Tektronix 7A26 User manual

Tektronix, Inc.
P.0. Box 500
Beaverton, Oregon 97005
TEKTRONIX®
7A26
DUAL TRACE
AMPLIFIER
ZOO /VI Ha.
INSTRUCTION MANUAL
Serial Number
070-1484-00 1072

') r’:: ft,
WARRANTY
All TEKTRONIX instruments are warranted against
defective materials and workmanship for one year. Any
questions with respect to the warranty should be taken up
with your TEKTRONIX Field Engineer or representative.
All requests for repairs and replacement parts should be
directed to the TEKTRONIX Field Office or representative
in your area. This will assure you the fastest possible
service. Please include the instrument Type Number or Part
Number and Serial Number with all requests for parts or
service.
Specifications and price change privileges reserved
Copyright ©1972 by Tektronix, Inc., Beaverton, Oregon.
Printed in the United States of America. All rights reserved.
Contents of this publication may not be reproduced in any
form without permission of Tektronix, Inc.
U.S.A. and foreign TEKTRONIX products covered by U.S
and foreign patents and/or patents pending.
TEKTRONIX is aregistered trademark of Tektronix, Inc

TABLE OF CONTENTS
Page
SECTION 1OPERATING INSTRUCTIONS
7A26 Features 1_1
Installation 1_1
General Operating Information 1-1
7A26 Front-panel Controls and Connectors 1-2
Basic Applications 1_4
SECTION 2SPECIFICATION
Introduction 2-1
Electrical Characteristics 2-1
Environmental Characteristics 2-2
Physical Characteristics 2-2
SECTION 3THEORY OF OPERATION
Introduction 3_1
Block Diagram 3_1
Detailed Circuit Description 3_1
SECTION 4MAINTENANCE
Introduction 4_1
Preventive Maintenance 4_1
Troubleshooting 4_1
Corrective Maintenance 4.3
SECTION 5CALIBRATION
Introduction 5_1
Performance Check 5-1
Test Equipment Required 5_1
Calibration Procedure 5.3
Index To Calibration Procedure 5.3
SECTION 6ELECTRICAL PARTS LIST
Parts Ordering Information
Abbreviations and Symbols
SECTION 7DIAGRAMS AND CIRCUIT BOARD ILLUSTRATIONS
Symbols and Reference Designators
SECTION 8MECHANICAL PARTS LIST
Mechanical Parts List Information
Index of Mechanical Parts Illustrations
Mechanical Parts List

linn ®push
VARIABLE (CAL INI
VOLTS/PIV
DISPLAY
MODE
CHI
TRIGGER
SOURCE
CH 2
'POLARITY
+UP FULL
20MHz
INVERT
POSITION rnn tj push
VARIABLE (CAL IN
VOLTS/PIV
TEKTRONIX® DUAL TRACE AMPLIFIER
Fig. 1-1. 7A26 Dual Trace Amplifier.

Section 1—7A26
OPERATING INSTRUCTIONS
7A26 Features
The 7A26 Dual Trace Amplifier plug-in unit is designed
for use with TEKTRONIX 7000-Series Oscilloscopes. The
7A26 is adual-channel wide-bandwidth amplifier. Internal
gain and compensation circuits are automatically switched
to correspond to the setting of the VOLTS/DIV switch.
Channel 2can be inverted for differential measurements.
PRELIMINARY INFORMATION
Installation
The 7A26 is calibrated and ready for use as received. It
can be installed in any compartment of TEKTRONIX
7000-series oscilloscopes, but is intended for principal use
in vertical plug-in compartments. To install, align the upper
and lower rails of the 7A26 with the oscilloscope tracks and
fully insert it. The front will be flush with the front of the
oscilloscope when the 7A26 is fully inserted, and the latch
at the bottom-left corner of the 7A26 will be in place
against the front panel. See Fig. 1-2.
To remove the 7A26, pull on the latch (which is
inscribed with the unit identification "7A26") and the
7A26 will unlatch. Continue pulling on the latch to slide
the 7A26 out of the oscilloscope.
GENERAL OPERATING INFORMATION
Introduction
For single-trace operation, either of the two identical
amplifier channels can be used independently by setting the
DISPLAY MODE and TRIGGER SOURCE switches to CH
1or CH 2and connecting the signal to be observed to the
appropriate input. In the discussions to follow, single-trace
operations using CH 1only apply equally to CH 2only.
Signal Connections
In general, probes offer the most convenient means of
connecting asignal to the input of the 7A26. A10X
attenuation probe offers ahigh input impedance and allows
the circuit under test to perform very close to normal
operating conditions.
Any TEKTRON IX probe, with areadout coding ring can
be used with TEKTRONIX 7A-series amplifier units
equipped with readout. The readout coding ring on the
probe connects to acircuit in the amplifier unit which
automatically corrects the readout displayed on the CRT to
the actual deflection factor at the tip of the probe being
used. For probes to be used with amplifier units with or
without readout, see the Tektronix, Inc. catalog.
Vertical Gain Check and Adjustment
To check the gain of either channel, set the VOLTS/DIV
switch to 10 mV and connect 40 millivolts, 1kilohertz
signal from the oscilloscope calibrator to the input connec-
tor of the channel being checked. The vertical deflection
should be exactly four divisions. If not, adjust the
front-panel GAIN for exactly four divisions of deflection.
The GAIN adjustment is engaged by pressing in the GAIN
control knob and turning the knob with anarrow-blade
screwdriver (see Fig. 1-3 Front Panel Controls and
Connectors). Turn the knob clockwise, then counter-
clockwise, until the GAIN control is engaged. When the
GAIN control is engaged, the vertical deflection will change
as the knob is turned. Turn the GAIN control knob with
the screwdriver until the deflection is set to exactly four
divisions, then remove the screwdriver.
Input Coupling
The Channel 1and Channel 2coupling (AC-GND-DC)
switches allow achoice of input coupling methods. The
type of display desired and the applied signal will determine
the coupling to use.
The DC coupling position must be used to display the
DC component of the signal. It must also be used to display
AC signals below about 30 hertz (ten hertz with a10X
probe) and square waves with low-frequency components as
these signals are attenuated in the AC position.
Fig. 1-2. Release latch.
RELEASE
LATCH
1-1

Operating Instructions—7A26
CH 1POSITION
Controls position of the CH 1
trace and "ADD” display mode
trace.
CH 1Input Connector
Provides signal connection to CH 1
CH 2POLARITY
Provides means of inverting the
CH 2display.
BANDWIDTH Switch
Provides ameans of limiting the
upper bandwidth.
CH 2POSITION
Controls position of the CH 2
trace only (disabled in ADD).
IDENTIFY (Same as CH 1)
Deflects trace about 0.2 division
for trace identification. In instru-
ments with Readout, also replaces
readout with the word
"IDENTIFY".
TRIGGER SOURCE
Selects source of the trigger
signal.
DISPLAY MODE
Selects the mode of operation.
VARIABLE VOLTS/DIV (same
asCH 1)
Provides continuously variable
uncalibrated settings between
calibrated steps.
GAIN Adjustment
When the VARIABLE control is
pushed in, it becomes afront
panel screwdriver adjustment for
calibration of deflection factor.
VOLTS/DIV (same as Ch 1)
Selects calibrated deflection
factors from 5mV/Div to 5
V/Div; ten steps in a1-2-5
sequence.
CH 2Input Connector AC-GND-DC (same as CH 1)
Provides signal connection to Ch 2. Selects signal input coupling
mode.
Fig. 1-3. 7A26 front-panel controls and connectors.

Operating Instructions— 7A26
In the AC coupling position, the DC component of the
signal is blocked by acapacitor in the input circuit. The AC
coupling position provides the best display of signals with a
DC component much larger than the AC components. The
precharge feature should be used with large DC inputs. To
use this feature, first set the coupling to GND. Connect the
probe to the circuit and wait about two seconds for the
coupling capacitor to charge. Then set the coupling to AC.
The GND position provides aground reference at the
input of the amplifier without externally grounding the
input connectors. However, the signals connected to the
inputs are not grounded, and the same DC load is presented
to the signal source.
VOLTS/DIV and VARIABLE Controls
The amount of vertical deflection produced by asignal is
determined by the signal amplitude, the attenuation factor
of the probe, the setting of the VOLTS/DIV switch, and
the setting of the VARIABLE control. Calibration deflec-
tion factors indicated by the settings of the VOLTS/DIV
switch apply only when the VARIABLE control is in the
calibrated (CAL IN) position.
The VARIABLE control provides variable, uncalibrated
settings between the calibrated steps of the VOLTS/DIV
switch. With the VARIABLE control fully counter-
clockwise and the VOLTS/DIV set to 5volts/division the
uncalibrated vertical deflection factor is extended to at
least 12.5 volts/division. By applying acalibrated voltage
source to the input connector, any specific deflection
factor can be set within the range of the VARIABLE
control.
CH 2POLARITY Switch
The CH 2POLARITY switch may be used to invert the
displayed waveform of the signal applied to the CH 2input.
This is particularly useful in added operation of the 7A26
when differential measurements are to be made. The CH 2
POLARITY switch has two positions, +UP and INVERT. In
the +UP position, the displayed waveform will have the
same polarity as the applied signal and apositive DC voltage
will move the CRT trace up. In the INVERT position, a
positive-going waveform at the CH 2input will be displayed
on the CRT in inverted form and apositive DC voltage will
move the trace down.
DISPLAY MODE Switch
For single-trace operation, apply the signal either to the
CH 1input or the CH 2 input and set the DISPLAY MODE
switch to the corresponding position: CH 1or CH 2.
To display asignal in one channel independently when a
signal is also applied to the other channel, simply select the
desired channel by setting the DISPLAY MODE switch to
the appropriate CH 1or CH 2position.
Alternate Mode. The ALT position of the DISPLAY
MODE switch produces adisplay which alternates between
channel 1and channel 2with each sweep on the CRT.
Although the ALT mode can be used at all sweep rates, the
CHOP mode provides amore satisfactory display at sweep
rates below about 0.5 millisecond/division. At slow sweep
rates alternate mode switching becomes visually percep-
tible.
Add Mode. The ADD position of the DISPLAY MODE
switch can be used to display the sum or difference of two
signals, for common-mode rejection to remove an undesired
signal. The overall deflection factor in the ADD mode with
both VOLTS/DIV switches set to the same position is the
deflection factor indicated by either VOLTS/DIV switch.
However, if the CH 1and CH 2VOLTS/DIV switches are
set to different deflection factors, the resultant amplitude
is difficult to determine from the CRT display. In this case,
the voltage amplitude of the resultant display can be
determined accurately only if the amplitude of the signal
applied to one channel is known. In the ADD mode,
positioning of the trace is controlled by the channel 1
POSITION control only.
Chop Mode. The CHOP position of the DISPLAY
MODE switch produces adisplay which is electronically
switched between channels at approximately a500 kilo-
hertz rate (controlled by mainframe). In general the CHOP
mode provides the best display at sweep rates slower than
about 0.5 millisecond/division or whenever dual-trace,
non-repetitive phenomena is to be displayed.
TRIGGER SOURCE Switch
CH 1. The CH 1position of the TRIGGER SOURCE
switch provides atrigger signal obtained from the signal
applied to the CH 1input connector. This provides astable
display of the signal applied to the CH 1input connector.
CH 2. The CH 2position of the TRIGGER SOURCE
switch provides atrigger signal obtained from the signal
applied to the CH 2input connector. This provides astable
display of the signal applied to the CH 2input connector.
MODE. In this position of the TRIGGER SOURCE
switch, the trigger signal for the time-base unit is dependent
on the setting of the DISPLAY MODE switch. The trigger
REV. B, MAR. 1975 1-3

Operating Instructions—7A26
source for each position of the DISPLAY MODE switch is
as follows:
MODE TRIGGER SIGNAL SOURCE
CH 1Channel 1
CH 2Channel 2
ADD Algebraic sum of channel 1and channel 2
CHOP Algebraic sum of channel 1and channel 2
ALT Alternates between channel 1and channel 2
Trace Identification
When the IDENTIFY button is pressed, the trace is
deflected about 0,2 division to identify the 7A26 trace.
This feature is particularly useful when multiple traces are
displayed. When the IDENTIFY button is pressed on instru-
ments with readout, the deflection factor readout is
replaced with the word "IDENTIFY".
BW Switch
Provides ameans of limiting the upper bandwidth.
FULL: Allows the 7A26 to operate at full bandwidth. 20
MHz: Reduces the upper bandwidth of the 7A26 to about
20 megahertz.
BASIC APPLICATIONS
General
The following information describes the procedures and
techniques for making basic measurements with a7A26 and
the associated TEKTRONIX oscilloscope and time-base.
These applications are not described in detail since each
application must be adapted to the requirements of the
individual measurements. This instrument can also be used
for many applications not described in this manual. Contact
your local TEKTRONIX Field Office or representative for
assistance in making specific measurements with this
instrument.
Peak-to-Peak Voltage Measurements (AC)
To make peak-to-peak voltage measurements, use the
following procedure:
1.Apply the signal to either input connector.
2. Set the DISPLAY MODE and TRIGGER SOURCE
switches to display the channel used.
3. Set the coupling switch to AC.
NOTE
For low-frequency signals below about 30 hertz use
the DC position to prevent attenuation of the signal.
4.
Set the VOLTS/DIV switch to display about five
divisions of the waveform vertically.
5.
Set the time-base Triggering controls for astable
display. Set the time-base unit to asweep rate which
displays several cycles of the waveform.
6. Turn the 7A26 POSITION control so the lower
portion of the waveform coincides with one of the graticule
lines below the center horizontal line, and the top of the
waveform is within the viewing area. With the time-base
Position control, move the display so one of the upper
peaks lies near the center vertical line (see Fig. 1-4).
7. Measure the divisions of vertical deflection peak-to-
peak. Check that the VARIABLE (VOLTS/DIV) control is
in the CAL IN position.
NOTE
This technique can also be used to make measure-
ments between two points on the waveform, rather
than peak to peak.
8.
Multiply the deflection measured in step 7by the
VOLTS/DIV switch setting. Include the attenuation factor
of the probe if used.
Fig. 1-4. Measuring the peak-to-peak voltage of awaveform.
1-4

Operating Instructions—7A26
EXAMPLE: Assume that the peak to peak vertical
deflection is 4.5 divisions (see Fig. 1-4) using a10X
attenuator probe, and the VOLTS/D IV switch is set to 1V.
Volts
Peak to Peak
vertical
deflection X
(divisions)
VOLTS/D IV
setting
probe
Xattenuation
factor
7. Measure the distance in divisions between the refer-
ence line and the point on the waveform at which the DC
level is to be measured. For example, in Fig. 1-5 the
measurement is between the reference line and point A.
8. Establish the polarity of the waveform. With the CH
2POLARITY switch in the +UP position, any point above
the reference line is positive.
Substituting the given values:
Volts Peak-to-Peak =4.5 X1X10
9. Multiply the distance measured in step 7 by the
VOLTS/DIV setting. Include the attenuation factor of the
probe, if used.
The peak-to-peak voltage is 45 volts.
Instantaneous Voltage Measurements (DC)
To measure the DC level at agiven point on awaveform,
proceed as follows:
EXAMPLE: Assume the vertical distance measured is
3.6 divisions (see Fig. 1-5) and the waveform is above the
reference line using a10X probe with aVOLTS/DIV setting
of 0.5 V.
Using the formula:
1. Connect the signal to either input connector.
2. Set the DISPLAY MODE and TRIGGER SOURCE
switches to display the channel used.
3. Set the VOLTS/DIV switch to display about five
divisions of the waveform.
Instan- vertical VOLTS/ probe
taneous =distance Xpolarity XDIV Xattenuation
Voltage (divisions) setting factor
Substituting the given values:
Instantaneous 3.6 X+1 X0.5 VX10
Voltage
4.
Set the coupling switch to GND and position the
trace to the bottom graticule line or other reference line. If
the voltage is negative with respect to ground, position the
trace to the top graticule line. Do not move the POSITION
control after this reference line has been established.
NOTE
To measure avoltage level with respect to avoltage
other than ground, make the following changes to
step 4. Set the coupling switch to DC and apply the
reference voltage to the input connector. Then
position the trace to the reference line.
5.
Set the coupling switch to DC. The ground reference
line can be checked at any time by switching to the GND
position.
6.
Set the time-base Triggering controls for astable
display. Set the time-base sweep rate for an optimum
display of the waveform.
The instantaneous voltage is 18 volts.
1
int A
—
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—
1
Verr
tical
anee
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Dis
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Fig. 1-5. Measuring instantaneous voltage with respect to some
reference.
1-5

Operating Instructions—7A26
Comparison Measurements
In some applications it may be desirable to establish
arbitrary units of measurement other than those indicated
by the VOLTS/DIV switch. This is particularly useful when
comparing unknown signals to areference amplitude. One
use for the comparison-measurement technique is to facili-
tate calibration of equipment where the desired amplitude
does not produce an exact number of divisions of deflec-
tion. The adjustment will be easier and more accurate if
arbitrary units of measurement are established so that the
correct adjustment is indicated by an exact number of
divisions of deflection. The following procedure describes
how to establish arbitrary units of measure for comparison
measurements.
6. Measure the vertical deflection in divisions and
calculate the amplitude of the unknown signal using the
following formula.
Signal =VOLTS/DIV
Amplitude setting
vertical vertical
Xconversion Xdeflection
factor (divisions)
EXAMPLE: Assume areference signal amplitude of 30
volts, aVOLTS/DIV setting of 5Vand the VARIABLE
control adjusted to provide avertical deflection of four
divisions. Substituting these values in the vertical conver-
sion factor formula (step 4):
To establish an arbitrary vertical deflection factor based
upon aspecific reference amplitude, proceed as follows:
1. Connect the reference signal to the input connector.
Set the time-base unit sweep rate to display several cycles
of the signal.
2. Set the VOLTS/DIV switch and the VARIABLE
control to produce adisplay which is an exact number of
vertical divisions in amplitude. Do not change the
VARIABLE control after obtaining the desired deflection.
Vertical
Conversion
Factor
30 V
4X5V1.5
Then with aVOLTS/DIV setting of 2V, the peak-to-
peak amplitude of an unknown signal which produces a
vertical deflection of five divisions can be determined by
using the signal amplitude formula (step 6):
Signal
Amplitude 2VX1.5 X5=15 volts
3.
To establish an arbitrary vertical deflection factor so
the amplitude of an unknown signal can be measured
accurately at any setting of the VOLTS/DIV switch, the
amplitude of the reference signal must be known. If it is
not known, it can be measured before the VARIABLE
VOLTS/DIV control is set in step 2.
4.
Divide the amplitude of the reference signal (volts)
by the product of the vertical deflection (divisions)
established in step 2and the setting of the VOLTS/DIV
switch. This is the vertical conversion factor.
reference signal
Vertical amplitude (volts)
Conversion =vertical VOLTS/DIV
Factor deflection Xswitch
(divisions) setting
5.
To measure the amplitude of an unknown signal,
disconnect the reference signal and connect the unknown
signal to the input connector. Set the VOLTS/DIV switch
to asetting that provides sufficient vertical deflection to
make an accurate measurement. Do not readjust the
VARIABLE control.
Dual-Trace Phase Difference Measurements
Phase comparison between two signals of the same
frequency can be made using the dual-trace feature of the
7A26. This method of phase difference measurement can
be used up to the frequency limit of the oscilloscope
system. To make the comparison, use the following
procedure:
1. Set the CH 1and CH 2coupling switches to the same
position, depending on the type of coupling desired.
2. Set the DISPLAY MODE to ALT or CHOP. In
general, CHOP is more suitable for low frequencies and
ALT is more suitable for high frequencies. Set the
TRIGGER SOURCE to CH 1.
3.
Connect the reference signal to the CH 1input and
the comparison signal to the CH 2input. Use coaxial cables
or probes which have similar time delay characteristics to
connect the signals to the input connectors.
4.
If the signals are of opposite polarity, set the CH 2
POLARITY switch to invert the channel 2display. (Signals
may be of opposite polarity due to 180° phase difference;
if so, take this into account in the final calculation.)
®
1-6