Tektronix 7a15a User manual

1 N STFR U
CTI
O
N
MANUAL .
Serial Number
Tektronix, Inc . 0
P .
O .
Box 500 " Beaverton, Oregon 97005 " Phone [phone] " Cables : Tektronix
070-1210-00

871

All
Tektrc
;mx

are '<arra
;-ited aaainst
defective r - r~ateriaic . end v~orknianship for one - gear .
Any questions vdit~o respect to the vurarrarrty,
mentioned above, should be taken up 'with your
Tektronix Field Ericw-er o ;' r~~presentaiii - r .
AIi
should
b,
repre5entatrVe ?n i . ,
you the

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instrument
airs and replaceniesit parts
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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
7A 15A/7A 15AN
SECTION 1
SPECIFICATION Page
SECTION5 CALIBRATION
Page
Introduction 1-1
Introduction
5-1
Electrical Characteristics 1-1
Test Equipment Required
5-1
Environmental Characteristics 1-2
Short Form Procedure 5-2
Physical Characteristics 1-2
Calibration Procedure
5-2
SECTION 2 OPERATING INSTRUCTIONS
SECTION 6
ELECTRICAL PARTS LIST
Installation
2-1
Controls and Connectors
2-1
Abbreviations and Symbols
Basic Operation
2-2
Parts Ordering Information
General Operating Information
2-2
Applications 2-3
SECTION 7 DIAGRAMS AND CIRCUIT
BOARD
SECTION 3 CIRCUIT
DESCRIPTION
ILLUSTRATIONS
Introduction 3-1
Symbols and Reference Designators
Block Diagram Description
3-1
Detailed Circuit
Description 3-1
SECTION 8 MECHANICAL
PARTS LIST
SECTION4 MAINTENANCE
Mechanical
Parts List Information
Preventive
Maintenance 4-1
Index of Mechanical Parts
Troubleshooting
4-1 Illustrations
Corrective
Maintenance
4-3 Mechanical Parts
List

Fig . 1-1 . 7A15A and 7A15AN Amplifier .

Introduction
The 7A15A/7A15AN Amplifier plug-in unit is a wide
band amplifier designed for use with Tektronix 7000-Series
oscilloscopes . The 7A15A and 7A15AN are electrically
identical except that encoding capabilities and an
"IDENTIFY" function are provided in the 7A15A . The
SECTION 1
SPECIFI
CA
T
ION
TABLE 1-1
ELECTRICAL
7A15A/7A15AN
7A15A/7A15AN can be operated in any plug-in compart-
ment of 7000-Series oscilloscopes .
The following electrical characteristics are valid over the
stated environmental range for instruments
calibrated at
an
ambient temperature of +20 ° C to +30 ° C, and after a five
minute warmup unless otherwise noted .
Characteristic Performance Requirement
Supplemental Information
Deflection Factor
Calibrated Range 5 mV/D iv to 10 V/D iv, 11 steps in a 1-2-5
sequence
Deflection Factor Accuracy Within 2% of indicated deflection factor
with GAIN adjusted at 10 mV/Div
Uncalibrated (VARIABLE) Continuously variable between calibrated
steps ; extends deflection factor to
at least
25 V/Div
X10
GAIN
Increases amplifier gain by a factor of 10
within 10%,
decreasing deflection factor
to 500 /1V/Div
Frequency Response
Upper Bandwidth Frequency
(at -3 dB)
X1 (System Dependent) With 7900-series With 7700-series
80 MHz 75 MHz
With 7500-series With 7400-series
60 MHz 50 MHz
X10 10 MHz
Low-Frequency Response 10 Hz or less without probe
(Lower -3dB point) 1 Hz or less with probe
AC (Capacitive) coupled

Specification-7A15A/7A15AN
TABLE 1-2
ENVIRONMENTAL CHARACTERISTICS
Refer to the specifications for the associated oscilloscope
TABLE 1-1 (cont)
TABLE
1-3
PHYSICAL
Characteristic
Performance Requirement Supplemental
Information
Maximum Input Voltage
DC Coupled
250 V (DC
+
Peak AC) ; AC component
500 V P-P maximum, 1 kHz or less
AC Coupled
500 V (DC
+
Peak AC)
;
AC component
500 V P-P maximum, 1
kHz
or
less
Input R and C
Resistance
Approximately 1 megohm
Capacitance
Approximately 20 pF
RC
Tolerance Within 1% at
all deflection factors
Overdrive Recovery Time
0 .1 ms or less to recover to within one
division after removal of overdrive signal
of up to +75 divisions to -75 divisions
regardless of overdrive signal duration
Size
Fits all 7000-series plug-in
compartments
Weight
1 pound 13 ounces (0 .82 kilogram)

General
To effectively use the 7A15A/7A15AN, the operation
and capabilities of the instrument must be known . This
section describes front-panel control functions, general
information on signal input connections, and other subjects
that pertain to various measurement applications .
Installation
SECTION 2
OPERATING
INSTRUCTIONS
The 7A15A/7A15AN is calibrated and ready for use as
received . It can be installed in
any
compartment of
Tek-
tronix 7000-Series Oscilloscopes, but
is
intended
for use
primarily in vertical plug-in
compartments . To install, align
the upper and lower
rails of the 7A15A/7A15AN with the
oscilloscope tracks
and insert the plug-in . The front panel
will
be
flush with the front of the oscilloscope when the
7A15A/7A15AN is fully inserted, and the latch at the
bottom-left
corner will be in place against the front panel .
MAG
Input Connector
To
remove the 7A15A/7A15AN, pull on the latch
(which is inscribed with the unit identification "7A15A" or
"7A15AN") and the 7A15A/7A15AN will unlatch .
Continue
pulling on the latch to slide the 7A15A/7A15AN

AC-GND-DC
out of
the oscilloscope .
GAIN Adjustment

Screwdriver adjustment permits
calibration of deflection factor .
FRONT PANEL CONTROLS
AND CONNECTORS
VOLTS/DIV

Selects calibrated deflection factors
from 5 mV/Div to 10 V/Div ; 11
steps in a 1-2-5 sequence .
VARIABLE

Provides continuously variable un-
calibrated settings between
calibrated steps . Extends the deflec
tion factor range to 25 volts/
division or
more .
POSITION
POLARITY

Provides
means of inverting the
display .
IDENTIFY
+UP : A positive-going signal at the

(7A15A Only)
Input connector deflects the CRT
display upward .
7A15A/7A15AN
INVERT : A positive-going signal at
the Input connector deflects the
CRT display downward .
Provides means of decreasing the
deflection factor .
X1 : Deflection factor is that
selected by VOLTS/DIV and
VARIABLE controls .
X10 REDUCED BANDWIDTH
:
Deflection factor is one-tenth of
the value selected by the VOLTS/
DIV and VARIABLE controls .
Provides signal connection to the
amplifier .
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 .
GND : Grounds the
amplifier input
while maintaining the same load for
the input signal . Provides a charge
path for the AC coupling capacitor
to pre-charge the input circuit
before switching the input to AC .
Controls position of the trace
.
Deflects
trace about 0 .3 division for
trace identification . In instruments
with readout, also replaces readout
with the word "IDENTIFY"
.

Operating
Instructions-7A15A/7A15AN
General . This procedure demonstrates the use of the
connectors and
controls of the 7A15A/7A15AN, while
at
the
same
time providing a means of checking the basic
operation of the instrument .
Preliminary Setup . Install the 7A15A/7A15AN into any
7000-series oscilloscope vertical compartment and set the
oscilloscope VERTICAL MODE and TRIGGER SOURCE
to the proper settings .
Install a 7-series time-base unit into a horizontal
compartment and set the oscilloscope HORIZONTAL
MODE to the proper setting . Set the time-base unit to a
sweep rate of one-millisecond per division and set the
triggering mode to AUTO .
Procedure
1 . Set the 7A15A/7A15AN AC-GND-DC switch to
GND and position the trace to the center of the graticule .
2 .
Set the
VOLTS/DIV switch to 10 mV and apply a 40
mV 1 kHz square-wave signal from the oscilloscope CALI-
BRATOR to the 7A15A/7A15AN INPUT connector . Set
the AC-GND-DC switch to DC and check for a four-division
display .
3 . Set POLARITY switch to INVERT and check that
the displayed signal is inverted as compared to its appear-
ance in step 2 .
4 . Set the AC-GND-DC switch to AC and check that the
trace is centered on the CRT graticule .
5 . Set the oscilloscope CALIBRATOR to 4 mV and the
7A15A/7A15AN
MAG switch to X10
.
Check for
a
four-
division display .
6 . (7A15A Only) Press the 7A15A IDENTIFY button,
check that the display moves upward approximately 0 .3
division and the readout (if so equipped) reads
"IDENTIFY" .
GENERAL OPERATINGINFORMATION
Signal Connections
BASIC OPERATION
In general, probes offer the most convenient means of
connecting a signal to the input of the 7A15A/7A15AN . A
10X attenuator probe offers a high impedance and allows
2-2
the circuit under test to perform very close to normal
operating conditions .
The Tektronix P6053 probe is equipped with a readout
coding ring which connects to a circuit in the amplifier
unit . This automatically corrects the readout displayed on
the CRT to the actual deflection factor at the tip of the
probe . This probe is recommended for use with the 7A15A
and an oscilloscope equipped with readout . The Tektronix
P6054 probe is electrically identical to the P6053 but is
intended
for
use with systems not equipped with
readout .
For more information refer to the Tektronix Inc . catalog
.
Vertical Gain Check
and Adjustment
To check the gain of
the 7A15A/7A15AN, set the
VOLTS/D IV switch
to 10 mV and connect a 40 mV, 1
kHz
signal from the
oscilloscope calibrator to the Input
connec-
tor . The
vertical deflection should be exactly four
divisions
.
If not, adjust
the front panel GAIN for exactly four
divi-
sions .
Input
Coupling
The AC-GND-DC switch allows a choice of input coup-
ling 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 applica-
tions . For AC signals with frequencies below about 30 Hz
(10 Hz with a 10X probe), and square waves whose
low-
frequency components are important to the display, it is
necessary to use DC coupling to obtain a satisfactory
presentation .
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 component .
The
pre-charge feature should be used when
there
is
a
possibility
of having a residual charge on the
input capacitor of the
opposite polarity to the
intended input, and when the
algebraic sum of the
combination of charges may be greater
than the maximum
input limitations 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 capacitor to charge, set the
coupling switch to AC .
The GND position provides a ground
reference at the
input of the amplifier without
externally grounding the
Input connectors . However, the signals
connected
to
the
inputs are not grounded,
and the same DC load is presented
to the signal source
.

VOLTS/DIV, VARIABLE and MAG Controls
The amount of deflection produced by a signal is deter-
mined by the signal amplitude, the attenuation factor of
the probe, the setting of the VOLTS/DIV switch, the
setting of the VARIABLE Control, and the setting of the
MAG
switch . Calibrated deflection factors represented by
the setting
of
the VOLTS/DIV switch
apply only when the
VARIABLE
control is in the CAL position (fully clock-
wise) and when the MAG switch is set to X1 .
The
VARIABLE
control provides variable uncalibrated
settings
between the calibrated steps
of
the
VOLTS/DIV
switch . With the
VARIABLE
control
set
fully counter-
clockwise and the VOLTS/DIV switch set to 5 volts/div,
the uncalibrated deflection factor is extended to at least 2 .5
times
the attenuator setting
.
By applying a calibrated
voltage source to
the Input connector, any
specific deflec-
tion factor can be set within the range
of
the VARIABLE
control .
The MAG switch increases the VOLTS/DIV switch
sensitivity by a factor of 10 when in the X10 position .
Thus, with the MAG switch set to X10 and the VOLTS/
DIV switch set to 5 mV/div, the deflection factor is
extended to 500 pV/div . The MAG switch also effects a
circuit in the 7A15A
which
corrects the readout . With the
MAG switch set to X10, the bandwidth of the amplifier
is
reduced . Refer to specifications in Section 1 .
Polarity Switch
The
POLARITY
switch provides a means of inverting
the displayed signal . With the POLARITY
set to +UP, a
positive-going signal at the INPUT produces an upward
deflection of the
CRT
display . With the POLARITY set to
INVERT,
a positive-going
signal will
produce a downward
deflection of the CRT display
.
Trace Identification (7A15A only)
When
the
IDENTIFY button is
pressed, the trace is
deflected upward about 0 .3 division
to
identify
the 7A15A
trace . This feature is particularly useful when
multiple
traces are displayed on the CRT . In mainframes with read-
out, it also replaces the deflection factor readout with the
word "IDENTIFY" .
General
APPLICATIONS
The following information describes the procedures and
techniques for making measurements with a 7A15A/
7A15AN and the associated Tektronix oscilloscope and
time-base . These applications are not described in detail,
since each application must be adapted to the requirements
Operating Instructions-7A15A/7A15AN
of the individual 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 measure-
ments with this instrument .
Peak-to-Peak Voltage Measurements (AC)
To make peak-to-peak voltage measurements, use the
following procedure :
1 . Apply the signal to the Input connector .
2 . Set the Coupling switch to AC .
NOTE
For low-frequency signals below about 30 hertz, use
the DC position to prevent attenuation of the signal .
3 . Set the VOLTS/DIV switch to display about five
vertical divisions of the waveform
.
4 . Set the time-base
Triggering controls
for a stable
display . Set the Time Base to a sweep rate which displays
several cycles of the waveform
.
5 .
Turn the 7A15A/7A15AN 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 .
2-1) .
6 .
Measure the divisions of vertical deflection peak to
peak . Check that the VARIABLE control is in the CAL
position .
NOTE
This technique can also be used to make measure-
ments between two points on the waveform, rather
than peak to peak .
7 . 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
deflection is
4
.5
divisions using a
10X attenuator probe,
and the
VOLTS/DIV switch is set to 1 V .
2- 3

Operating Instructions-7A15A/7A15AN
Vertical
deflection
Position to
center vertical
line
MOMEME
EMENHEMENE
OWNER MENOM
EL FAE MK
EMEMSEENEE
Fig . 2-1 . Measuring the Peak-to-Peak voltage of a waveform .
Substituting the given values :
The peak-to-peak voltage is 45 volts .
If you are using a 7A15A with a coded probe and an
oscilloscope equipped with readout, simply multiply the
distance
measured in step 7 by the deflection
factor
displayed on the CRT .
Instantaneous Voltage Measurements (DC)
To measure the DC level at a given point on a waveform,
proceed as follows :
2 . Set the VOLTS/DIV switch to display about five divi-
sions .
3, 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- 4
Volts
vertical
VOLTS/DIV
probe
deflection X

X attenuator
(divisions)
setting
factor
Volts Peak to Peak =4 .5 X 1 X 10
1 . Connect the signal to the Input connector .
NOTE
To
measure a voltage level with respect
to
another
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 .
4 . Set the Coupling switch to DC . The ground reference
line can be checked at any time by switching to the GND
position .
5 . Set the time-base Triggering controls for a stable
display . Set the Time Base sweep rate for an optimum
display of the waveform .
6 .
Measure the distance in divisions between the
reference line and the point on the waveform at which the
DC level is to be measured . For example, in Fig . 2-2 the
measurement is between the
reference
line and point A .
7 . Establish the polarity of the waveform . With the
+UP/INV switch in the +UP position, any point above the
reference line is positive .
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 the vertical distance measured is 3 .6
divisions (see Fig . 2-2) and the waveform is above the
reference line using a 1OX probe with a VOLTS/DIV switch
setting of 0 .5 V .
Using the formula :
Instan- vertical

VOLTS/ probe
taneous =

distance

X Polarity X

DIV

X attenuation
Voltage (divisions)

setting factor
Substituting the given values :
Instantaneous Voltage = 3 .6 X 1 X 0 .5 V X
10
The instantaneous voltage is 18 volts .

MEMO =0
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MMEMEMEMEM
"""OMEN"
i"
MOMOMMINNION
1NNMMMMMWM
MMMEME/
" SE
MOMMEMOMME
MEMEMEMEMIN
Vertical
distance
Reference line
Fig . 2-2 . Measuring instantaneous
voltage with respect to same
reference .
Comparison
Measurements
In
some applications it may be desirable to
establish
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
facili-
tate
calibration of equipment where the desired amplitude
does
not produce an exact number of divisions of deflec-
tion
. The adjustment will be easier and more accurate if
arbitrary units of measurement are established, so that the
correct
adjustment is indicated by an exact number of divi-
sions of deflection
. The following procedure describes
how
to
establish arbitrary units of measure for
comparison
measurements .
To establish a
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
VARIABLE
control to produce a display which is an exact
number of
vertical divisions in amplitude . Do
not change the
VARIABLE control after obtaining
the desired deflection .
3 . To establish an arbitrary vertical
deflection factor so
the amplitude of an unknown
signal can be measured
accurately at any setting of the
VOLTS/DIV switch, the
amplitude of the reference
signal must be known . If it is
Vertical
Conversion
Factor
Operating Instructions-7A15A/7A15AN
not known,
it can be measured before the
VARIABLE
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 .
reference signal
amplitude ( volts)
vertical
deflection

X VOLTS/DIV
(divisions)

setting
5 . To
measure the amplitude
of an unknown signal,
disconnect the reference
signal and connect the unknown
signal to the Input connector
. Set the VOLTS/DIV switch
to a setting that
provides sufficient vertical deflection to
make an accurate
measurement . Do not re-adjust
the
VARIABLE control
.
6 . Measure the vertical deflection in divisions
and calcu-
late the
amplitude of the unknown signal
using the
following formula :
Signal VOLTS/DIV
vertical vertical
Amplitude

setting

X
conversion X deflection
factor (divisions)
EXAMPLE : Assume a reference signal
amplitude
of
30
volts, a VOLTS/DIV setting of 5
V and the VARIABLE
control adjusted to
provide a vertical deflection of four
divisions .
Substituting these values in the vertical conversion factor
formula (step 4) :
Signal
Amplitude
Vertical Conversion

30 V

=

1
.5
Factor

4 X 5 V
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)
:
=

2 V

X

1 .5

X

5

=

15
volts
2-5

NOTES

Introduction
SECTION 3
CIRCUIT
DESCRIPTION
This section of
the manual contains a description of the
circuitry used in
the 7A15A/7A15AN amplifier . The
description
begins with a discussion of the instrument using
the block diagram
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 throughout the
following circuit description for electrical values and
relationship .
BLOCK DIAGRAM
The following discussion is provided to aid
in under-
standing the overall concept of
the 7A15A/7A15AN before
the individual
circuits are discussed in detail . Only the basic
interconnections between the individual blocks are shown
on
the block diagram (see Diagrams section) . Each block
represents a major circuit within the instrument . The
numberon each block refers to the
schematic on which the
complete circuit is
found .
The
signal
to
be displayed on the CRT is applied to the
Input connector . The signal passes through the input
coupling switch, where the appropriate coupling is selected,
to the attenuators . The VOLTS/DIV switch selects the
correct amount of attenuation and the signal is passed to
the Input Amplifier .
The Input Amplifier provides signal polarity inversion in
addition to gain setting, variable gain
control, and trace
positioning . The output of this circuit is applied
push-pull
to the Signal and Trigger Amplifiers .
The signal and trigger outputs
are provided to the
oscilloscope via the Interface
connector .
The Readout encoding
circuit (7A15A only) provides
readout
logic for the oscilloscope readout system . Logic is
supplied
identifying the polarity, deflection factor, and the
7A15A/7A15AN
uncalibrated symbol (when
the VARIABLE knob is
not
fully clockwise) . When
the IDENTIFY button is pressed,
the trace is deflected about 0 .3 division and the deflection
factor readout is replaced by the word "IDENTIFY"
(7A15A only) .
General
DETAILED CIRCUIT DESCRIPTION
Attenuator
The Attenuator circuit determines the input coupling
and the deflection factor . A diagram of this circuit is shown
on Diagram 2 in the Diagrams section .
AC-GND-DC Switch
Input signals connected to the Input connector can be
AC-coupled, DC-coupled, or internally disconnected .
S100A is a cam-type switch ; a contact-closure chart show-
ing the operation is given on Diagram 1 .
The
dots on
this
chart indicate
when
the
associated contacts are in the
position shown (open or closed) .
When the AC-GND-DC
switch is
in the DC position, the input signal is coupled
directly
to
the Input Attenuator stage . In the AC position,
the input signal passes through capacitor C10 . The
capacitor prevents the DC component of the signal from
passing to the amplifier . The GND position opens the signal
path and connects the input circuit of the amplifier to
ground . This provides a ground reference without the need
to disconnect the applied signal from the Input connector .
Resistor R102, connected across the AC-GND-DC switch,
allows C10 to be pre-charged in the GND position so the
trace remains on screen when switching to the AC position
if the applied signal has a high DC level .
Input Attenuator
The effective overall deflection factor of the 7A15A/
7A15AN is determined by the setting of the VOLTS/DIV
switch, S100B . The basic deflection factor is five millivolts
per division of CRT deflection (with Mag switch set to X1) .
To increase the basic deflection factor to the values indica-
ted on the front panel, precision attenuators are switched
into
the circuit
.
S100B is
a cam-type
switch and the dots on
the
contact-closure chart
(see Diagram 1)
indicate when the
associated contacts are in the position shown (open or
closed) . In the 5 mV/Div position, input attenuation is not
used ; the input signal is connected directly to the input
amplifier .

Circuit
Description
-7A15A/7A15AN
For switch positions above five millivolts, the attenua-
tors are switched into the circuit singly or in pairs to pro-
duce the deflection factor indicated on the front panel .
These hybrid
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 . How-
ever, at higher frequencies, the reactance of the capacitors
decreases and the attenuator becomes primarily a capaci-
tance divider .
In
addition to
providing constant attenuation at all
frequencies within the bandwidth of the instrument, the
Input
attenuators are designed to maintain the same input
RC
characteristics (one megohm X 20 pF) for each setting
of
the VOLTS/DIV switch . Each attenuator contains an
adjustable series capacitor to provide correct attenuation at
high frequencies, and an adjustable shunt capacitor to pro-
vide correct input capacitance .
General
Input Amplifier
The Input Amplifier
converts the single-ended signal
applied to the Input
connector
to
a differential (push-pull)
output . A schematic of this circuit is shown on Diagram
2
in the Diagrams section .
Input Source
Follower
The Input Source
Follower 0210A provides
a
high input
impedance with a low
impedance drive for the following
stage . R210 limits
the current drive to the gate of Q210A .
Dual-diode
CR210 provides circuit protection by limiting
the voltage
swing at the gate of Q210A to about ± (positive
or negative) 15 volts . Q210B provides a constant current
source for 0210A . Q210A and Q210B are encapsulated in
the same case so that Q210B temperature compensates the
circuit .
Amplifier
The signal from the Input Source Follower is applied to
paraphase amplifier Q220-Q320 .
The
paraphase
amplifier
converts the single-ended
input
to a
differential (push-pull)
output . It also provides a
means
of
compensating for stray
currents throughout the entire
amplifier
by
varying the DC
Level at the base of Q320
via the DC Bal control R322 . The
differential
signal from the paraphase amplifier is cascoded
to the
Inverting Amplifier, a set of common base differen-
tial
amplifiers Q230-Q330 and Q235-Q335 . With the
POLARITY switch set to +UP, Q230 and Q330 are forward
biased while Q235
and Q335 are reverse biased . The signal
is
therefore allowed to pass un-inverted through
Q230-Q330 . By setting the POLARITY switch to INVERT,
3-2
0230 and Q330 are
reverse biased and Q235 and Q335 are
forward biased . The
signal is inverted through Q235-Q335 .
Current gain
for amplifiers Q230-Q330 and Q235-Q335 is
controlled by
the GAIN potentiometer R238 and
VARIABLE
control R239 . The output from Q230-Q330 or
Q235-Q335 (depending
on the POLARITY switch) is
connected to
the
X1
amplifier, 0240-Q340, and the X10
amplifier,
Q245-Q345 . The MAG switch determines which
amplifier (X1 or X10) is on, by switching their emitter
supply voltages . Current gain for the X10
amplifier is
adjusted by R245 . R341
and
C341
provide frequency com-
pensation for the X1
amplifier . The signal from the X1 or
X10 amplifier (depending
on the MAG switch) is cascoded
through
the common base amplifier, Q250-Q350, to the
Signal Amplifier, Q260-Q360, and Trigger Amplifier,
Q270-Q370 .
General
The Connectors and Readout circuit consists of the
power supply and signal distribution from the Interface
Connector and the Readout Encoding circuit . A schematic
of this circuit is shown on Diagram 3 in the Diagrams
section .
Connectors
Connectorsand Readout
All
the connections made to the mainframe by the
7A15A/7A15AN are
shown
on
the Connectors portion
of
Diagram 1 .
Also
shown are the power supply decoupling
components .
Readout Encoding (7A15A only)
The Readout Encoding circuit consists of switching
resistors and probe sensing stage Q620 . This circuit encodes
the Row and Column output lines for readout of deflection
factor, uncalibrated deflection factor (VARIABLE) infor-
mation, and signal inversion . Data is encoded on these
output lines by switching resistors between them and the
time-slot input lines or by current added through Q620
.
R647-C647 are switched
between time-slot three (TS-3)
and the Column output
line when the CAL switch
is
in the
uncal
position . This results in the symbol > (greater than)
being displayed preceding the deflection factor readout .
R648 is switched between TS-2 and the Column output line
when
the
POLARITY
switch is in the
INVERT
position
.
This results in the symbol f (inverted) being displayed
preceding the deflection factor readout .
Switching resistors are used to indicate the setting of the
VOLTS/DIV switch to the mainframe readout system . The
dots on the contact-closure chart (see Diagram 3) indicate
when the associated contacts on the VOLTS/DIV cam
O

switch
are closed . R633, R634, and
R635 select the
number 1, 2, or 5 depending on
the combination that is
switched in . R637 selects
the m (milli-) prefix and R639
selects the symbol
V (volts) in the 5 mV through .5 V (500
mV) positions of
the VOLTS/DIV switch . R638 selects
the
symbol V
in the 1, 2, and 5 V positions . R630,R631,
and
the output of the probe sensing stage
(Q620) select the
decimal point (number of
zeroes), again depending on the
resistor combination
switched in by the VOLTS/DIV
switch .
Probe sensing stage Q620 identifies the attenuation
factor of
the probe connected to the Input connector by
sensing the
amount
of
current flowing through the probe
coding resistor
located in the probe connector . The output
of this
circuit corrects the mainframe readout system to
include the probe attenuation factor . The third
contact
of
the Input connector provides the input to
the probe sensing
device from the probe coding
resistance (coded probes
only ; see Operating Instructions)
. The third contact is also
used for the
IDENTIFY input . The coding resistor forms a
voltage
divider with R621 through CR621 to the -15 V
supply . The resultant voltage sets the bias on
Q620 and
determines the collector
current, along with emitter resistor
R622 . When the -15 volt
time-slot pulse is applied to I nter-
face Connector B33, Q620
is interrogated and its collector
current is added to
the column current output through
Interface
Connector A37 .
Circuit Description-7A75A/7A75AN
With a 1X probe (or no probe) connected to
the Input
connector, Q620 is turned off . The deflection factor
read-
out
is determined by the VOLTS/DIV switch position . With
a
10X probe connected, the bias on Q620 allows 100
microamperes of collector current to flow . This increases
the deflection factor readout by a factor of 10 .
The IDENTIFY button (S45 on Diagram 1) does
two
things when pressed :
1 . It causes the trace representing
the appropriate
channel of the 7A15A to move (see the discussion
on
the
Input Amplifier) .
2 . Forward biases
CR621 and Q620 to result in a
sufficient amount of
collector current which replaces the
deflection factor
readout with the word "IDENTIFY" .
These two actions
aid in identifying the 7A15A trace
when
multiple traces are displayed . When the IDENTIFY
button
is released, the deflection factor readout is restored .
For further information on the operation of the
readout
system, see the oscilloscope instruction manual
.

NOTES