Tektronix 7A13 User manual

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a
Tektronix, Inc.
P.O. Box 500
. Beaverton, Oregon
070-1948-00
Product Group 42
Tektronix
COMMITTED TO EXCELLENCE
PLEASE CHECK FOR CHANGE INFORMATION
AT THE REAR OF THIS MANUAL... —
7A13
DIFFERENTIAL
COMPARATOR
SN B200000-up
INSTRUCTION: MANUAL |
97077 Serial Number
First Printing APR 1975
Revised JUN 1986

Copyright © 1975 Tektronix, Inc. All rights reserved.
Contents of this publication may not be reproduced in any
form without the written permission of Tektronix, Inc.
Products of Tektronix, Inc. and its subsidiaries are covered
by U.S. and foreign patents and/or pending patents.
TEKTRONIX, TEK, SCOPE-MOBILE, and are
registered trademarks of Tektronix, Inc. TELEQUIPMENT
is a registered trademark of Tektronix U.K. Limited.
Printed in U.S.A. Specification and price change privileges
are reserved.
INSTRUMENT SERIAL NUMBERS
Each instrument has a serial number on a parel insert, tag,
or stamped on the chassis. The first number or letter
designates the country of manufacture. The last five digits
of the serial number are assigned sequentially and are
unique to each instrument. Those manufactured in the
United States have six unique digits. The country of
manufacture is identified as follows:
Bo00000 Tektronix, Inc., Beaverton, Oregon, USA
100000 Tektronix Guernsey, Ltd., Channel Islands
200000 Tektronix United Kingdom, Ltd., London
300000 Sony/Tektronix, Japan
700000 Tektronix Holland, NV, Heerenveen,
The Netherlands
—

Section 1
Section 2
7A13 (SN B200000-up)
TABLE OF CONTENTS
Page
SPECIFICATION
Introduction ...... 0.0... ce eee eee 1-1
Electrical Characteristics ............ 1-1
System Characteristics ............. 1-4
Environmental Characteristics ........ 1-4
OPERATING INSTRUCTIONS
Introduction ......... 0.0.0.0 0. e eee 2-1
Front Panel Description ............. 2-1
Test Setup Chart .................. 2-2
Familiarization Procedure ........... 2-2
General Operating Information ....... 2-5
THE FOLLOWING SERVICING INSTRUCTIONS
ARE FOR USE BY QUALIFIED PERSONNEL ONLY.
TO AVOID PERSONAL INJURY, DO NOT PERFORM
ANY SERVICING OTHER THAN THAT CONTAINED
IN OPERATING INSTRUCTIONS UNLESS YOU ARE
QUALIFIED TO DO SO.
Section 3
CIRCUIT DESCRIPTION
INtrOGUCTION .52.6 6c a:sty es ea eee ese 3-1
Block Diagram Description .......... 3-1
Detailed Circuit Description .......... 3-3
REV MAY 1982
Section 4
Section 5
Section 6
Section 7
Section 8
Page
MAINTENANCE
Preventive Maintenance............. 4-1
Troubleshooting ................... 4-2
Corrective
Maintenance ............. 4-8
PERFORMANCE CHECK/
CALIBRATION
INtFOGUCTION «0s es eee sc nimi memsmen 5-1
Test Equipment Required............ 5-1
Short-Form Performance
Check/Calibration Procedure and Index 5-5
Performance Check/Calibration
Procedure ........... 0.00. eee eee 5-8
ELECTRICAL PARTS LIST
Abbreviations and Symbols
Parts Ordering Information
DIAGRAMS AND CIRCUIT BOARD
ILLUSTRATIONS
Symbols and Reference Designators
Voltage and Waveform Conditions
MECHANICAL PARTS LIST
Mechanical Parts List Information
Index of Mechanical Parts Illustrations
Mechanical Parts List
Accessories
CHANGE INFORMATION

Rin = &
COMPARISON
ty
@
VOLTAGE
(Vc)
VAR IN ONLY COARSE -®- FINE
ac Lag VOLTS/DIV
UNCAL
MO BAL y, mer -10KWT
@ Position
BW — |
: FULL ts Ve cos Migs
DIFFERENTIAL COMPAR
ATOR}
Fig. 1-1. 7A13 Differential Comparator.
7A13 (SN B200000-up)
1948-1
o

Section 1—7A13 (SN B200000-up)
SPECIFICATION
Introduction
The 7A13 Vertical Plug-In is a DC coupled differential
comparator with excellent common-mode rejection and
medium gain characteristics for medium level
applications. The 7A13 is designed for use in Tektronix
7000 series oscilloscopes. It may be used as a differential
input preamplifier or conventional preamplifier in addition
to its use as a comparator.
In the differential input mode, the dynamic range allows
the application of common-mode signals up to +10 or
—10 volts to be applied to the unit without attenuation.
Common-mode rejection ratio of at least 20,000:1 at DC to
100 kHz permits measurements of differential signals less
than 1 mVin amplitude on 10 volt common-mode signals.
When used as a differential comparator, the 7A13 has
an
effective offset range of 10,000 divisions.
Electrical Characteristics
The electrical characteristics described in Table 1-1 are
valid over the stated environmental range of instruments
calibrated at an ambient temperature of +20°C to +30°C
and after a 20-minute warmup period unless otherwise
noted.
TABLE 1-1
ELECTRICAL CHARACTERISTICS
TABLE 1-1 (cont)
Characteristic Performance Requirement
Common Mode
Signal Range
1 mV/Div to
50 mV/Div;
At least +10 V and —10 V
X10 Ve In
10 mV/Div to
50 mV/Div;
X10 Ve Out
At least +100 V and —100 V
0.1 V/Div to
0.5
V/Div;
X10 Ve In
0.1 V/Div to
0.5
V/Div;
X10 Vc Out
At least +500 V and —500 V
1 V/Div to 5 V/Div At least +500 V and —500 V
X10 Ve In
Characteristic Performance Requirement
Deflection Factor
(VOLTS/DIV)
Calibrated Range 1 mV/Divto5 V/Div, 12stepsina1,
2, 5 sequence
Frequency Response
(8 Div Reference)
FULL Bandwidth
Upper Limit
See Table 1-4, System Charac-
teristics
AC (Capacitive) 10 Hz or less
Coupled Input
Lower Bandwidth
Frequency
5 MHz Bandwidth |DC to 5 MHz within 500 kHz
Gain Ratio Within 1.5% of GAIN adjusted at 1
Accuracy mV/Div
Uncalibrated Continuously variable: extends
(Variable) deflection factor to at least
12.5 V/Div
Overdrive Recovery Recovers to within 2 mV in 1 us
(1X Attenuator at (and 1 mV in 0.1 ms) after a pulse
1 mV/Div) of +10V or -—10V, or less,
regardless of pulse duration.
1-1

Specification—7A13 (SN B200000-up)
TABLE 1-1 (cont)
TABLE 1-1 (cont)
Characteristic
Common Mode
Rejection Ratio
1 mV/Div to
50 mV/Div
X10 Ve In
DC to 100 kHz
Performance Requirement
See Fig. 1-2.
At least 20,000:1, 20 V P-P or less
test signal
Characteristic
Input R and C
Resistance
Performance Requirement
1 MQ +0.15%
Capacitance
R and C Product
Approximately 20.0 pF
Within +1% between all deflection
factors.
100 kHz to At least 10,000:1, 10 V P-P or less
1 MHz test signal
1 MHz to Decreases to 500:1 at 10 MHz with
20 MHz 1V P-P, then 200:1 at 20 MHz
at 1 V P-P. See Fig. 1-2.
10 mV/Div to
50 mV/Div; X10
Ve Out;
Maximum Gate
Current
0°C to +35°C
Both Inputs
0.2nA or less
mV/Div)
(0.2 Div at 1
+35°C to +50°C
Both Inputs
DC Drift
Drift
With Time
2 nA or less (2 Div at 1 mV/Div)
0.1 V/Div to 5
V/Div: (Ambient Temp-
X10 Ve In or Out erature and Line
Voltage Con-
DC to 10 kHz At least 2,000:1 stant)
Short Term 1 mV P-P or less or 0.1 Div or less
XG GOUpISd it me teas} EUW (whichever is greater) any 1 minute
60 Hz interval within 1 hour after 20
minutes from turn-on.
Maximum Input Long Term 1 mV P-P or less or 0.1 Div
Voltage
DC (Direct)
Coupled
DC + Peak AC
1 mV/Div to
50 mV/Div;
X10 Vc In
40 VDC, 40 V Peak AC, 1 kHz or
less
10 mV/Div to
50 mV/Div;
X10 Vc Out
400 VDC, 400 V Peak AC, 1 kHz
or less
0.1 V/Div to
0.5
V/Div;
X10 Vc In
(whichever is greater) during any
hour after the first hour and 20
minutes from turn-on.
Drift With Ambient
Temperature
Line Voltage Con-
stant)
2 mV/10°C or less, 0.2 Div/10°C
or less (whichever is greater)
Amplifier Crosstalk
1% or less shift within 20 ns of step
of fast rise squarewave when
switching undriven input from
GND to AC or DC
0.1 V/Div to
0.5 V/Div;
X10 Vc Out
500 VDC. 500 V Peak AC, 1 kHz
or less
Displayed Noise
400 uV or less at 1 mV/Div in
1 V/Div to 5 V/
Div;
X10 Vc In
AC (Capacitive
Coupled Input
500 VDC
Tangentially Meas- |Type 7700-Series indicator os-
ured) cilloscope
Comparison Voltage
Range OV to +10 V
Accuracy
+ (0.1% of setting +3 mV)
Electrical Zero 0.5 mV or less
Vc OUT Resistance 2 kQ to 5.5 kQ
1-2
®

Specification—7A13 (SN B200000-up)
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ZH
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ZH
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Fig.
1-2.
Common
mode
rejection
ratio
graph.
It
pertains
to
1
mV/Div
through
20
mV/Div
deflection
factors.
1-3

Specification—7A13 (SN B200000-up)
TABLE 1-2
7A13 tested alone (separate from
oscilloscope mainframe)
ENVIRONMENTAL CHARACTERISTICS
Characteristic Performance Requirements
TABLE 1-4
SYSTEM CHARACTERISTICS
(Indicator Oscilloscope: 7500-Series with P6053B Probe.)
‘Accuracy (%) Sig Out
BW T. 7EXT *INT “INT BW T,
Temperature (MHz) (ns) CAL CAL CAL (MHz) (ns)
Operating re hen" 7 |4a7]15 25 35 55 64
Storape 55°C to +75°C 75 47/ 15 25 35 55 64
Altitude (Indicator Oscilloscope: 7700-Series with P6053B Probe.)
Operating 19,000 'Teet 100 [35] 15 25 35 55 64
Storage 50,000 feet too 35 15 25 35 55 64
Qualified under National Safe
Transit Committee test procedure
1A, Category Il.
Transportation
TABLE 1-3
PHYSICAL CHARACTERISTICS
Size Fits all 7000-Series plug-in com-
partments.
Weight 3.1 Pounds (1.4 kilograms).
System Characteristics
The system characteristics listed in Table 1-4 specify
the performance of the plug-in with various combinations
of probes and in various indicator oscilloscopes.
1-4
‘Accuracy percentages apply to all deflection factors. Plug-in
GAIN must be set at the deflection factor designated at the
applicable position of the VOLTS/DIV switch. When a probe is
used, the GAIN must be set with the calibration signal applied to
the probe tip.
*EXTernal CALibrator, 0°C to +50°C: The plug-in GAIN is set
(within 10°C of the operating temperature) using an external
calibrator signal whose accuracy is within 0.25%.
3INTernal CALibrator, +15°C to +35°C: The plug-in GAIN is set
using the oscilloscope’s own calibrator and the instrument is
operating within the +15°C to +35°C range.
“‘INTernal CALibrator, 0°C to +50°C: The plug-in GAIN is set
(within 10°C of the operating temperature) using the os-
cilloscope’s own calibrator, and the instrument is operating
within the 0°C to +50°C range.

INTRODUCTION
This section opens with a brief functional description of
the front panel controls and connectors (see Fig. 2-1).
Following the front-panel description is a familiarization
procedure and finally a general discussion of the opera-
tion of the 7A13.
FRONT-PANEL DESCRIPTION
Rin = oo
1-50 mV
VAR IN ONLY
Lamp
IIluminates when switch S10,
located on left side of plug-in, is
turned cw. This indicates a +
INPUT and — INPUT impedance of
approximately infinity whenever
the VOLTS/DIV switch is set
between 1 and 50 mV, VARIABLE
knob is pushed in and 1X probe is
used.
Section 2—7A13 (SN B200000-up)
OPERATING INSTRUCTIONS
NOTE
With VOLTS/DIV switch set from .1 V to 5 V, lamp
remains on but input impedance is ~1 MQ and the
input attenuator is uncompensated.
COMPARISON
VOLTAGE (Vc)
+ and —
Pushbuttons
VOLTS DISPLAY
COARSE Control
Selects polarity of comparison vol-
tage.
Reads out the equivalent voltage
selected by the COMPARISON
VOLTAGE (Vc) COARSE, FINE,
VOLTS/DIV, and PULL VAR for
X10 Vc controls.
Varies the Vc voltage from zero to
ten volts.
A
Rin
Switch a ‘
“7A3
tins
USE IN @
+50mvV
VARIN ONLY _
COMPARISON
VOLTAGE (Vc}
COARSE -® FINE
VOLTS
VOLTS/DIV
UNCAL
i STEP ATTEN BAL |
}
oc i ® QO
Ve Gg KO UAL oy nee.
r POSITION
s
> Ve OUT O-10V
te @
DIFFERENTIAL COMPARATOR] |
1948-2
Fig. 2-1. Front panel of 7A13 and location of Rin switch S10.
2-1

Operating Instructions—7A13 (SN B200000-up)
FINE Control Provides more precise control of
the Vc voltage.
+ INPUT Connector Provides a means of connection
for signal measurement. It also
contains a third contact for probe
attenuation information. This
enables proper deflection factor
display on the CRT screen, and
proper volts display on 7A13.
+ INPUT Mode
Switch
Selects AC, DC, GND or Vc Mode
of coupling for the — INPUT
channel.
VOLTS/DIV Switch Selects one of twelve volts per
division calibrated deflection fac-
tors.
VOLTS/DIV VARI-
ABLE CONTROL
Selects an uncalibrated deflection
factor somewhere between the
twelve settings. A minium of 2.5
times the VOLTS/DIV switch set-
ting is provided. The UNCAL lamp
lights when the VARIABLE control
is out of the CAL detent.
PULL VAR FOR X10
Vc Switch
Extends the deflection factor of the
VOLTS/DIV Switch. This occurs
only for 10, 20, and 50 mV/DIV and
.1, .2, and .5 V/DIV settings of the
VOLTS/DIV switch.
— INPUT Connector Same as for + INPUT connector.
INPUT Mode
Switch
Selects AC, DC, GND or Vc Mode
of coupling for the — INPUT
channel.
STEP ATTEN BAL
Adjustment
Adjusts for no vertical trace move-
ment as. the VOLTS/DIV switch
setting is varied from 10 to
50 mV/DIV.
GAIN Adjustment Adjusts the amplifier gain for dis-
play of four divisions upon receipt
of a 4mvV_ signal when the
VOLTS/DIV switch is set to 1 mV
and the VARIABLE control is set to
CAL.
2-2
X10 BAL
Adjustment
Adjusts for no vertical trace move-
ment as VARIABLE (VOLTS/DIV)
knob is pulled out.
VAR BAL Control Adjusts for no vertical trace move-
ment as VARIABLE (VOLTS/DIV)
knob is varied throughout its
range.
Release Latch Pull to withdraw plug-in from in-
dicator oscilloscope.
BW Switch Selects either the FULL bandwidth
or 5 MHz.
Vc OUT 0-10 V Jack Provides a convenience outlet for
the comparison voltage.
POSITION Control Positions display vertically on the
CRT face.
Vc REF-IDENT Internally disconnects both sig-
Pushbutton nals and applies Vc to both inputs.
Readout display is replaced by the
word “IDENTIFY”.
TEST SETUP CHART
Fig. 2-2 shows a drawing of the front panel controls and
connectors. This chart can be reproduced and used as a
test setup record for special measurements and
applications, or it may be used as training aid for
operation of the 7A13.
FAMILIARIZATION PROCEDURE
First-Time Operation
The following steps are intended to help get the trace
on the CRT screen quickly and to prepare the unit for
immediate use. These steps are intended to acquaint you
with some of the basic functions of the 7A13.
1. Insert the unit into the oscilloscope vertical plug-in
compartment.
a,

DATA:
Operating Instructions—7A13 (SN B200000-up)
7A13 DIFFERENTIAL COMPARATOR SETUP CHART
ae COMPARISON
USE IN VOLTAGE (Vc)
VAR IN ONLY COARSE —o@— FINE
VOLTS
AX INP | 45 J 480 ]] 500 |
AC
> IheeT C oo wom
eno(_] Ge
ee ot FOR
oc(_] vas ar
ac{_]
eno[_|
STEP ATTEN BAL
| oc(_]
ve[_] © BAL .
POSITION
DIFFERENTIAL ow
C2
gO
[7ats
|
(1113) 1948-3
Fig. 2-2. Test set-up chart.
2-3

Operating Instructions—7A13 (SN B200000-up)
2. Set the 7A13 front panel controls as follows:
COMPARISON VOLTAGE
(Vc) Polarity Pushbutton canceled
+ INPUT Mode Gnd
— INPUT Mode Gnd
VOLTS/DIV 1V
VARIABLE In (CAL)
STEP ATTEN BAL As is
GAIN As is
X10 BAL As is
VAR BAL As is
BW 5 MHz
POSITION Midrange
3. Turn the Intensity control fully counterclockwise
and turn the oscilloscope Power ON. Preset the time-base
plug-in controls for a .5 ms/div sweep rate and automatic
triggering.
4. Wait about five minutes for the 7A13 and the
oscilloscope to warm up.
NOTE
About five minutes is sufficient warmup time when
using the 7A13 for short-term DC measurements.
For long-term DC measurements using the lower
deflection factors, allow at least one hour.
5. Adjust the Intensity control for normal viewing of
the trace. The trace should appear near the graticule
center.
6. Using the POSITION control, position the trace two
divisions below graticule center. Set VOLTS/DIV to 1 mV
position.
NOTE
If trace is off screen, perform Front Panel Ad-
justments outlined below.
7. Vary the VARIABLE control throughout its range
while observing the CRT trace.
8. Adjust VAR BAL so that there is no trace movement
while varying the VARIABLE control.
2-4
9. Vary the VOLTS/DIV switch from 10 mV to 50 mV
while observing the CRT trace.
10. Adjust STEP ATTEN BAL so that the trace does not
move while varying the VOLTS/DIV switch.
11. Set the VOLTS/DIV switch to 10 mV. Pull out the
PULL VAR FOR X10 Vc range knob while observing the
CRT trace.
12. Adjust X10 BAL so that there is no trace movement
while moving the PULL VAR FOR X10 Vc range knob in
and out.
13. Repeat steps 7 through 12 until optimum settings
are achieved.
14. Pushin the PULL VAR FOR X10 Vc Range knob and
set the VOLTS/DIV switch to 1 mV; position the trace two
divisions below graticule center.
15. Apply a 4mV_ peak-to-peak calibrator signal
through a coaxial cable to the + INPUT connector on the
7A18.
16. For DC coupled, single-ended operation, set the +
INPUT Mode Switch to DC. The display should be 4
divisions of square wave amplitude.
NOTE
If the display amplitude is not 4 divisions, adjust
GAIN control until it is.
17. For AC coupled, single-ended operation, re-
position the display to place the bottom of the display at
the graticule center line.
18. Set the + INPUT Mode switch to AC and note that
the display shifts downward about two divisions to its
average level.
19. Disconnect the calibrator signal from the + INPUT
connector. Set both the + and — INPUT Mode switches to
GND.

Front Panel Adjustments
These adjustments must be accomplished each time
the 7A13 is placed in a different oscilloscope and should
be checked in the given sequence prior to any Critical
measurement of waveforms.
Preset 7A13 controls as follows:
COMPARISON VOLTAGE
+ INPUT Mode GND
— INPUT Mode GND
VOLTS/DIV 1V
VARIABLE In (CAL)
STEP ATTEN BAL Midrange or as is
GAIN As is
X10 BAL Midrange or as is
VAR BAL Midrange or as is
BW 5 MHz
POSITION Midrange
Allow 20 minutes warmup time.
VAR BAL Adjustment
1. Set the VOLTS/DIV switch to 1 mV.
2. Vary the VARIABLE control throughout its range
while observing the CRT trace.
3. Adjust VAR BAL so that there is no trace movement
while varying the VARIABLE control.
STEP ATTEN BAL Adjustment
1. Vary the VOLTS/DIV switch from 10 mV to 50 mV
while observing the CRT trace.
2. Adjust STEP ATTEN BAL so that the trace does not
move while varying the VOLTS/DIV switch.
X10 BAL Adjustment
1. Set the VOLTS/DIV switch to 10 mV. Pull out the
PULL VAR FOR X10 Vc Range knob while observing the
CRT trace.
2. Adjust X10 BAL so that there is no trace movement
while moving the knob in and out.
3. Repeat all balance adjustments until optimum set-
tings are achieved.
@
Operating Instructions—7A13 (SN B200000-up)
GAIN Adjustment
1. Using the POSITION control, position the CRT
trace two divisions below graticule center.
2. Set VOLTS/DIV to 1 mV position.
3. Apply a 4mV_ peak-to-peak calibrator signal
through a coaxial cable to the + INPUT connector on the
7A13.
4. Set the + INPUT Mode switch to DC.
5. Adjust GAIN control so that the display is four
divisions of square wave amplitude with the bottom of the
display at the reference established in step 1.
GENERAL OPERATING INFORMATION
Signal Connection
In general, probes offer the most convenient means of
connecting signals to the inputs of the 7A13. The
Tektronix probes are shielded to prevent pickup of
electrostatic interference. A 10X attenuator probe offers a
high input impedance and allows the circuit under test to
perform very close to normal operating conditions. (For
further information on the use of probes, see Use of
Attenuator Probes in this section of the manual, and inthe
probe instruction manual).
In high-frequency applications requiring maximum
overall bandwidth, use a coaxial cable terminated at both
ends in the characteristic impedance of the cable. To
maintain the high-frequency characteristics of the applied
signal, use high-quality, low-loss cable. Resistive coaxial
attenuators can be used to reduce reflections if the applied
signal has suitable amplitude.
High-level, low-frequency signals can be connected
directly to the 7A13 input with short unshielded leads. This
coupling method works best for signals below about one
kilohertz and deflection factors above one volt/division.
When this method is used, establish a common ground
between the 7A13 and the equipment under test (Common
ground provided by line cords is usually inadequate).
Attempt to position the leads away from any source of
interference to avoid errors in the display. If interference is
excessive with unshielded leads, use a coaxial cable,
probe, or differential amplifier operation. (Differential
amplifier operation may require special considerations for
signal connections. See Differential Operation in this
section of the manual).
2-5

Operating Instructions—7A13 (SN B200000-up)
Bandwidth Limiter
The BW (bandwidth) switch provides a method of
reducing interference from unwanted high-frequency
signals when viewing low-frequency signals. With the
FULL button pressed, the full bandwidth capabilities of the
amplifier are available. When the 5 MHz button is pressed,
the upper—3 dB bandwidth point of the amplifier is limited
to about 5 MHz. The unwanted high-frequency signals are
reduced in the displayed waveform.
Display Identification
When the 7A13 Vc REF-IDENT button is pressed, the
signal is internally disconnected. This feature is par-
ticularly useful when the 7A13 is used with an indicator
unit designed for use with more than one vertical amplifier.
When using an oscilloscope system equipped with the
readout feature, the 7A13 deflection factor is displayed on
the CRT. When the Vc REF-IDENT button is pressed, the
readout information pertaining to the 7A13 is replaced by
the word IDENTIFY.
Rin Switch
The Rin switch S10 (located on left side of the unit)
selects the input resistance to ground (+ and — INPUT
simultaneously) to be either 1 MQ or ~ °%. The Rin switch is
normally set to the 1 MQ position. A front-panel lamp is
illuminated to indicate when the Rin switch is set to ~ °°.
The high input impedance obtained by setting Rin to ~ °°
is useful for measuring voltages in high-impedance cir-
cuits where minimum loading is necessary, and the
voltage to be measured is within the 10 volt range of the
7A13. Differential comparator operation is used to make
the measurement. At null, the comparison voltage is equal
to the voltage being measured.
Several precautions must be observed when using the
~ 0 position of the Rin switch (1) the VOLTS/DIV control
must be set to one of the six positions from 1 to 50 mV;
(2) the VARIABLE (VOLTS/DIV) knob must be pushed to
the IN position; (3) the INPUT Mode switch of the signal
channel must be set to DC.
If the external device does not provide a DC return path
for the input FET gate, an external resistance (adequately
shielded) must be connected between the input connector
and ground.
2-6
CALIBRATED DIFFERENTIAL
COMPARATOR OPERATION
Introduction
When one of the INPUT switches is set to Vc and the
other is set to AC or DC, the 7A13 is operating as a
calibrated differential comparator or slide-back voltmeter.
The calibrated comparison voltage Vc, can be added
differentially to the input signal to obtain a null. For linear
operation, Table 1-1 in the Specification section lists the
maximum input signal or voltage that can be applied to the
7A13 INPUT connector at a given VOLTS/DIV switch
position.
In differential comparator operation the calibrated DC
comparison voltage is internally applied, to differentially
offset any unwanted portion of the applied signal. This
allows measurements of relatively small AC or DC signals
riding on top of relatively large AC or DC signals.
The DC comparison voltage is set by the two COM-
PARISON VOLTAGE (Vc) controls: COARSE and FINE.
Equivalent Vc
When a signal is applied to the 7A13 INPUT connec-
tor(s), it is attenuated in the Input Attenuators before
being applied to the + or — Input stage. In contrast, the Vc
Supply voltage is not attenuated, but is applied (via relays)
directly to the Input stage.
Direct application of the Vcto the Input stage makes the
+10 V Vc appear equivalent to the product of the Vc and
the attenuation factor of the Input Attenuator. Likewise,
the use of an attenuator probe multiplies the Vc by the
attenuation factor of the probe.
Vc LED Readout
The front panel, light-emitting-diode VOLTS readout
array displays the four-digit Vc value selected by the
COMPARISON VOLTAGE (Vc) COARSE and FINE con-
trols. The decimal point placement is switched
automatically by internal circuitry for display of the
equivalent Vc. Special probes that correct the Vc display
(by changing decimal point placement) for the probe
attenuation may be used. Attenuator probes not so
equipped may be used with the instrument, but the VOLTS
display will not be correct. The operator must take this into
account when viewing the VOLTS display.
—~

Vc OUT 0-10 V Jack
The Vc OUT 0-10 V jack, mounted on the front panel of
the 7A13, permits monitoring of the comparison voltage.
The voltage at the jack is set up by the COMPARISON
VOLTAGE (Vc) COARSE and FINE controls, and polarity
switch. Impedance is 2 kQ to 5.5 kQ.
When monitoring the voltage at the jack, the voltage is
not affected if an "infinite-impedance type” voltmeter
(such as a digital voltmeter or any null type meter which
draws negligible current) is used for monitoring purposes.
If the Vc-OUT 0-10 V jack is loaded by an external meter,
the comparison voltage available at the jack and applied to
the input of the amplifier will not be the same as indicated
by the VOLTS display.
Differential Operation
Differential measurements are made by applying the
signals to the + INPUT and — INPUT connectors. Then,
both input Mode switches should be set to the same
position: AC or DC, depending on the method of signal
coupling desired. When using the 7A13 for differential
operation, only the voltage difference between the two
signals is amplified and displayed. Common-mode signals
(signals that are common in amplitude, frequency, and
phase) are rejected and not displayed.
The 7A13 differential input provision may be used to
eliminate interfering signals such as AC line-frequency
hum. Single-ended measurements often yield unsatisfac-
tory information because of interference resulting from
ground-loop currents between the oscilloscope and the
device under test.
These limitations of single-ended measurements are
virtually eliminated in differential measurements. A
differential measurement is made by connecting each of
the two inputs to selected points in the test circuit. Since
the chassis of the 7A13 need not be connected in any way
to the test circuit, there are few limitations to the selection
of these test points.
Amplitude and Common-Mode Rejection
In the text which follows, the term “Input Signal Range”
means the common-mode operating range of voltage
through which the amplifier will produce a usable output.
This should not be confused with the maximum (non-
destructive) input voltage, which is related to the break-
down limits of the amplifier components.
@
Operating Instructions—7A13 (SN B200000-up)
Factors That Affect CMRR
Frequency. Since the common-mode output voltage is
a factor of phase differences as well as gain between
channels, the frequency of the input common-mode
signal has a direct bearing on the CMRR. Generally, as the
frequency of the input signal increases, the CMRR
decreases. (Exception: with AC-coupled input, the CMRR
will become higher as frequency is increased from DC to
over 100 Hz.)
Source Impedance. The specified CMRR assumes that
the points being measured have identical source im-
pedance. The source impedance and the amplifier input
impedance form an RC divider which determines the
portion of the signal that appears across the amplifier
input, and the apparent effect on CMRR.
The user may desire to construct a graph of CMRR
versus frequency for specific applications where the
source or signal tranporting lead impedances are un-
balanced.
Signal Transporting Leads. A principal requirement for
maximum CMRR is that the signals arrive at the amplifier’s
two inputs with no change in phase or amplitude. Slight
differences in attenuation factors, or phase shift between
two input attenuators may reduce the CMRR 20% or more.
Attenuator probes extend the usable voltage range ofa
differential amplifier by reducing the input signal level
below the maximum common-mode input voltage.
However, a reduction in the apparent CMRR will usually
occur because of component value differences within the
probes. (See Use of Attenuator Probes in this section).
Ground Connections. Proper grounding reduces
signals generated from ground loop currents. It is usually
best to electrically connect the probe or signal lead shields
together at the probe body or signal source, but not to the
instrument ground.
USE OF ATTENUATOR PROBES
General
Attenuator probes reduce the resistive and capacitive
loading of the signal source and extend the measurement
range to include substantially higher voltages. Passive
attenuator probes having different attenuation factors as
well as special-purpose types are available from
Tektronix, Inc.
2-7

Operating Instructions—7A13 (SN B200000-up)
Special probes that correct the oscilloscope deflection
factor display, VOLTS display, and MAX INPUT VOLTS
indicator to match probe attenuation may be used. (The +
and — INPUT connectors have an outer ring that is
connected to the Probe Sensing circuit). Attenuator
probes not so equipped may be used with the instrument,
but they will not operate the sensing circuit. Therefore, the
operator must supply the probe attenuation factor for
measurements involving the three indicators affected.
NOTE
If two probes with different attenuation are con-
nected to the INPUT connectors (@.g., 10X and
100X), the deflection factor readout, Vc Display, and
MAX INPUT VOLTS indicator will be corrected for
the probe with the larger division ratio (100X).
Probe Selection
The P6055 probe is recommended for measurements
where CMRR up to 20,000:1 must be maintained (100:1 at
20 MHz). The attenuation ratio is adjustable to X10 to
compensate for differences in input resistance of the
amplifier.
The P6053B probe is recommended for measurements
requiring the full bandwidth of the 7A13 (see Table 1-4 in
the Specification section). The P6053B also has the
sensing capability for deflection factor and Vc Display
readout compatibility.

WARNING
THE FOLLOWING SERVICING INSTRUCTIONS
ARE FOR USE BY QUALIFIED PERSONNEL
ONLY. TO AVOID PERSONAL INJURY, DO NOT
PERFORM ANY SERVICING OTHER THAN THAT
CONTAINED IN OPERATING INSTRUCTIONS
UNLESS YOU ARE QUALIFIED TO DO SO.
REFER TO OPERATORS SAFETY SUMMARY
AND SERVICE SAFETY SUMMARY PRIOR TO
PERFORMING ANY SERVICE.
Be
pQ
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WmWA
DBA

Section 3—7A13 (SN B200000-up)
CIRCUIT DESCRIPTION
INTRODUCTION
A block diagram description covering the general
configuration of each circuit in the 7A13 is included in this
section. Following the block diagram description is amore
detailed description, particularly for circuits unique to this
instrument.
A block diagram and complete schematic diagrams are
included in the Diagrams section at the back of this
manual. These should be referred to throughout the
circuit description.
For more information relating to the function or
adjustment of variable components, refer to the Perfor-
mance Check/Calibration procedure in Section 5.
BLOCK DIAGRAM DESCRIPTION
(See Block Diagram Pullout preceding schematics.)
INPUT Mode Switches
A signal applied to either the + or — INPUT connector,
as
shown by the heavy lines on the Block Diagram, passes
through the INPUT Mode switches to the Input
Attenuators. The signals can be AC coupled, DC coupled
or disconnected internally. (See schematic diagram No.
1.)
Input Attenuators
The Input Attenuators for the + and — inputs are
identical and are conventional RC type attenuators. The
adjustable resistive elements facilitate matching the — and
+ attenuators to obtain optimum DC common-mode
rejection and precise attenuation ratios.
The attenuators (Schematic Diagram No. 1) are fre-
quency compensated voltage dividers that provide con-
stant attenuation at all frequencies within the bandwidth
of the instrument. This is done while maintaining the same
input time constant (20 us) for all positions of the
VOLTS/DIV switch.
Input Source and Emitter Followers
The Input Source and Emitter Followers are designed
to present a very high input impedance at the attenuator’s
output. Bootstrapping is incorporated in these stages for
each of the + and — inputs. A Current Source is used to
supply the proper amount of current for these stages.
Each side also contains an overload protection circuit to
guard against signals larger than approximately +15 volts.
Differential and Common Mode Signal Clamps
The Differential and Common Mode Signal Clamps
allow the following Differential Comparator stage to
operate linearly for all input conditions. The Differential
Mode Signal Clamp limits the output levels to ap-
proximately +1 volt, whereas the Common Mode Signal
Clamp allows a window of at least +10 volts.
Differential Comparator
The Differential Comparator operates upon the limited
output of the Differential and Common Mode Signal
Clamps. The amplified signal is then applied to the gain-
switching amplifiers. The gain of the Differential Com-
parator is approximately one.
1X, 2X, and 5X Gain-Switching Amplifier
The 1X, 2X, and 5X Gain-Switching Amplifier receives
the differential signals from the Differential Comparator.
Gain switching is accomplished by the VOLTS/DIV switch
through relays.
3-1

Circuit Description—7A13 (SN B200000-up)
1X and 10X Gain-Switching Amplifier
The 1X, 10X Gain-Switching Amplifier receives the
differential signal from the 1X, 2X, and 5X Gain-Switching
Amplifier. The gain of this stage is switched through relays
by the VOLTS/DIV switch in conjunction with the PULL
VAR FOR X10 V. switch.
Driver Amplifier
The Driver Amplifier receives the differential signals
from the 1X, 10X Gain-Switching Amplifier. The gain of the
Driver Amplifier is approximately 2.5. Its output signals
are applied to the Output Amplifier.
Output Amplifier
The Output Amplifier further amplifies the differential
signals and then applies them to the input of the os-
cilloscope through pins A11 and B11 of the interface
connector.
Trigger Amplifier
The Trigger Amplifier receives a portion of the Output
Amplifier signal in order to provide + and — internal
triggers to drive the Trigger Generator circuit in the time-
base plug-in unit.
Feedback Limiter
The Feedback Limiter is driven by signals from the
Trigger Amplifier when the output amplitude exceeds a
certain limit. These signals are fed back to the 1X, 2X, and
5X Gain-Switching Amplifier stage to limit the incoming
signal and prevent the signal from over-driving the
succeeding stages into saturation.
3-2
Uncal Readout Logic
The Uncal Readout Logic circuit provides the
necessary logic to indicate the uncalibrated symbol on the
CRT display, and illuminates the UNCAL lamp on the
7A13 when the VARIABLE control is not in the CAL
position.
V. Decimal Point Logic
The V. Decimal Point Logic provides a logic signal to
the DVM circuit, which is determined by the total attenua-
tion of the input signal by the Input Attenuator and divider
probe. This provides a decimal point placement signal to
the V. Readout Driver circuit, enabling it to display the
equivalent comparison voltage.
V. Generation and Digital Voltmeter
The comparison voltage (V.) is derived from a Zener
diode reference, and is selectable by the COARSE and
FINE front panel controls. The selected value of V. is
measured by the Digital Voltmeter circuit and displayed
by the front panel light-emitting-diode array. V. may also
be measured externally at the V. OUT output jack on the
front panel.
Probe Sensing and Trace Identify
The Probe Sensing circuit senses the attenuation
factor of an attenuator probe connected to an input. The
output of this circuit then adjusts the output of the readout
logic and the V. Decimal Point Logic circuits to include the
probe attenuation factor. The inputs to this circuit also
provide the trace identify signals to the readout output line
and Output Amplifier circuits.
—_