Tektronix 7A12 User manual

1 N
ST
FR
U GTI O N
MAN
VAL . Serial

Number
J~
l
)
V
~ 1 '~Cr
7A 12
DUAL TRACE
AMPLIFIER
Tektronix, Inc . 0
P .
O .
Box 500 " Beaverton,
Oregon97005 " Phone [phone]
"
Cables : Tektronix
070-0977-01

971

WARRANTY
All Tektronix
instruments are warranted
against
defective materials
and workmanship
for one year .
Any questions
with respect to the
warranty,
mentioned above,
should he taken up
with your
Tektronix
Field Engineer or
representative .
All
requests for repairs
and replacement parts
should
he directed to
the Tektronix Field
Office or
representative in your
area . This procedure
will assure
you the fastest
possible service . Please
include the
instrument Type
(or Part Number) and
Serial or
Model Number
with all requests for parts
or service .
Specifications and price change privileges
reserved .
Copyright © 1971 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 the
copyright owner .
U .S .A . and
foreign Tektronix products covered
by U .S .
and foreign patents and/or patents pending .

TABLE OF
CONTENTS
7A12
SECTION 1 SPECIFICATION Page
SECTION
5 CALIBRATION
Page
Introduction 1-1
Introduction
5-1
Electrical Characteristics 1-1
Test Equipment Required
5-1
Environmental Characteristics 1-3
Performance Check
5-3
Physical Characteristics
1-3 Adjustment
5-7
SECTION2 OPERATING INSTRUCTIONS
General 2-1
SECTION 6 ELECTRICAL PARTS LIST
Front Panel Controls and Connectors 2-1
Abbreviations and Symbols
General Operating Information 2-3
Parts Ordering Information
Basic Applications 2-5
Index of Electrical Parts List
SECTION 3 CIRCUIT DESCRIPTION
Introduction 3-1
SECTION 7 DIAGRAMS AND CIRCUIT BOARD
Block Diagram 3-1
ILLUSTRATIONS
Detailed Circuit
Description
3-1
SECTION 4 MAINTENANCE SECTION 8
MECHANICAL PARTS
LIST
Introduction
41
Mechanical Parts List
Information
Preventive Maintenance
41
Index of Mechanical Parts
Illustrations
Troubleshooting 41
Mechanical Parts
List
Corrective Maintenance 44
Accessories

Fig . 1-1 . 7A12
Dual Trace
Amplifier .

Introduction
The 7A12 vertical
amplifier plug-in is
designed for use
with Tektronix
7000-Series Oscilloscopes
. The 7A12 is a
dual-channel,
wide band amplifier
with the upper frequency
limit mainly
determined by
the oscilloscope mainframe
in
which it is
used . For example,
the 7A12 used with
the 7504
oscilloscope
will have an upper
bandwidth limit of
about 75
MHz
. When used with a
7704 oscilloscope, the upper band-
width limit is about 105
MHz . . Internal gain and
compen-
sation circuits are auotmatically
switched to correspond
to
the
setting of the
VOLTS/DIV switch . A
+UP/INV switch
for each channel
allows either channel to
be inverted for
differential
measurements . The 7A12
features a DC offset
circuit with a
DC offset of at least
±500 divisions for
SECTION 1
SPECIFICATION
TABLE 1-1
ELECTRICAL
7A12
viewing low level AC signals in the presence of a
high-level
DC component .
This instrument will meet the electrical
characteristics
listed in Table 1-1 following
complete calibration as given
in Section 5 . The performance
check procedure given in
Section 5 provides a convenient method of checking instru-
ment performance withoutmaking internal adjustments .
The following
electrical characteristics are valid over the
stated
environmental range for instruments calibrated at an
ambient temperature of +20 ° C to +30 ° C, after a five
minute warmup unless otherwise stated .
Characteristic
Performance Requirement Supplemental
Information
Deflection Factor
Calibrated Range 5 mV/div to 5 V/div, 10 pushbuttons
in
a
I, 2, 5 sequence
Deflection Factor
Within
2%
with GAIN adjustment at 10
Accuracy
mV/div
Uncalibrated
Continuously variable ; VAR V/DIV
extends
deflection factor
to at
least 12 .5
V/div
+UP to . INVERT Gain
Ratio
1 :1 within 1%
Frequency Response See Table A
AC (Capacitive)
10 Hz or less without probe, 1 Hz or less
Coupled Input
Lower with P6053
Bandwidth
Frequency
Time Delay Between
Channels
500 ps or less
Channel Isolation At least 3000 :1 or 0 .3 div with 5
div
reference signal at 75
MHz . At least
100 :1 with equal deflection factors at
50
MHz or less

Specification-7A12
TABLE 1-1 (cont)
Characteristic
Performance
Requirement Supplemental
Information
Maximum Input
Voltage
DC Direct
Couple
5 mV/div &
350 V, DC or DC + Peak AC at 1 kHz or
10
mV/div less
20 mV/div to
500 V, DC or DC
+
Peak AC at 1 kHz or
5 V/div
less
AC (Capacitive)
Coupled Input
5 mV/div &
350 V, DC or DC
+ Peak AC
at
1 kHz or
10 mV/div
less
20 mV/div to
5 V/div
500 V, DC or DC + Peak AC
Input R and C
Resistance 1 Megohm ±2%
Capacitance Approximately 24 .0 pF
R and C
Product Within ±1% between all deflection factors
Recommended X10 Probe P6053, 6ft
Maximum Input Gate
0 .4 nA at 0 to +35 ° C2 nA at +35 ° C to
Current
+50 ° C
IDENTIFY
Trace deflects up 0 .2 div to 0 .4 div
Display Noise
250
pV
at
5 mV/div . 0 .1 div or less at
(Tangentially Measured)
other deflection factors
Chopped Mode
(when
installed
in vertical
compartment
. Mainframe
dependent .)
Repetition Rate
500kHz within 20%
Channel Time Segment
0 .8 ps to 1 .2 ps
DC Drift
Drift With
Ambient
100 pV/ ° C or 0 .01 div/ ° C
whichever is
Temperature
(Line
greater
Voltage
Constant)
DC OFFSET Range
At least +1000 div to -1000
- div at 5 mV/div
At
least +500 div to -500
div from
10 mV/div to 5 V/div
Common Mode Rejection
Ratio
At least 10 :1 at 40 MHz or less

scope .
TABLE
A
7A12 AND MAINFRAME
FREQUENCY RESPONSE (five division reference)
With 7900
Series

With 7700 Series

With 7500 Series

With 7400
Series
120
MHz

105 MHz

I

75 MHz

I

55 MHz
TABLE
1-2

TABLE 1-3
ENVIRONMENTAL

PHYSICAL
Specification-7A12
Refer

to

the

Specification

for the

associated

oscillo-

Size

Fits all 7000-Series Plug-in
Compartments
Weight

I

2 .8 Ibs (1 .3 kg)

General
SECTION
2
OPERATING
INSTRUCTIONS
To effectively use the
7A12, the operation and
capa-
bilities of the instrument
must be know . This
section de-
scribes front-panel
control functions, general
information
on signal input
connections, and other subjects that
pertain
to various measurement
applications .
Installation
The 7A12 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 7A12 with the
oscilloscope tracks and
insert it .
The front panel will be
flush with the front of the
oscilloscope when the 7A12 is fully
inserted, and the latch
at the
bottom-left corner of the
7A12 will be in place
against the
front panel .
To remove
the 7A12, pull on the latch (which is
in-
scribed with the
unit indentification "7A12") and the
7A12 will unlatch .
Continue pulling on the latch to slide
the 7A12 out of the
oscilloscope .
FRONT PANEL CONTROLS
AND CONNECTORS
The following descriptions apply to the controls and
connectors of
both Input Amplifier channels when appli-
cable . See
Fig . 2-1 .
Input
Connector

Provides signal connection to
the
channel
.
AC-GND-DC-

Four position pushbutton switch
DC &
OFFSET

selects signal input coupling
mode .
AC-The
AC component of the
signal is coupled to the amplifier
input while the DC component
is blocked .
DC-Both AC and
DC components
of the signal
are coupled to the
amplifier input .
POSITION
IDENTIFY
VOLTS/D I V
Pushbuttons
VAR V/DIV
+UP/INV
Pushbuttons
7A12
GND-Grounds the
amplifier input
while maintaining the same load
for the input signal . Provides a
charge path for the AC coupling
capacitor to precharge the
input
circuit before
switching the
input to AC .
DC
&
OFFSET-Direct couples the
signal to the amplifier and pro-
vides DC offset of at least ± 500
divisions (± 1000 divisions at 5
mV/Div) .
Controls
vertical position of the
trace
.
Deflects trace up about 0 .3 div for
trace identification . Also replaces
readout with the word
"IDENTIFY" when used with
oscilloscopes equipped with a read-
out system .
Volts per major
graticule division .
Selects calibrated deflection factors
from 5 mV/Div to 5 V/Div ; 10
steps in a 1-2-5 sequence .
Provides continuously
variable un-
calibrated deflection factor
settings
between calibrated steps . Extends
deflection factor range to at least
12 .5 volts/division .
Provides
means of inverting the
display .
+UP-A positive-going signal at the
input deflects the CRT display
upward
.
INV-A positive-going
signal at the
input deflects the
CRT
display
downward .

Operating Instructions-7A12
2
.
2
nld rwmDe
x0n cx
Fig . 2-1 .Front-Panel Controls
=uConnectors .

OFFSET
TRIGGER
SOURCE
Pushbuttons
Variable
control permits on-screen
display
of small signal variations
superimposed
upon relatively large
DC
levels .
DISPLAY MODE

Pushbutton

selection

of

the
Pushbuttons

following modes of operation :
CH 1-A single trace display of the
signal applied to Channel
1 input
connector .
ALT-A dual trace
display of the
signals
applied to both channels .
The channels are alternately dis-
played and switching occurs at
the
end of each sweep .
ADD-Algebraically adds the signals
applied to the CH 1 and CH 2 input
connectors and the algebraic sum is
displayed
on the CRT .
CHOP-A dual trace display
of the
signal applied to both
channels . The
two channels time-share the
sweep
at a rate determined by the
main-
frame . An output signal is provided
for trace blanking during the
channel switching interval .
Selects source of trigger signal . The
trigger signals
provide internal trig-
gering
for the oscilloscope time-
base .
CH 1-Internal
triggering signal ob-
tained
from signal applied to
Channel 1
Input connector .
Mode-(Center
pushbutton) in-
ternal trigger
automatically follows
DISPLAY MODE
selection . In
ADD or CHOP
display modes, the
trigger signal
is the algebraic sum of
CH 1 and CH
2 trigger .
CH 2-Internal trigger
signal ob-
tained from signal
applied to
Channel 2 Input connector .
Operating Instructions-7A12
GENERAL OPERATINGINFORMATION
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
1 or CH 2 and
connecting the signal to be observed to the
appropriate
input . In the discussions to follow, single-trace
operation using CH 1 applies equally to CH 2 .
Signal Connections
In
general probes offer the most convenient means of
connecting a
signal to the input of the 7A12 . A X10 atten-
uator
probe offers a high input impedance and allows the
circuit under test to perform very close to operating con-
ditions . The Tektronix P6053 6 foot probe, with its
readout-coding ring, is designed specifically for use with the
Tektronix 7A series vertical amplifiers . The readout coding
ring on the probe connects to a circuit in the 7A12 which
automatically corrects the readout displayed on the CRT to
the actual deflection factor at the tip of the probe . The
Tektronix P6053 3 .5 foot probe is not recommended be-
cause it may compromise the aberration limits of this am-
plifier .
Vertical GainAdjustment
To check the gain of either
channel, set the VOLTS/D IV
switch to 10 mV and connect a 40
mV,
1
kHz signal from
the
oscilloscope calibrator to the Input connector 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 . Both channels
should be checked
and adjusted, if necessary, before
making measurements .
If the gain of the two
channels must be closely
matched (as in
differential measurements using the
ADD MODE), use
the adjustment procedure in
Section 5 .
Input Coupling
NOTE
The
Channel 1 and Channel 2 Input Coupling switches
allow a choice of
input coupling methods . The type of
display
desired and the applied signal will determine the
coupling to use .
The DC coupling position can be used
for
most appli-
cations . It must be used to display AC signals below about
30 hertz (10 hertz with a X10 probe) and squarewaves
with
low frequency components, as these signals will be atten-
uated in the AC position .
2- 3

Operating
Instructions-7A12
In the AC
coupling position, the DC component of the
signal is blocked by a capacitor 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 when there is a possibility
of having a
residual charge on the input capacitor of po-
larity opposite to
that of the intended input, and whose
algebraic
sum may be greater than the maximum input lim-
itations
of
the amplifier . To use this feature, first set
the
coupling switch to GND, then connect the probe to the
circuit and wait about two seconds
for the coupling ca-
pacitor to become charged . Push
the AC coupling switch to
AC
GND,
The GND
position provides a ground reference at
the
input of
the amplifier without externally grounding
the in-
put
connectors . However, the signals
connected to the
inputs
are not grounded, and the same DC load
is presented
to
the signal source .
With the DC &
OFFSET position, it is possible to use the
7A12 to
observe small signals whose DC component
may
be
quite
large
.
The OFFSET control is continuously
variable
and has a
range of at least +500 divisions to -500
divisions
except
in the 5 mV/DIV position, the range
is at least
±1000 divisions .
VOLTS/DIV Pushbuttons and VAR V/DIV Con-
trol
The amount of vertical deflection produced by a signal is
determined by the signal amplitude, the attenuation factor
of the
probe, the setting of the VOLTS/DIV switch and the
setting
of
the VAR V/DIV control . Calibrated deflection
factors indicated by the settings of the VOLTS/DIV switch
apply only when the VAR V/DIV control is in the CAL IN
position .
The VAR V/DIV control provides variable, uncalibrated
settings between the calibrated steps of the VOLTS/DIV
switch . With the VAR V/DIV switch set to 5 Volts/Div, the
uncalibrated vertical deflection factor
is extended to at
least 12 .5 volts/div . By
applying
a
calibrated voltage source
to the
input connector, any specific deflection factor can
be set within the range of the VAR V/DIV control .
+UP/INV Switch
The polarity of the displayed waveform
may be inverted
by the +UP/INV switch . This is
particularly useful in added
operation of the 7A12 when
differential measurements are
to be made .
2-
4
In the +UP position, the
displayed waveform will have
the same polarity as the
applied signal, and a positive DC
voltage will move the
CRT trace up . In the INV position, a
positive-going waveform at
the input of the 7A12 will be
displayed in an inverted
form and a positive DC voltage will
move the trace down .
DISPLAY
MODE Switch
The
CH 1 position of the DISPLAY
MODE switch is
used to display only that signal
present at the CH 1 Input
connector . To display
the signal present at the CH 2 Input
connector, simply
select the CH 2 mode of operation .
NOTE
Source of triggering
signal does not necessarily follow
the DISPLAY MODE, To
obtain stable display, refer
to TRIGGER SOURCE,
The
ALT position of the DISPLAY MODE switch pro-
duces a display
which alternates between Channel 1 and
Channel 2
with each sweep on the CRT . Although the
ALT
mode can be used at all sweep rates, the CHOP
mode pro-
vides a more satisfactory display
at sweep rates belowabout
0 .5 millisecond/division
. At slow sweep rates, alternate
mode switching
becomes visually perceptible .
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, or
for applying DC offset (apply
a DC voltage to one channel
to offset a DC
component on the other channel) . The over-
all 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 1 and CH 2 VOLTS/DIV
switches are set to dif-
ferent 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 ac-
curately only if the amplitude of the
signal applied to one
channel is known .
The CHOP position of
the DISPLAY MODE switch pro-
duces a display which is
electronically switched between
channels at a rate determined by the
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 are to be displayed .
At faster sweep rates, the chopped switching
transients may
cause improper triggering of the display .
TRIGGER SOURCE
Switch
The Ch
1 position of the TRIGGER SOURCE switch
provides a trigger signal obtained from the signal applied to

the CH 1 Input connector . This provides a stable display of
the signal applied to the CH 1 Input connector .
The CH 2 position of the TRIGGER SOURCE switch
provides a trigger signal obtained from the signal applied to
the CH 2 Input connector
. This provides a stable display of
the signal applied to the CH
2 Input connector .
In the Mode (center pushbutton)
position
of the
TRIG-
GER
SOURCE
switch, the trigger signal for the time-base
unit is dependent on the
setting of the DISPLAY MODE
switch . The trigger
source for each position of the
DISPLAY MODE switch is as follows :
Trace Identification
When the
POSITION Control is pressed, the trace is de-
flected about 0 .3 division to identify the 7A12 trace . This
feature
is particularly useful when multiple traces are dis-
played . In mainframes with readout, it also replaces de-
flection factor with the word "IDENTIFY ."
General
BASIC APPLICATIONS
The following information describes the procedures and
techniques for making basic measurements with a 7A12 and
the associated Tektronix oscilloscope and time-base . These
applications are not described in detail, since each ap-
plication must be adapted to the requirements of the indi-
vidual measurements . This instrument can also be used for
many applications which are 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 :
3 . Set the Coupling switch to AC .
Operating Instructions-7A12
1 .
Apply
the signal to
either Inputconnector .
2 . Set the DISPLAY MODE and TRIGGER SOURCE
switches to display the channel used .
NOTE
For low-frequency signals belowabout 30 hertz, use
the DC position to prevent attenuation of the signal .
4 . Set the VOLTS/DIV switch to display about five
vertical divisions of the waveform .
5 . Set the time-base Triggering controls for a stable dis-
play . Set the Time Base to a sweep rate which displays
several cycles of the waveform .
6 . Turn the 7A12 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 . 2-2) .
7 . Measure the divisions of vertical deflection peak-to-
peak . Check that the VAR V/DIV control is in the CAL IN
position .
Vertical
deflection
Position to
center vertical
line
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INNNENNEW
~rrr~rirr~r~~r~rrirrrr~r~
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EMMUMEONE
mmmmmmmmm
Fig .
2-2 . Measuring the Peak-to-Peak Voltage of a
waveform .
2-5
MODE TRIGGER SIGNAL SOURCE
CH 1 Channel l
CH 2 Channel 2
ADD
Algebraic sum
of
Channel 1 and
Channel 2
CHOP Algebraic sum of Channel 1 and
Channel 2
ALT Alternates
between Channel 1 and
Channel 2,

Operating
Instructions-7A12
This technique can
also be used to make measure-
ments between
two points on the waveform,
rather
than peak to peak
.
8 . Multiply the
distance measured in
step 7 by the
VOLTS/DIV
switch setting . Include the
attenuation factor
of the probe
if used .
EXAMPLE : Assume that the
peak-to-peak vertical
de-
flection is
4 .5 divisions (see Fig .
2-2) using a 10X atten-
uator probe and
the VOLTS/DIV
switch is set to 1 V .
vertical
VOLTS/DIV
probe
Volts P-P =

deflection

X

X
attenuation
(divisions)
setting factor
Substituting the
given values :
Volts Peak-to-Peak
=4 .5 X 1 X 10
The peak-to-peak
voltage is 45 volts .
Instantaneous
Voltage Measurements
(DC)
To measure
the DC level at a given point on a
waveform,
proceed as
follows :
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 .
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 .
2-6
NOTE
1 .
Connect the signal to either
Input connector .
NOTE
To measure a voltage
level with respect to a voltage
rather 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 and disconnect
the reference voltage
.
1
EMENEENEEN
EMENEEMENE
Vertical
distance
Reference line
Fig . 2-3 . Measuring
instantaneous voltage with
respect to some
reference .
5 . Set the Coupling
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 a stable
dis-
play . Set the
Time Base sweep
rate for an optimum display
of the
waveform .
7 . Measure the distance
in divisions between
the ref-
erence line and the
point on the waveform at
which the DC
level is to be measured
. For example, in Fig .
2-3 the meas-
urement is between
the reference line and
point A .
8 . Establish the
polarity of the waveform
. With the
+UP/INV switch
in the +UP position, any point
above the
reference line is
positive .
9 . Multiply
the distance measured
in step 7 by the
VOLTS/DIV
switch setting . Include the
attenuation factor
of the probe, if
used .
EXAMPLE
: Assume the vertical distance
measured is
3 .6 divisions (see
Fig . 2-3) and the waveform
is above the
reference line
using a 10X probe with a
VOLTS/DIV switch
setting of
0 .5 V .
Using the formula :
vertical

VOLTS/DIV
probe
distance X
polarity X

setting

X
attenuation
(divisions)

factor

Substituting the given values :
Instantaneous Voltage =3 .6 X 1 X 0 .5 VX 10
The instantaneous voltage is 18
volts
.
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 a reference amplitude . One
use for the
comparison-measurement technique is to facil-
itate calibration of equipment where the desired amplitude
does
not produce an exact number of divisions of de-
flection . 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 .
To establish an arbitrary vertical
deflection factor based
upon a specific reference amplitude, proceed
as follows :
1 . Connect the reference
signal to the Input connector .
Set the time-base sweep
rate to display several cycles of the
signal .
2 . Set the VOLTS/DIV switch and the VAR V/DIV
control to produce a display which is an exact number
of
vertical divisions in amplitude . Do not change the
VAR
V/DIV control after obtaining the desired deflection .
3
.
To establish an arbitrary vertical deflection factor so
the amplitude of an unknown signal can be measured accu-
rately at any setting of the VOLTS/DIV switch, the am-
plitude of the reference signal must be known . If it is not
known, it can be measured before the VAR V/DIV
control
is set in step 2 .
4 . Divide the amplitude of the reference signal (volts)
by the product of the vertical deflection
(divisions) estab-
lished in step 2 and the setting of the VOLTS/DIV
switch .
This is the vertical conversion factor .
Vertical
Conversion =
Factor
reference signal amplitude (volts)
vertical deflection X VOLTS/DIV setting
(divisions)
5 . To measure the amplitude
of an unknown signal, dis-
connect the reference signal
and connect the unknown
signal to the Input connector .
Set the VOLTS/DIV switch
Factor

4X
5 V
Operating Instructions-7A12
to a setting that provides sufficient vertical deflection to
make an accurate measurement . Do not readjust the VAR
V/D IV
control
.
6 . Measure the vertical deflection in divisions and cal-
culate the amplitude of the unknown signal using the
following formula :
Signal VOLTS/DIV
vertical vertical
Amplitude

setting

X
conversion X deflection
Ampl
factor (divisions)
EXAMPLE : Assume a reference signal amplitude of 30
volts, a VOLTS/DIV setting of 5 V and the VAR V/DIV
control
adjusted to provide a vertical deflection of four
divisions .
Substituting these values in the vertical conversion factor
formula (step 4) :
Vertical Conversion

=

30 V

=
1
.5
Then with a VOLTS/DIV
setting of 2 V, 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

=
2
V

X 1 .5 X
5 = 15 volts
Amplitude
Dual-Trace Phase Difference Measurement
Phase comparison between two signals
of
the same
frequency can be made using the dual-trace feature
of
the
7A12 . 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 pro-
cedure .
1 . Set the CH 1 and CH 2 Coupling 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 TRIG-
GER
SOURCE to
CH
1 .
3 . Connect the reference signal to the CH 1 Input and
the comparison signal to the CH 2 Input . Use coaxial cables
2-7

Operating
Instructions-7A12
or probes which
have similar time delay
characteristics to
connect the
signals to the Input connectors .
4 . If the signals
are of opposite polarity, push
the
channel 2 INV
button to invert the channel
2 display .
(Signals may be
of opposite polarity due to
180 ° phase
difference ; if
so, take this into account in
the final calcu-
lation .)
5 . Set the
VOLTS/DIV switches and the VAR
V/DIV
controls of the
two channels so the displays are equal
and
about five divisions in
amplitude .
6 . Set the time-base
for a sweep rate which
displays
about one cycle of the
waveforms . Set the Triggering con-
trols for a stable display .
7 . Center the
waveforms on the graticule
with the 7A12
POSITION controls
.
8 . Adjust the
time-base Variable Sweep Rate
until one
cycle of the reference
signal occupies exactly
eight hori-
zontal
divisions between the second and
tenth vertical lines
of the
graticule (see Fig . 2-4) . Each
division of the graticule
represents 45 ° of the cycle (360°
-
8 divisions =
45 ° /division) .
The sweep rate can now be stated
in terms of
degrees as 45 °
/division .
9 .
Measure the horizontal difference
between corre-
sponding
points on the two waveforms .
10 . Multiply
the measured distance
(in divisions) by
45 ° /division to obtain the
exact
amount of phase dif-
ference .
EXAMPLE : Assume a
horizontal difference of 0 .3
division with a sweep rate of
45 ° /division as shown in
Fig .
2-4 . Using the formula :
horizontal

sweep rate
Phase
Difference = difference X
(divisions)
(degrees/division)
Substituting the given values
:
2-8
Phase
Difference = 0 .3 X 45 °
The phase difference is 13 .5 ° .
Fig . 2-4
.
Measuring phase
difference between two signals .
HighResolutionPhase Measurements
More accurate dual-trace
phase measurements can be
made by increasing
the sweep rate (without changing the
Variable
Sweep rate control) . One of the easiest ways to
increase the
sweep rate is with the time-base Magnifier
switch . Set
the Magnifier to X10 and determine the mag-
nified sweep
rate by dividing the sweep rate obtained pre-
viously by the
amount of sweep magnification .
EXAMPLE : If the sweep rate is increased 10
times by
the
Magnifier, the magnified sweep rate is
45°/division
-
10
= 4 .5 ° /division . Fig . 2-5 shows the same
signals as used in
Fig . 2-4 but with the Magnifier set to X10 .
With a hori-
zontal difference of 3 divisions, the phase difference is :
horizontal magnified
Phase Difference =difference
X sweep rate
(divisions)
(degrees/division)
Substituting the given values
:
Phase Difference = 3 X 4 .5 °
The phase
difference is 13 .5 ° .
Common Mode Rejection
The ADD
feature of the 7A12 can be used
to display
signals which
contain undesirable components .
These un-
desirable
components can be eliminated through
common-
mode
rejection . The procedure is as
follows :
Channel 1
. ., .
NVANNNIN
NNE
I
nw~
Horizontal
Diffe rence a
ESSEN
8 Divisions
(360
°
) (360 0
)
1

Channel l
(reference)
Channel 2
EMEMMMMMMM
. .
.~
----------------
saw

. ., .
MENEM
MEMEMEMEME
EMMEMEMEME
- Horizontal,
I difference I
Fig . 2-5 . High resolution phase measurement using
sweep magnifier .
1 . Set the DISPLAY
MODE switch
to
ALT
or
CHOP
and
TRIGGER SOURCE switch to MODE .
2 . Connect the signal containing both the desired and
undesired information
to the CH 1 Input .
3 . Connect a signal similar to the unwanted portion of
the CH 1 signal to the CH 2 Input . For example,
in Fig . 2-6
Operating Instructions-7A12
a line-frequency
signal is connected to Channel 2 to cancel
out the line-frequency
component of the
Channel 1 signal .
4 . Set both Coupling switches to
the same setting, DC
or AC, depending on the applied
signal .
5 . Set the VOLTS/D
IV switches so the signals are about
equal in amplitude .
6 . Set the
DISPLAY MODE switch to ADD . Push the
channel 2
INV pushbutton so the common-mode signals are
of opposite polarity .
7 . Adjust the
channel 2 VOLTS/DIV and VAR V/DIV
control for
maximum cancellation of the common-mode
signal . The signal which remains should be only the
desired
portion of the channel 1 signal .
EXAMPLE : An example of
this mode of operation is
shown in Fig . 2-6 .
The signal applied to channel 1 contains
unwanted
line frequency components (Fig . 2-6A) . A corre-
sponding line frequency signal is connected to channel 2
(Fig . 2-6B) . Fig . 2-6C shows the desired portion of the
signal as displayed when
common-mode rejection is used .
The above
procedure can also be used for examining a
signal superimposed on
some DC level when DC Coupling
must be used .
ADC voltage of the proper polarity applied
to channel 2 can be
used to cancel out the DC portion of
the
signal applied to channel 1 .
EMEMEMMEMN
ZMEMEMEMEN
mmmmmmmm
MONMEEMMM
EMEMEMMEMM
Desired
sign
EMEMEMEMEN
EMEMEMMEME
MWOMEMEMEN
NONE MENEM
MEMMEMEMEM
EMEMMEMMEM
(A)
Channel 1 signal .

(B) Channel 2
signal .
(C) Resultant display .
Fig . 2-6 . Using the
ADD
feature
for common-mode rejection . (A) Channel 1 signal contains desired information along
with line-frequency
component, (B) Channel 2 signal contains
line-frequency only,
(C)
CRT display using common-mode
rejection .

Introduction
SECTION 3
CIRCUIT DESCRIPTION
This
section of the manual contains a
description of the
circuitry used in the 7A12
dual-trace amplifier . The de-
scription begins with a discussion of
the instrument using
the block diagrams shown in the
Diagrams Section . Then,
each circuit is described in
detail using block diagrams to
show the interconnections between stages
in each major
circuit
and the relationship of the
front-panel controls to
the
individual stages .

.
Complete schematics
of each circuit are given in the
Diagrams Section . Refer to
these schematics thoughout the
following circuit description
for electrical values and rela-
tionship .
BLOCK DIAGRAM
The following discussion
is provided to aid in under-
standing the overall concept of
the 7A12 before the in-
dividual circuits are discussed
in detail . Only the basic inter-
connections between the
individual blocks are shown on
the block diagram (see
Diagrams Section) . Each block rep-
resents a major
circuit within the instrument . The number
on
each block refers to the schematic on which the
complete circuit is found .
Block Diagram
Description . The signal to be
displayed
on the CRT is
applied to the Input connector . The signal
passes
through the input coupling relay, where the appro-
priate
coupling is selected, to the attenuators . The
VOLTS/
DIV switch selects the correct attenuator(s) . The
signal is
passed through an isolation stage to the input
amplifier . DC
offset signal is provided by
this
stage
.
The amplifier stage
provides signal polarity inversion,
gain setting, variable gain
circuit, and vertical' positioning .
The output of this stage
is applied push-pull to the signal
and trigger channel
switching stages .
The channel switching
stage electronically selects the
proper signal
and trigger according to' the logic supplied by
the switching
logic state . The signal and trigger outputs of
the channel
switching stages correspond to the setting of
the DISPLAY MODE and
TRIGGER SOURCE switches .
The switching logic stage provides the control signals for
the channel switching stages . The digital logic techniques
used
in this stage will
be described under Detailed
Circuit
Description .
In addition
to the functions of the
amplifier just ex-
plained, the 7A12
also provides readout logic
for the
oscilloscope . Logic is
supplied identifying the polarity,
the
deflection factor, the
uncalibrated symbol (when the
VAR
V/D IV control is in
the out position) and the plug-in
mode .
When the IDENTIFY
button is pressed, the readout
is re-
placed by the
word IDENTIFY and the trace representing
the plug-in is
deflected upward about 0 .3 division
for iden-
tification .
DETAILED CIRCUIT
DESCRIPTION
Introduction
The 7A12
consists of two identical channels .
Therefore,
the following description of
channel 1 applies equally to
channel 2 . Any functional differences
will be pointed out in
the description . Throughout
the discussion, you should
refer to the block and
circuit diagrams located near the
back of this manual .
Input Coupling
Input signals applied to the Input connector can be
AC-
coupled,
DC-coupled or internally disconnected . The input
coupling relay,
K5 is actuated by S9 (S10 for CH 2) . When
the
AC button is pressed, K5B is actuated, providing a
signal path through C1-R1 and C2 to the attenuators . C2
blocks the DC component of the signal
.
With
the GND
button pressed, K5 is not actuated
and the signal path is
through C1-R1, C2 and R3 to ground . In this position,
the
amplifier input is grounded while the impedance presented
to the load
remains the same . R3 allows C2 to be pre-
charged in the GND position so the trace remains on screen
when the AC button is pressed with a high DC level applied .
With the DC button pressed, K5A is actuated,
providing a
signal path through C1-R1 to the attenuators .
The signal
path for DC & OFFSET is
the
same
as for DC .
Input Attenuator
ATTENUATORS
7A12
The effective overall
deflection factor of the 7A12 is
determined by the
setting of the VOLTS/DIV switch . The

Ciicuit
Description-7A12
basic deflection factor is 10 millivolts per division of
CRT
deflection . To increase this basic deflection
factor to the
values indicated on the front panel,
precision attenuators
are switched into the circuit . In the 5 and 10
millivolt
positions, input attenuation is not used . Instead, the gain of
the integrated circuit (U225) is changed in the 5
millivolt
positions by switching off 0205, which
turns on emitter-
coupled Q215 to decrease
the deflection factor .
For switch positions above 10 millivolts,
the attenuators
are switched into the circuit
singly
or
in pairs to produce
the vertical deflection factor
indicated on the front panel .
These attenuators
are frequency-compensated voltage
dividers . For DC and
low-frequency signals, the attenuators
are primarily resistance
dividers and the voltage attenuation
is determined by
the resistance ratio in the circuit . The
reactance of the capacitors in the circuit is so high at low
frequencies that their effect is negligible .
However, at
higher frequencies, the reactance of the capacitors decreases
and the attenuator becomes primarily a capacitance
divider .
In addition to providing constant attenuation at
all
fre-
quencies within the bandwidth of the instrument, the input
attenuators are designed to maintain the same input RC
characteristics (one megohm X 24 pF) for each setting of
the VOLTS/DIV switch . Each attenuator contains an
ad-
justable series capacitor to provide correct attenuation
at
high frequencies, and an adjustable shunt capacitor to pro-
vide correct input capacitance .
INPUT
AMPLIFIER and OFFSET GENERATOR
The input amplifier and offset generator incorporates a
single ended input for use in driving a paraphase type
dif-
ferential amplifier (integrated circuit U225) .
The outputs
include a signal line, an offset
line, and
a
bootstrap line .
The DC mode is used to
balance DC currents through the
entire amplifier
. The DC OFFSET mode works in a similar
manner to balance out
the DC component of the input
signal . This
allows an offset range of at least +1000
divisions to -1000 divisions .
The signal from the attenuator is
coupled to the gate of
the high input impedance, unity gain
amplifier Q25A-
027A . Current stabilization
for
the
pair is provided by
Q29 . R21, along with the attenuators, provides
a constant
one megohnh input . C23 and R28 provide frequency
com-
pensation . Dual diode clipper CR23 and CR24
protects the
input against being overdriven by limiting the gate
of Q25A
to ±15 volts .
Offset generator Q25B
and Q27B is
a
unity gain
amplifier stabilized by constant
current source 035 . In the
DC & OFFSET coupling mode, an offset
voltage from
about +5 volts to about -5 volts is
supplied
to
the gate of
3- 2
Q25B
by variable resistor R56 . Emitter follower Q27B
supplies the offset voltage used to balance the differential
input of U225 . CR33 and CR34 limit the differential input
to U225 to ±0 .6 volt, thus protecting the integrated circuit .
In addition to a balanced output, the generator provides
a bootstrap voltage to the integrated circuit U225 . This
voltage maintains the quiescent operating parameters of the
paraphase amplifier (see Fig . 3-1) . 032 is a constant
current
generator giving a fixed potential across
R33 . Therefore, in
a balanced condition the potential
between the signal out
and the bootstrap out is kept
constant .
The potential
from the base of the floating power supply,
Q30
to
the emitter of Q27B is held constant by constant=
current generator Q32 and resistors R32 and R33
.
The
voltage between
drain and source
of 025A
and Q25B is
therefore kept at a
constant potential . Temperature
stablization is
increased through the use of dual FET (Field
Effect
Transistor) 025 and dual transistor Q27 .
AMPLIFIER CIRCUIT
The amplifier circuit converts the single-ended signal
input to a differential output for use in oscilloscope vertical
deflection system . It also furnishes a trigger signal for
internal triggering of the time base unit .
Gain-switching circuit Q205-Q215 increases the
sensitivity in the 5 mV position of the VOLTS/DIV switch .
For all positions of the VOLTS/DIV switch except 5
mV/div, Q205 is conducting and Q215 is turned off . The
current for U225 is from Pins 2 and 15 through 0205 .
When the 5 mV pushbutton is pressed, the junction of
R200-R201 is grounded through CR60 on the VOLTS/DIV
switch board . This makes the base of 0205 more negative,
turning it off . Now the current generator for U225 is from
Pins 1 and 16 through Q215 . R217 is adjustable to DC
balance the amplifier in the 5 mV position . CR215 is ad-
justable for high-frequency compensation in the 5 mV
position of the
VOLTS/DIV switch .
C205
provides high-
frequency compensation for the remaining positions of the
VOLTS/DIV switch .
The Tektronix-made integrated circuit (IC) U225
provides gain control, polarity switching, and vertical
positioning . Refer to the block diagram in Fig . 3-1, keeping
in mind that it is only a representation to be used in
explaining the overall operation of the IC .
The signal from the Input Amplifier is applied to U225
at Pin 14 . A DC level is applied to Pin 3 to balance out DC
levels present at Pin 14 . Proper quiescent operating
conditions for the Paraphase Amplifier are provided by the

Fig .
3-1
.
Block representation of U225
.
Circuit
Description-7A12
3-3

Circuit
Description-7A12
ALT
ADD
CHOP
~i I

I
l
I
15 V

I

I
[
C
H 72
II___2L ?J
SSih
y y
4
14
Fig . 3-2 . Logic diagram and input/output table for trigger selection .
(
I
CH 1 HI LO CH 1
CH 1 Mode HI LO CH 1
CH 2 LO LO CH 2
CH 1 HI LO CH 1
ALT Mode HI-LO LO Alternates between CH 1 and CH 2
CH 2 LO LO CH 2
CH 1 HI LO CH 1
ADD
Mode LO
HI CH 1 and CH 2 added
CH 2 LO LO CH 2
CH 1 HI LO CH 1
CHOP
Mode
LO HI CH 1 and CH 2 added
CH 2 LO LO CH 2
CH 1 HI LO CH 1
CH 2
-
Mode
,
-Lo
1 Lo
l
CH 2
CH 2 LO LO CH 2
--
CH
2
I Trigger
ource wtcO
INPUT OUTPUT
a
Display
Mode a
o
.~o
c
a° 0 .o~
U300 (CH 2)
Oscilloscope
Switch ~ o
Co
3
o
A y~y3 Pins Trigger
Signal
Source