed the maximum safe input voltages The and AC buttons should both be set to the same positions depending on the method of signal coupling required 1 5 A Operating Instructions 5A26 Only the voltage difference between two signals is amplified and displayed in differential measurements while the common mode signals common in amplitude frequency and phase are rejected The ability of the 5A26 to reject common mode signals is indicated by the common mode rejection ratio emer Cmrr is at least 100 000 1 at the input connectors from 50 mierovolts division to 50 millivolts division when signals between de and 30 kilohertz are de coupled to the inputs To illustrate this characteristic assume that a single ended input signal consists of an unwanted 60 hertz signal of 1 volt peak to peak plus a desired signal
12. eflection Connectors include coded probe input rings for activating the readout when using readout coded probes BASIC OPERATION Preparation The following procedures are intended to heip place the trace on the screen quickly and prepare the instrument for immediate use a Insert 5A26 all the way into oscilloscope plug in compartment b Turn oscilloscope Intensity control to midrange and turn oscilloscope system power on Preset time base and triggering controls for 2 millisecond division sweep rate and automatic triggering c Set 5A26 front panel controls as follows DISPLAY ON button in TRIGGER OUT CH 1 button out POS Midrange BW LIMIT OUT 1 MHz button out VOLTS DIV 5 STEP ATTEN BAL Midrange CH 1 and CH 2 Input Coupling AC OUT DC button out GND GND button in Operating Instructions 5A26 CH 1 and CH 2 Input Coupling AC OUT DC button out GND GND button in MODE CH 1 and CH 2 buttons in d Adjust intensity control for normal viewing Traces should appear near graticule center e Set the VOLTS DIV switches to 1 If traces shift adjust the STEP ATTEN BAL contro as described in Step Attenuator Balance Adjustment procedure f Move CH 1 trace 2 divisions below graticule horizontal centerline and CH 2 trace to bottom of graticule with POS controls g Apply 400 millivolt peak to peak signal from oscillo scope calibrator loop with a test lead or 1X probe to CH 1 input c
13. in practice any mismatch will cause a differential output Floating Power Supply A Floating Power Supply consisting of Q39 044 048 and Zener diodes VR40 and VR42 minimizes inherent common mode difficulties and therefore improves the common mode rejection ratio refer to Fig 2 1 Transistor Q44 is a constant current high impedance source and Q39 is the current return The input to the bootstrap X1 gain amplifier is connected to the junction of R58 and R78 The bootstrap amplifier portion of the supply consists of emitter follower 048 and dc level shifting Zener diodes VR40 and VR42 The collec tor impedance of Q39 presents minimum loading to the 048 output and maintains the gain of the amplifier boot strap efficiency very close to one The entire power supply and amplifier voltages move an amount equal to the common mode voltage to maintain a Theory of Operation 5A26 constant operating characteristic of the elements in the Preamplifier stage Since no common mode signal current is developed the output at the collectors of Q64A and 064B remains unchanged that is the common mode signal is rejected Variable capacitor C59 HF CMR is adjusted to normalize the effective capacitance of the active devices on one side of the amplifier to the other OUTPUT AMPLIFIERS Gain Switching Amplifier The Gain Switching Amplifier consists of two identical operational amplifiers The operational amplifiers consist of 0
14. isting the two leads together over as much of their length as possible Voltage Probes in general probes offer the most convenient means of connecting a signal to the input of the 5A26 Tektronix probes are shielded to prevent pickup of electrostatic inter 7 ference A 10X attenuator probe offers a high input i impedance and allows the circuit under test to perform very close to normal operating conditions See your Tektronix Inc catalog for characteristics and compatibility of probes for use with this system CODED PROBES The 5A26 is designed for compatibility with coded probes such as the Tektronix P6062A 1X 10X Passive Probe The and input connectors have an outer ring to which the coding pin on the probe connector makes contact This type of probe allows the vertical deflection factor indicated by the readout to correspond with the actual voltage at the probe tip eliminating the need to consider the attenuation factor when measuring the signa amplitude on the graticule scale Differential Measurements The following adjustment procedure is recommended when preparing to use two Tektronix P6062A probes for differen tial measurements in the 10X mode a Connect one probe for de coupled single ended operation to CH 1 input Obtain triggered display of an appropriate square wave such as that from a calibrator Fa or square wave generator Compensate probe square U wave response with probe compensation adjustment
15. led to the mainframe through interface connector pins A7 and B7 30 V i CONSTANT CURRENT SOURCE 044 BOOTSTRAP AMPLIFIER X1 GAIN CURRENT RETURN 039 1947 4 Fig 2 1 Floating Power Supply detailed block diagram showing standing current paths through the Preamplifier 2 4 While 0286 is off Q386 is on and draws current from the 5 volt de supply through R386 The resultant voltage across R386 turns off 0382 and 0392 in the Channel Switch stage Thus the channel 2 signal is not coupled to the mainframe When the CH 2 MODE button is pressed in the bias level on 0286 is higher less negative than that at 0386 which turns 0286 on and 0386 off The conditions described for the channel 1 mode are then reversed For dual trace operation both CH 1 and CH 2 MODE buttons pushed in a 0 to 5 volts repetitive signal is applied l from the mainframe to the base of Q386 through pin B21 Thus the bias level on Q386 alternates above and below the bias level on Q286 which turns on channel 1 then channe 2 at a rate determined by the repetition rate of the channel switch signal at pin B21 Trigger Amplifier Differential amplifier 0245 and 0255 receives the triggering signal from the emitters of Q242 and 0252 in the isolation stage of the Output Amplifier The triggering signal is amplified and passed through TRIGGER switch 401 to transistor Q404 where it is made available to an associated
16. mplifier outputs From the Preamplifier the signal is coupled directly to the Gain Switching Amplifier Two identical operational ampli fiers and a series paraliel resistor network provide the proper gain for each VOLTS DIV switch setting The BW LIMIT 10 kHz switch places a capacitor across the two output lines to reduce the bandwidth and thus limit the noise The signal is then passed to the Output Amplifier through an emitter follower isolation stage The output differential amplifier is operated push pull presenting a signal to the output terminal that is of the same polarity as that applied to the Preamplifier input The front panel Variable VOLTS DIV CAL and internal gain controls provide a means of varying the gain of the 5A26 A positioning current driver is connected to the output lines to alter the quiescent trace position A triggering signal is tapped from the emitter follower isola tion stage amplified and made available to an associated time base plug in unit Triggering signal amplitude is about 0 25 volts per displayed division Channel switching is accomplished by the plug in MODE buttons and the channel switch signal from the oscilloscope Electronic switching provides a display of either or both channels dual trace The switching rate in dual trace operation is determined by the channel switch signal repeti tion rate from the oscilioscope The VOLTS DIV switch contro is made up of a series of cams and con
17. n factor with 20 V p p sine wave applied to and inputs of either channel TABLE 1 2 Environmental Specifications Characteristics Performance Requirement Temperature Operating 0 C to 50 C Storage 55 C to 70 C Altitude Operating To 15 000 Feet Storage To 50 000 Feet Vibration Operating 15 minutes along each of 3 major axes at a total dis placement of 0 015 inch with frequency variad from 10 Hz to 50 Hz to 10 Hz in 1 minute sweeps Hold for 3 minutes at 50 Hz All major resonances must be above 50 Hz Shock Operating and Storage To 30 g s 1 2 sine 11 ms duration 2 shocks per axis in each direction for a total of 12 shocks TABLE 1 3 _Physical Specifications Characteristics Description Weight Approximately 2 lbs 3 oz 1 kg Dimensions See Fig 1 3 Dimensions 1 8 Operating Instructions 5A26 1947 3 1 2 30 5 cm o OLO cooo 0000 U Fig 1 3 Dimensions 1 9 Errante na Innen O aminami Section 2 5A26 THEORY OF OPERATION This section of the manual describes the operation of the circuits in the 5A26 Dual Differential Amplifier unit Complete schematic diagrams and an overall block diagram of the unit are given on pullout pages at the back of this manual The channel 1 and channel 2 circuitry is essentially identical The following block diagram and circuit operation discussions apply to channel 1 with exceptions included where applicabie BLOC
18. onnector h Release CH 1 GND button Display will be a square wave 4 divisions in amplitude with bottom of display at reference established in step f i Position bottom of square wave to graticule horizontal centerline j Push in CH 1 AC button and note that display shifts downward 2 divisions k Disconnect test lead from CH 1 input and connect to CH 2 input L Push in TRIGGER button for CH 2 triggering and repeat steps f through j for CH 2 operation Step Attenuator Balance Adjustment When the STEP ATTEN BAL contro is not properly adjust ed the display will shift vertically due to a de imbalance in the amplifier as the VOLTS DIV switch is rotated through out its range The shift is more noticeable on the most sensitive positions 1 3 Operating Instructions 5A26 a Set VOLTS DIV switch to 5 Ground both and inputs GND buttons pushed in and position trace to graticule horizontal centerline b Adjust STEP ATTEN BAL contro for minimum trace shift as VOLTS DIV switch is rotated throughout its range Gain Check When the 5A26 is first inserted into a piug in compartment the amplifier gain may be checked and adjusted if necessary See Adjustment Procedure in Section 4 of this manual for complete instructions DETAILED OPERATING INFORMATION Applying Signals gt CAUTION If the maximum input voltage rating at the gates of the input FET s is exceeded the gates are clamped at abou
19. ope instruction manual for information on X Y operation To instali align the upper and lower grooves of the 5A26 with the plug in compartment tracks and fully insert the 5A26 until it locks into place To re move pul the release latch to disengage the 5A26 from the gt oscilloscope CONTROLS CONNECTORS AND INDICATORS The following describes the function of the front panel controls connectors and indicators Refer to Fig 1 2 for the location of the controls connectors and indicators More information is given under Detailed Operating information DISPLAY Push Button Applies and removes logic levels to the oscilloscope system to enable or disable plug in operation Switch is functional oniy when plug in is operated in one of the vertical deflection plug in compartments POS Controls Positions display vertically ORO BW LIMIT Push Buttons Allows reduction of the upper bandwidth limit to increase the signal to noise ratio for low frequency applications Selects upper bandwidth limit at either 1 megahertz button out or 10 kilohertz button in VOLTS DIV Switches Volts per major graticule division Selects calibrated deflection factors from 50 microvolts division to 5 volts division 16 steps in a 1 2 5 sequence Knob skirt is illuminated to indi cate deflection factor and 10X scaling of both knob skirt and crt readout is provided automatically when a 10X coded probe is used CAL Variable VOLTS DIV Control
20. rovides additional open loop gain Total gain of the stage is determined by R58 and R78 between the two amplifiers and resistors R64 and R84 Quiescently the two sides of the amplifier are balanced When a differentia signal is applied to the gates of 054A and O54B signal current is developed through resistors R58 and R78 Conduction of O64A and 064B is changed by the amount of signal current with the output voltage develop ed across R64 and R84 The output is a push pull signal opposite in polarity to the signal applied to the inputs To minimize trace shift as different vertical deflection factors are selected the outputs at the 064A and 064B collectors are dc balanced at equal potentials so that the voltage across the gain setting resistors in the Gain Switching Amplifier is zero at all settings of the VOLTS DIV switch This de balancing is achieved by adjusting R89 Atten Bal Range and R72 STEP ATTEN BAL Common Mode Rejection One of the primary functions of the Preamplifier is to reject any common mode component of the input signal and amplify only the difference Assume that the inputs are tied together and a voltage is applied to the common input The amplifier differential output is ideaily zero and would actually be zero provided that the characteristics of all corresponding elements on the two sides of the amplifier were matched e g O54A and 054B transconductance and u Q60A and Q60B beta current sources etc
21. s Provides un calibrated continuously variable deflection factors between the calibrated settings of the VOLTS DIV switch and extends the deflection factor range to 12 5 volts division STEP ATTEN BAL Adiustments Balances the input amplifier for minimum trace shift throughout the deflection factor gain switching range Input Coupling Push Buttons AC OUT DC Button pushed in selects capacitive coupling of signal applied to associated or input connector button out selects direct coupling of input signal GND Grounds the preamplifier input and connects input signal to ground through a 1 megohm resistor PRE CHG Both AC and GND buttons pushed in permits precharging of the coupling capacitor to the input signal dc level Release GND button for measurement MODE Push Buttons Select channel to be displayed and illuminate the proper deflection factor setting 1X or 10X on the VOLTS DIV switch knob skirt Both buttons pushed in provides a dual trace display fi yee fi Operating Instructions 5A26 1 2 Fig 1 2 Front panel controls connectors and indicators y TRIGGER Push Button Selects time base triggering signal from either channel and Input Connectors BNC connectors for appli cation of external voltage signals Connector labeled indicates that a positive going signal will cause upward deflection connector labeled indicates that a positive going signal will cause downward d
22. sing it becomes increasingly important to use shielded signal cables In all cases the signal transporting leads should be kept as short as practical When making single ended input measurements conventional amplifier operation be sure to establish a common ground connection between the device under test and the 5A26 The shield of a coaxial cable is normally used for this purpose In some cases differential measurements require no common ground connection and therefore are less susceptible to interference by ground loop currents Some problems with stray magnetic coupling into the signal transporting leads can also be minimized by using a differential rather than a single ended measurement These considerations are dis cussed later in this section under Differential Operation It is always important to consider the signal source loading and resulting change in the source operating characteristics due to the signal transporting leads and the input circuit of the 5A26 The circuit at the input connectors can normally be represented by a 1 megohm resistance to ground parallel ed by about 47 picofarads A few feet of shielded cable 20 to 40 picofarads per foot may increase the parallel capacitance to 100 picofarads or more In many cases the effects of these resistive and capacitive loads may be too great and it may be desirable to minimize them through the use of an attenuator probe Attenuator probes not only decrease the resisti
23. t or 10 volts If the signal source can supply more than 1 16 ampere the input protective fusefs will open When measuring unknown voltages use the highest deflection factor first H the deflection is too small to make the measurement switch to a lower deflection factor Pre Charging When only the ac component of a signal having both ac and de components is to be measured the pre charge circuit permits charging the coupling capacitor to the input signal dc level MA a a Ze a TAL CAUTION If the 5A26 input is connected to a large de voltage source 10 volts peak or more use the precharge provision Damage to the signal source could result since the peak current through the ac coupling capacitor and the input protection circuitry is limited only by the signal source internal resistance a Before connecting 5A26 to a signal containing a dc component push in AC and GND buttons Then connect input to circuit under test 1 4 b Wait about 1 second for coupling capacitor to charge c Release GND button and ac component can be measured in usual manner Signal Input Connectors When connecting signals to the and input connectors on the 5A26 consider the method of coupling that will be used Sometimes unshielded test leads can be used to connect the 5A26 to a signal source particularly when a high level low frequency signal is monitored at a low im pedance point However when any of these factors are mis
24. tacts that switch input attenuators select the proper gain setting resistors in the Gain Switching Amplifier and provide the proper ert readout data Either of two lamp bulbs behind the knob skirt on the VOLTS DIV switch illuminate the selected deflection factor One lamp lights for 1X deflection factor the other lights when a readout coded probe is attached to the or input connectors to indicate a 10X deflection factor 2 1 Theory of Operation 5A26 CIRCUIT OPERATION Circuits unique to this instrument are described in detail in this discussion Circuits commonly used in the electronics industry are not described in detail If more information is desired on these commonly used circuits refer to the following textbooks Phillip Cutler Semiconductor Circuit Analysis McGraw Hill New York 1964 Jacob Millman and Herbert Taub Pulse Digital and Switching Waveforms McGraw Hill New York 1965 Albert Paul Malvino Transistor Circuit Approximations McGraw Hill New York 1973 Gordon V Deboo Integrated Circuits and Semiconductor Devices McGraw Hill New York 1971 Lloyd P Hunter Ed Handbook of Semiconductor Electronics third edition McGraw Hill New York 1970 The numbers inside the diamonds after a heading refer to the schematic diagram for that circuit located at the back of this manual PREAMPLIFIER Plug In Logic When DISPLAY button is pressed a logic level is applied to
25. the electronic switching circuit in the oscilloscope to enable plug in operation Power is applied to illuminate the VOLTS DIV knob skirt behind the proper deflection factor when the MODE button is pressed in input Coupling Signals applied to the front panel and input connectors may be capacitive coupled fac direct coupled dc or internally grounded through a capacitor and resistor Input coupling is selected by means of two push button switches at each input S2C and D for the input and SZA and B for the input A signal applied to the input with both buttons in the out position dc coupied is passed directly to the Input Attenu ator When the AC button is pressed in C12 is placed in the circuit to ac couple signals of about 2 hertz 3 dB point and higher to the attenuator Capacitor C12 blocks any dc component of the signal When the GND button is pressed in a ground reference is provided to the input of the amplifier without the need to remove the applied signal from the input connector NOTE When ac coupled with de levels of 10 volts or more both the AC and GND buttons should be pressed in PRE CHG while input connec tions are made or broken or when voltage levels are changed This allows the coupling capacitor to charge without opening the input fuses or overdriving the amplifier 2 2 Input Attenuators The Input Attenuators consist of frequency compensated voltage dividers that provide 100X attenua
26. their associated emitter resistors deter mines the crt readout mn ent i Nina ZE
27. tion factor settings are calibrated only when the Variable control is rotated fully clockwise into the detent position The range of the Variable control is at least 2 5 1 it provides uncalibrated deflection factors covering the full range between the calibrated settings of the VOLTS DIV switch The Variable control can be set to extend the deflection factor to at least 12 5 volts division To reduce noise and obtain a more usable display when the VOLTS DIV switch is set to the more sensitive positions push in the BW LIMIT button to reduce the bandwidth to 10 kHz if this limit does not appreciably distort the desired features of the signal under observation Voltage Comparison Measurements Some applications require deflection factors other than the fixed values provided by the VOLTS DIV switch One such application is comparison of signal amplitudes by ratio rather than by absolute voltage To accomplish this apply a reference signal to either or inputs of the 5A26 and set the VOLTS DIV switch and Variable control so that the reference display covers the desired number of graticule divisions Do not change the Variable control settings throughout the subsequent comparisons However the settings of the VOLTS DIV switch can be changed to accommodate large ratios In doing so regard the numbers which designate the switch positions as ratio factors rather than voltages Differential Operation Single ended measurements often yiel
28. tion in the 1 to 5 positions of the VOLTS DIV switch For dc and low frequency signals the dividers are essentially resistive attenuation ratio determined by the resistance ratio Balance between the attenuators of the two inputs for low frequency common mode signals is set by adjusting R9 LF CMR At higher frequencies the capacitive reactance becomes effective and the impedance ratio determines the attenuation in addition to providing constant 100X attenuation at ail frequencies within the bandwidth capabilities of the instru ment the Input Attenuator maintains a constant input RC characteristic 1 megohm paralleled by about 47 picofarads for the 1 to 5 positions of the VOLTS DIV switch input Protection Input protection consists of fuses F20 and F30 and diodes CR21 CR24 CR31 and CR34 If the signal should reach a level sufficient to forward bias one of the protection diodes a potential greater than about 12 5 volts current will be conducted through that diode protecting the input FET s if that current should exceed the fuse rating the fuse s will open Preamplifier The Preamplifier consists of two identical operational amplifiers connected in a differential configuration The operational amplifiers consist of Q54A O60A and Q64A on the input side and 054B 0608 and 064B on the input side Transistors Q54A and 0548 provide a voltage follower input to output transistors Q64A and Q64B 77 while Q60A p
29. ve capacitive loading of a signal source but also extend the measurement range of the 5A26 to include substantially higher voltages Passive attenuator probes having attenuation factors of 10X 100X and 1000X as well as other special purpose types are available through your Tektronix Field Engineer or Field Office Some measurement situations require an extremely high resistance input to the BA26 with very little source loading _ or signal attenuation In such situations a passive attenuator i probe cannot be used However this problem may be solved by using a FET Probe or the high impedance input provision of the 5A26 as described under High Impedance Input High Impedance Input In the 50 m through 50 u positions of the VOLTS DIV switch where the input attenuator is not used the internal gate return resistors alone establish the 1 megohm input resistance The removal of the strap from the Preamplifier eircuit board see Fig 4 4 Channel 1 and 2 Preamplifier circuit board component and test point locations in the Diagrams and Parts Lists section disconnects these resis tors from ground and permits the input FET gates to float providing a very high input impedance The input signal must be kept to relatively low amplitudes since the deflection factor is restricted to 50 millivolts division through 50 microvolts division and de coupling must be used to provide a de path for the FET gate current NOTE In the 0 1 volt to 5
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