Tektronix 7A13 User manual

Tektronix, Inc .
P.O . Box500
Beaverton, Oregon
97077

Serial Number
070-1948-00

First Printing APR 1975
N MANUAL

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CHANGE INFORMATION
'7A13 (SN B200000-up)
SECTION 1 SPECIFICATION
Page
Introduction 1-1
Electrical Characteristics 1-1
System Characteristics 1-4
Environmental Characteristics
1-4
SECTION 2 OPERATING INSTRUCTIONS
Introduction
2-1
Front Panel Description 2-1
Test Setup Chart 2-2
Familiarization Procedure 2-2
General
Operating Information 2-5
SECTION 3 CIRCUIT DESCRIPTION
Introduction
3-1
Block Diagram Description
3-1
Detailed Circuit Description 3-3
SECTION 4 MAINTENANCE
Preventive Maintenance 4-1
Troubleshooting 4-2
Corrective Maintenance 4-8
SECTION 5 PERFORMANCE CHECK/CALIBRATION
Introduction 5-1
Test Equipment Required 5-1
Short-Form Performance Check/Calibration
Procedure and Index
5-5
Performance Check/Calibration Procedure 5-8
SECTION 6 ELECTRICAL PARTS LIST
Abbreviations and Symbols
Parts Ordering Information
SECTION 7 DIAGRAMS AND CIRCUIT BOARD ILLUSTRATIONS
Symbols and Reference Designators
Voltage and Waveform Conditions
SECTION 8 MECHANICAL PARTS LIST
Mechanical Parts List Information
Index of Mechanical Parts Illustrations
Mechanical
Parts List
Accessories

7A13 (SN B200000-up)
Fig . 1-1 . 7A13 Differential Comparator .

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

Common Mode
7000
series oscilloscopes . It may be used as a differential

Signal Range
input preamplifier
or conventional preamplifier in addition

1 mV/Div to
to its use as a comparator .

50 mV/Div ;
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
thanl mV in amplitude on10volt 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 fable 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 (cont)
yV` ~Characteristic

-

Performance Requirement
X10
Vc In
10 mV/Div
to
50 mV/Div ;
X10 VC Out
0 .1 V/Div to
0 .5 V/Div ;
X10V
c In
0 .1 V/Div to
0 .5 V/Div ;
X10V
c Out
1
V/Div to 5 V/Div
X10 V
c
In
Frequency Response
(8 Div Reference)
Section 1---7A13 (SN B200000-up)
At least +-10 V and -10 V
At least -+-100 V and ---100 V
At least
-i-500 V and ---500 V
At least +500 V and -500 V
ELECTRICAL
Characteristic
TABLE 1-1
CHARACTERISTICS
PerformanceRequirement
FULL Bandwidth
Upper Limit
AC
(Capacitive)
Coupled Input
See Table 1-4, System
Charac-
teristics
10 Hz or less
Lower
Bandwidth
Deflection Factor
Frequency
(VOLTS/DIV)
Calibrated Range 1 mV/Div to 5 V/Div,12 steps in a 1,
5 MHz Bandwidth DC to 5 MHz within 500 kHz
2,
5 sequence
Gain Ratio
Within 1 .5% of GAIN adjusted at 1
Accuracy mV/Div
Overdrive Recovery Recovers to within 2 mV
in 1 us
Uncalibrated
Continuously variable : extends (1X Attenuator
at (and 1 mV in 0 .1 ms) after a pulse
(Variable) deflection factor
to at least 1 mV/Div) of
-110V or
--10V, or
less,
12 .5 V/Div
regardless of pulse duration .

Specification---7A13 (SN B200000-up)
Common Mode
Rejection Ratio
1 mV/Div to
50 mV/Div
X10Vc In
DC to 100 kHz
Maximum Input
Voltage
DC
(Direct)
Coupled
DC t- Peak AC
1 mV/Div to
50 mV/Div
;
X10
Vc
In
10 mV/Div to
50 mV/Div ; X10
VC
Out ;
0 .1 V/Div to 5
V/Div ;
"TABLE 1-1 (cont)
At least 20,000 :1, 20 V P-P or less
test signal
At least
10,000
:1,
10
V P-P
or less
test signal
Decreases to 500 :1 at 10 MHz with
1 V P-P, then 200 :1 at 20 MHz
at 1 V P-P . See Fig . 1-2 .
At least 2,000 :1
At least 500 :1
400 VDC, 400 V Peak AC, 1
kHz
or
less
TABLE 1-1 (cont)
Characteristic

Performance Requirement
Input R and C
Resistance
Maximum Gate
Current
0°C to -1-35°C
Both Inputs
DC Drift
Drift With Time
(Ambient Temp-
erature and
Line
Voltage Con-
stant)
Short Term
Long Term
Drift With Ambient
Temperature
Line Voltage Con-
stant)
Displayed Noise
Tangentially Meas-
u red)
1 Mfg ±0 .15%
Approximately 20 .0 pF
Within + - 1% between all deflection
factors .
1 mV P-P
or
less or 0 .1 Div or less
(whichever is greater) any 1 minute
interval within 1 hour
after 20
minutes from turn-on .
1 mV P-P or less or 0 .1 Div
(whichever is greater) during
any
hour
after the first hour and 20
minutes from turn-on .
2 mV/10° C or less, 0 .2 Div/10° C
or
less (whichever is greater)
1 % or less shift within 20 ns of step
of fast rise squarewave
when
switching undriven
input from
GND to AC or DC
400 NV

or

less

at

1 mV/Div

in
Type 7700-Series
indicator os-
cilloscope
0Vto±10V
(0
.1%
of setting +3 mV)
0
.5
mV
or less

Specification-7A13 (SN B200000-up)
V
N
F
Jd
Oa
noMME
MISM
EMMI
WEEM
OEMIMM
ME
111NOW1110 so
" 0 6MI'll
ININMEMSE-11MM
n
nnuuW
n Ism
nmn
. . : . . .
.l
. . : __ .
Y
O
O
r
0
O
O
r
g
r
2
O
N
N
O
O
2
Y
O
Fig . 1-2 . Common mode rejection ratio
graph . It pertains to 1 mV/Div through
20 mV/Div deflection factors .

Specification--7A13 (SN B200000-up)
Storage
Weight
TABLE
1-2
7A13 tested alone (separate from
oscilloscope mainframe)
ENVIRONMENTAL CHARACTERISTICS
Characteristic
.M . .M
Performance
Requirements
a___ ._._ ._____ .___ . .~ .- .-~- .~- ._ .~ :-T
Temperature
Operating
System Characteristics
O' C to +50 0 C
-55° C to +75'C
15,000 feet
50,000 feet
Qualified
under National Safe
Transit Committee test procedure
TABLE 1-3
PHYSICAL CHARACTERISTICS
Size___
. . . _
il Fits all 7000-Series plug -in
com-
partments
.
3 .1 Pounds (1 .4 kilograms) .
The system characteristics listed in Table
1-4 specify
the performance
of the plug-in with various combinations
of
probesand in various indicator oscilloscopes .
~'EXT
CAL
1 .5
1 .5
TABLE 1-4
SYSTEM CHARACTERISTICS
(Indicator Oscilloscope : 7500-Series with P6053B
Probe .)
'Accuracy (%)
3 1 NT
ry CAL -
__
2 .5
__2
.5
._
4 1NT
CAL
3 .5
3 .5
100 I 3 .5 I 1 .5 I 2 .5 I 3 .5
(MHz) (ns)
55 6 .4
__ . . .55_ ._6
.4
(Indicator Oscilloscope : 7700-Series with P6053B Probe .)
__ i 00 __ . ._1_ 3 .5 __j___1, :5 _1__ .5___1 ._
3-555

L6
.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 mustbe set with the calibration signal
applied to
the probe tip .
2 EXTernal CALibrator, 0°C to - h-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% .
3 INTernal CALibrator, -1-15°C to 1-35°C :
The plug-in GAIN is set
using the oscilloscope's
own calibrator and the instrument is
operating within the
+15o
C to -F 35 0 C range .
4 INTernal CALibrator, 0°C to -1-50°C : The plug-in
GAIN is set
(within 10 0 C of the operating
temperature) using the os-
cilloscope's own calibrator, and the instrument is
operating
within the 0° C to
-1_50°
C range .

R ;
n
~
~
00

Illuminates

when

switch

S10,
1-50 mV

located on left side
of
plug-in,
is
VAR IN ONLY

turned cw . This indicates a -!
Lamp

INPUT and -- INPUT impedanceof
approximately infinity whenever
the VOLTS/DIV switch
is
set
between 1 and 50 mV, VARIABLE
knob is pushed in and 1X probe is
used .
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
procedureand finally a general
discussion of
the opera-
tion of the 7A13 .
FRONT-PANEL DESCRIPTION
Fig . 2-1 . Front panel of 7A13 and location of R,, switch S10 .
Section 2----7A13
(SN
B200000-up)
NOTE
With
VOL
TS/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)
-I- and --

Selects polarity of comparison vol-
Pushbuttons tage
.
VOLTS DISPLAY

Reads out the equivalent voltage
selected by the COMPARISON
VOLTAGE (Vc) COARSE, FINE,
VOLTS/DIV, and PULL VAR for
X10
Vc
controls .
COARSE Control

Varies the V
c
voltage from zero
to
ten volts .
2-'i

Operating Instructions-7A13 (SN B200000-up)
FINE Control

Provides more
precise control of
the V
c
voltage .
I- 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

Selects AC,DC, GND or V c Mode
Switch

of coupling for the - INPUT
channel .
VOL"I`S/DIV Switch

Selects one of twelve volts per
division calibrated deflection fac-
tors .
VOLTS/DIV VARI-

Selects an uncalibrated deflection
ABLE CONTROL

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

Extends the deflection factor of the
V
cSwitch

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

Selects AC,DC, GND or
Vc
Mode
Switch

of coupling for the --- INPUT
channel .
STEP ATTEN BAL

Adjusts for no vertical trace move
Adjustment

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 4 mV signal when the
VOLTS/DIV switch is set to
1 mV
and the VARIABLE control is set to
CAL .
2-2
X10 BAL

Adjusts for no vertical trace move
Adjustment

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 .
V c OUT 0-10 V Jack

Provides
a
convenience outlet for
the comparison voltage .
POSITION Control

Positions display
vertically
on
the
CRT face .
V c REF-I DENT

Internally disconnects both
sig
Pushbutton

nals and applies V
c
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 a 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
immediateuse . 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 .

7A13 DIFFERENTIAL COMPARAT P SETUP CHART
COMPARISON
VOLTAGE (Vc)
COARSE -
s9--
FINE
Operating Instructions--7A13 (SN B200000-up)
GAIN
0 (00)
X10 BAL
~_
POSITION
Fig . 2-2 . Test set-up chart .
(1113) 1948-3
2-
3

Operating Instructions-7A13 (SN B200000-up)
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 .
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 .
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
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
.
NOTE
If trace is off screen, perform
Front Panel Ad-
justments outlined below .
13 . Repeat
steps 7 through 12 until optimum settings
are achieved .
14 . Push in the PULL VARFOR X10
Vc
Range knob and
set the VOLTS/DIV switch to 1 mV ; position the trace two
divisions below
graticule center .
15
.
Apply a 4 mV peak-to-peak calibrator signal
through
a coaxial cable to the -F INPUT connector on the
7A13 .
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 -i- INPUTMode switch to AC and note that
the display
shifts downward about two divisions to its
average level .
19 .

Disconnect the
calibrator signal from the -i- INPUT
connector . Set both the -i- and -- INPUT
Mode switches to
GND .
2 . Set the 7A13 front panel
controls as follows : 9 . Vary the VOLTS/DIV switch from 10 mV
to 50 mV
while observing the CRT trace .
COMPARISON
VOLTAGE
(Vc)
Polarity Pushbutton
canceled
10 . Adjust STEP ATTEN BAL
so
that the trace
does not
+- INPUTMode Gnd
move while varying the VOLTS/DIV switch .
INPUTMode Gnd
VOLTS/DIV 1 V
VARIABLE
In (CAL)
11 . Set the VOLTS/DIV switch
to 10 mV
.
Pull out the
STEP ATTEN BAL As
is
PULL VARFOR X10 V c range knob while
observing the
CRT trace .
GAIN As is
X10 BAL As is
VAR BAL
As is
12 . Adjust X10 BAL so that there is no trace movement
BW
5 MHz while moving the
PULL VARFOR X10
Vc
range knob in
POSITION Midrange and out .

Front Panel Adjustments
These
adjustments mustbe 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 :
Allow 20 minutes warmup time
.
VAR BAL Adjustment
1 . Set the VOLTS/DIV switch to 1 mV .
STEP ATTI=N BAL Adjustment
X10 BAL Adjustment
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 .
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 .
1 . Set the
VOLTS/DIV switch to 10 mV . Pull out the
PULL VAR FOR X10 V
c
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 4 mV

peak-to-peak calibrator signal
through a coaxial cable to the + INPUT connector on the
7A13 .
4 . Set the
-I-
INPUTMode 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 impedanceand 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 in the
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
COMPARISON VOLTAGE
-f-- INPUT Mode GND
--- INPUT Mode GND
VOLTS/DIV 1 V
VARIABLE In (CAL)
STEP ATTEN BAL Midrange
or as is
GAIN As is
X10 BAL Midrange or as is
VARBAL Midrange
or as is
BW 5 MHz
POSITION Midrange

Operating Instructions--7A13 (SN B200000-up)
Bandwidth Limiter
'T - he 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 V
c
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 V
c
REF-IDENT button is pressed, the
readout information pertaining to the 7A13 is replaced by
the word IDENTIFY .
Ri,,
Switch
The R . switch S10 (located on left side of the unit)
selects the input resistance to ground
(_1-
and - INPUT
simultaneously) to be either 1 Mfg or
=
- . The R ; . switch is
normally set to the 1 Mfg position . A front-panel lamp is
illuminated
to
indicate when the R ; . switch is set to
-
oo .
The high input impedance obtained by setting Ri . 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 mustbe observed when using the
-
position of the R, . 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 mustbe set
to DC .
If the external device does not providea DC return path
for the input FET gate, an external resistance (adequately
shielded) must be connected between the input connector
and ground .
2- 6
Introduction
Equivalent Vc
CALIBRATED DIFFERENTIAL
COMPARATOR OPERATION
When one of the INPUT switches is
set
to
V
c
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 .
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 V c
Supply voltage is not attenuated, but is applied (via relays)
directly to the Input stage .
Direct application of the V c to the Input stage makes the
1-10 VV c appear equivalent to the product of the V c 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 .
V c LI=D Readout
The front panel, light-emitting-diode VOLTS readout
array displays the four-digit V
c
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 V c . Special probes that correct the V
c
display
(by changing decimal point placement) for the probe
attenuation may be used . Attenuator probes not so
equipped may be used with the instrument, butthe 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 V
c
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 VOL'T'S 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
I n the text which follows, theterm
"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 impedanceand 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
requirementfor
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
of a
differential amplifier
by reducing the input signal level
below
the maximum common-mode
input voltage .
However, a
reduction in the apparent CMRR will usually
occurbecause 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 .
General
USE (3F ATT
NUATOP PRO
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 ----7Al3 (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 -F-
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 operatethe 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 (e .g ., 1OX 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 V
c
Display
readout compatibility .

Input Attenuators
INTRO ACTION
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 a more
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
INPUT Mode Switches
CK
DIAGRAM
(See Block Diagram Pullout preceding schematics .)
A signal applied
to either the -+- or -- INPUT connector,
as shown by the heavy lines
on
the
Block Diagram, passes
through the INPUTMode switches
to the Input
Attenuators . The signals can be AC coupled, DC coupled
or disconnected internally . (See schematic diagram No .
1 .)
The Input Attenuators for the -'r- and --- inputs
are
identical and are conventional RC type attenuators . The
adjustable resistive elements facilitate matchingthe--and
1- 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 isdone while maintaining
thesame
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
Section 3-7A13 (SN B200000-up)
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 .
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 againstsignals largerthan 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 bythe
VOLTS/DIV switch
through relays .

Circuit Description--7A13 (SN
8200000-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
VARFOR 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 .
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 lampon 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 c ) 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 c
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 theV c Decimal Point Logic circuits to includethe
probe attenuation factor
. The inputs
to
this circuit
also
providethe trace identify signals
to
the readout output line
and Output Amplifier circuits .

AIL
I
ATT NUATiOR AND SWITCHING
The Attenuator and Switching circuitry determines
the
operating mode and deflection factor of the 7A13 . Also
included is the circuitry that provides the readout logic to
the indicator oscilloscope, and encodes the decimal point
placement for the Digital Voltmeter .
INPUT Mode Switches
NOTE
The separate
-~
and - INPUT switches
control the
mode of operation for the 7A13 . S1 controls the -f- input
side of the amplifier, and S21 controls the --- input side .
These switches permit
connecting one input or the other,
or both, to the Input Source and Emitter Follower
stages .
The V . position
of
the INPUT
switches applies the
comparison voltage, instead
of
the signal, to one
or both
sides of the amplifier .
The -+- and - input channels of the 7A 13 are similar .
To avoid duplication, only the + input channel is
described in detail throughout this description .
When the INPUT" switch is in the DC position, the input
signal is coupled directly to the Input Attenuator . In the
AC position, the AC component of the signal is coupled
through capacitor C1, while the DC component is block-
ed . 'T'he GND position internally disconnects the signal
from the 7A13
and connects ground to the Input
Attenuator . This provides a ground reference for the
amplifier without requiring removal of the input probes .
When the INPUT
switch is set to Vc, relay
K10 (relay
K30 for --- input) disconnects the signal while the com-
parison voltage V c is applied to the gate of the input FET .
'This provides a DC reference for one input of the
Differential Comparator while the signal is applied to the
other . The front-panel V . REF-IDENT
pushbutton S540,
when pressed, actuates both relays K10andK30 to apply
V c to both input channels simultaneously .
VOLTS/DIV and PULL VARFOR X10 V c Switches
The VOLTS/DIV switch, S50, sets the overall deflection
factor of the 7A13 . The PULL VARFOR X10 V c switch, S80,
multiplies the
equivalent V c range ten times and increases
the maximum input voltage rating of the 7A13 . S80
is
actuated by pulling the VARIABLE (VOLTS/DIV) knob to
Circuit Description---7A13 (SN B200000-up)
1
'TI N
its outer position . A latch on the control shaft prevents the
knob from being pulled out unless the VOLTS/DIV
switch
is in one of its center six positions (10 mV to .5 V) .
The functions of these two controls are accomplished
by switching relays to switch input attenuators and set the
gain of the two gain-switching amplifier stages . The
switches also provide signals indicating deflection factor
and V . range to the Readout Logic and V . Decimal Light
Driver stages .
S50 and S80 control the switching relays associated
with the following circuits
:
1 . Input Attenuators . Relays K6-K26 and K7-K27
switch the input
signal
(both +
and -- input channels
simultaneously) to pass directly
to
the input FET (1X),
pass through one attenuator network (10X), or pass
through two attenuator networks (100X)
.
The current to
energize the switching relays passes through contacts on
both S50 and S80 . With S80 pushed in, the Input
Attenuators are selected by
S50 to be
1X for
S50 settings
of 1-50 mV ; 10X for S50 settings of .1- .5 V ; and 100X for
S50 settings of 1-5 V . Pulling S80 out increases the input
attenuation to 10X for S50 settings of 10
to
50 mV,and
100X for settings of .1 to .5 V .
2 . 1X-10X
Gain-Switching Amplifier
.
The gain of this
stage is switched by relays K60-K61 . The
stage gain is
maximum when the relays are in the 1 X setting . The
relays
in the 10X position switch in an attenuator to divide the
gain by a factor of 10 . The relay energizing current passes
through contacts
on S50andS80 .
3 . 1X, 2X, and 5X Gain-Switching Amplifier . Relays
K47-K48 switch attenuator networks to reduce thegain of
this stage by a factor of 2 or 5 . These relays are controlled
by S50 only,
giving the VOLTS/DIV switch the 1-2-5
sequence .
Thus, S50selects the amplifiergains and input attenua-
tion as necessary for any specific deflection factor . Pulling
out S80 multiplies the equivalent Vc by switching in an
attenuator to divide the input signal 10 times ;
simultaneously, the amplifier gain is multiplied by 10 to
retain the
deflection factor set by S50 .
3-3

Circuit Description---7A13 (SN B200000-up)
Input Attenuators
The Input Attenuator circuit consists of two 10X RC
divider networks in each
input
channel .
Switching
relays
K6-K26 and K7-K27 switch the input signals to bypass the
RC divider networks for 1 X attenuation ; pass through one
divider network for 10X attenuation ; or pass through both
networks for 100X attenuation . ('The amount of input
signal attenuation
is
determined by the setting
of
the
VOLTS/DIV and PULL VARFOR X10 Vc switches . For
more information, refer to the discussion of these
switches and to the schematic diagram .)
In the 1X attenuator setting, the input resistance
consists of two resistors in series returned to ground
through R ; . switch S10 . The ground return is disconnected
when S10 is set
to
R ; . -= .
Input
Source and Emitter Followers
INPUT AM
The Input Source and Emitter Followers stageprovides
a very high input impedance, so as not to load the
attenuator circuit elements . This stage includes
Q120,
source follower ; Q130A, emitter follower ; and the
following : Q140-Q145, constant current source ; Q115-
0118, bootstrap emitter followers ; Q105, constant current
source ; and Q110-CR110-CR112-CR101-CR102-CR134,
overdrive protection . The overdrive protection circuitry
acts to prevent component damage within this stage . The
Differential and Common Mode Signal Clamps protect the
rest of the amplifier from an input overdrive .
A simplified schematic diagram of this stage is shown
in Fig . 3-1 . The primary signal path, through Q120 and
Q130A, is shown by a heavy line on the diagram . The
secondary signal path is from the emitter of Q130A
through emitter followers 0118 and Q115 . This part of the
circuit, called "bootstrapping", allows the
drain and
collector of Q120 and Q130A to follow the input ; main-
taining approximately quiescent (or a constant) voltage
across Q120 and Q130A . The resistive
tails
of
Q118,Q120,
and 0130A all connect to current source Q145 through
CR134 . The constant
current source,
in
conjunction with
the bootstrapping, results in minimal transistor parameter
changes, particularly at high frequencies .
The operation of the protection circuitry depends on
the polarity of the overdrive signal . Quiescently, Q110 is
saturated, forward biasing CR110 to connect -1-25 V to the
input
stage . 0105 is operating as a constant current
Variable components
in the input
to
the FET adjust to
establish the input R and C at 1 MO and 20 pF . Variable
components in the attenuator networks adjust to provide
the following characteristics :
1 .

The input
R and C is
normalized at all settings of the
VOLTS/DIV switch .
2 . Precise attenuation ratios and high-frequency com-
pensation . With these adjustments accurately set, op-
timum common-mode rejection through
the attenuators
is
achieved .
L .I I
source for biasing Q115, and CR112 is conducting .
Current is returned to the power supply through forward
biased
CR134
and
constant current
source
Q145 . 47 volt
zener diode VR103 is in a non-conducting state, with
approximately 40 volts DC across it . CR102 and CR101 are
reverse biased, and thus non-conducting .
A positive-going over-excursion at the input will
forward bias the gate-source junction of 0120 and will
saturate Q130A, Q118, and Q115 . Asthe input is rising,
the
voltages
at the cathodes
of
CR110 and CR112 will reach a
level which will reverse bias the diodes . This essentially
disconnects the input stage from the -1 power supplies,
allowing the input stage to float with the input . The voltage
strain is now across Q145 .
If a negative signal is applied to the input, the junction
of R124, R126, R134, and CR134 follows . When the voltage
at this point is approximately equal to or more
negative
than the
voltage
at
the
collector of
Q145, CR134 becomes
reverse biased and thus disconnects the input stage from
the supply return . If the signal becomes more negative
than about ----15 volts, CR101 and CR102 become forward
biased, thus locking the anode end of VR103 to ap-
proximately the same potential as the input . If the input
becomes equal to or more negative than about- 21 volts,
zenerdiode VR103 begins to conduct, locking the base of
Q110 and the base of Q105 to a level about 48
volts above
the input . As the input goes more negative, Q110 goesout
of saturation and is turned off . This disconnects the input
stage from the -I-25 V supply . The collector of Q105
had
been
going negative becuase of the bootstrapping . Now