Tektronix 7A18 User manual

INSTRUCTION
MANUAL
7A18/7A18N .
DUAL .. TRACE
AMPLIFIER
Tekfronix, Inc. p, 0. Box 500 • Beaverton, Oregon 97005 • Phone 644·0161 • Cables: Tektronix
070-1126-01 1271

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"
All TEKTRONIX ,in$#uments are warranted against
defective materIals !ilfldworkmanship for one year. Any
questions withresp'eC't
to the warranty should be taken up
with
your TE'KTRel'UXField Engineer or representative.
~ 0 x>" , ,"
All requests for"repai';s and replacement parts should be
directed
to tbeTEKrRONJ~FieldOfficeor representati
in your areai~~This wilt'!'~~~~re you tti~';ii~stestPos' •
service. Pleas~:jnchlfle tb~)ilfl~rument Ty~.~~umber or
Number and Serial all
'Cop¥~~:'!®<~~;~,'~~~ material 1974, by Tektronix,
Inc., Beaverton, Oregon. Printed in the United States of
America.
All rights reserved. Contents of this publication
may
not be reproduced in any form without permission of
Tektronix, Inc.
U.S.A. and foreign TEKTRONIX products covered by U.S.
and foreign patents and/or patents pending.
TEKTRONIX is a registered trademark of Tektronix, Inc.

7A18/7A18N
TABL.E OF CONTENTS
SECTION 1 SPECIFICATION
Introduction
Electrical Characteristics
7A18 And Mainframe Frequency
Response
Environmental Characteristics
Physical Characteristics
SECTION 2 OPERATING INSTRUCTIONS
Installation
Front Panel Controls and Connectors
General Operating Information
Basic Applications
SECTION 3 CIRCUIT DESCRIPTION
I ntrod uction
Block Diagram Description
Detailed Circuit Description
SECTION 4 MAINTENANCE
Preventive Maintenance
T rou blesh ooting
Replacement Parts
Component Replacement
SECTION 5 CALIBRATION
©I
Recalibration Interval
Test Equipment Required
Part I - Performance Check
Part II - Adjustment
Page
1·1
1·1
1·3
1·3
1·3
2·1
2·1
2·2
2·4
3·1
3·1
3·1
4·1
4·1
4·3
4·4
5·1
5·1
5·3
5·7
OPTION INFORMATION
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
CHANGE INFORMATION
Abbreviations and symbols used in this manual are based on
or taken directly from IEEE Standard 260 "Standard
Symbols for Units", MIL·STD·12B and other standards of_
the electronics industry. Change information, if any, is
located at the rear of this ma-nual.
REV_ JUNE 1974

• -
1 to
•
=(@) H'
,
•
•
~.~
I::"
•
~
-
CH'
~
,
•
•
_"",!"",o
'A"
-
CH'
=~
:J
... ~.
Fig. 1·1. 7A18 ~nd 7A 1 8N Amplifier.
-
--
@
7A18nAl~8N ____________________________________________________________________ ____ ______ __ __

7A1817A18N
SECTION 1
SPECIFICA TION
Change information, if any, affecting this section will be found at the rear of the manual.
Introduction
The 7A18 and 7A18N Dual Trace Amplifier plug-in
units are designed for use with Tektronix 7000-Series
Oscilloscopes. The 7A18 and 7A18N are electrically iden-
tical except that readout encod ing capab ilities and an
"IDENTIFY" function are provided only in the 7A18. All
references made to the 7A18 apply equally to the 7A18N
unless otherwise noted. The 7A 18 is a dual-channel,
medium-bandwidth amplifier. Internal gain and compensa-
tion circuits are automatically switched to correspond to
the setting of the VOL TS/DIV switch. Channel 2 can be
inverted for differential measurements. The 7A18 can be
operated in any plug-in compartment of the 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.
TABLE 1-1
ELECTRICAL
Characteristic
Performance Requirement
Supplemental Information
Deflection Factor
Calibrated Range
5 mV/Div to 5 V/Div; ten steps in a 1,
2,5 sequence.
~~
f---~--
Deflection Factor Accuracy Within 2% with GAIN adjusted at
10 mV/Div.
-------~~-------
--
Uncalibrated
Continuously variable between calibrated
(VARIABLE)
steps; extends deflection factor to at
least 12.5 V/Div.
-----~--------~~-
~~--------~--- .. -.-~---- .- ---------
1---
GAIN
Permits adjustment of deflection factor
for calibrated operation with all 7000-
series oscilloscopes.
~~----~ .. ---------.------~----- -~--
"._--" ---
... _--------
-.- ------~----~ f------------.-~-----
Frequency Response
System Dependent
(8 div reference signal)
Upper Bandwidth
DC (Direct) Coupled
See Table A
-~--.--- ~--
Lower Bandwidth
10 Hertz or less
AC (Capacitive) Coupled
--------
With lOX Probe
1 Hertz or less
----
©
1-1

Specification-7A 18/7 A18N
TABLE 1-1 (cont)
Characteristic Performance Requirement Supplemental Information
-
Maximum Input Voltage
DC Coupled 250 volts, (DC + Peak AC); AC com-
ponent 500 volts peak-to-peak maxi-
mum, one kilohertz or less.
AC Coupled
500 volts, (DC + Peak AC); AC compo-
nent 500 volts peak-to-peak maxi-
mum, one kilohertz or less.
Channel I solation
50: 1 display ratio up to 50
megahertz.
Input Rand C
Resistance
1 Mn ± 2%
Capacitance
Approximately 20_0 pf
RC Product
Within ±1%
between
all
deflection
factors.
Displayed Noise
(Tangentially Measured)
300 microvolts or less at 5 mV /Div in
7000-Series Oscilloscope_
Overdrive Recovery Time 0.1 ms or less to recover to within one
division after the removal of an over-
drive signal
of up to + 75 divisions or
-75 divisions regardless of overdrive
signal duration.
Common Mode Rejection
At least 10:1 up to 50 megahertz_
Ratio
DC Drift
Drift with Time (ambient 0_02 division or less in anyone minute,
temperature and line voltage
after one hour warmup.
constant)
Drift with Temperature
No more than 0.01 division per degree C.
(line voltage constant)
Time Delay between Channels
700 picoseconds or less_
Display Modes
Channell only_
Dual-trace, alternate between channels.
Added algebraically.
Dual-trace chopped between channels_
Channel 2 only.
1-2 @

Specification-7 A 18/7 A18N
TABLE
1-1 (cont)
----.--.----.-.-.----.-.------.-.-- .. ----.~~-.----.-.----;---.-.- .... ----.--~.---.------.---~-.-~--~-
Characteristic
Trigger source Selection
Performance Requirement
Channell only.
Follows DISPLAY MODE selection.
Channel 2 only
TABLE A
7A18 AND MAINFRAME
FREQUENCY RESPONSE
Supplemental Information
.~ __ W_;th7:7:::::,~~J_~_~~h6:5::::'" -_1 __
With 7400 Series
50 MHz
TABLE 1-2
ENVIRONMENTAL CHARACTERISTIC
Refer to the Specification for the associated oscilloscope.
TABLE 1-3
PHYSICAL
__ :_i:_~_9_h_t ~~~--_- - - t: :'::~~~~~'~:~:::'~:~"" -- -----~-----------
© 1-3

NOTES
-----------.------------~----------------------------------------
.- ... ----~.-------- --------------------------------------
--~-.- ..- ..•. -..... ---- .. ~~~~~~- -------------------------------
--.--------------.--~~~~-

-
7A18 / 7A18N
SECTION 2
OPERA TING INSTRUCTIONS
Change information, if any, affecting this section will be found at the rear of the manual.
General
To effectively use the 7A 18, the operation and capabili·
ties of the instrument must be known. Thi s section des·
cribes front·panel co ntrol functions, ge neral information on
signal input connections, an d other su bjects that pertain to
va rious measurement application s.
Installation
The 7A 18 is calibrated and ready for use as received. It
ca n be installed in any compartment of T ektron ix 7000 ·
Ser ies oscilloscopes, but is intended for principal use in
vertical plug·in compar
tment s. To install, al i gn the upper
and lower rails
of the 7A18 with the oscilloscope tracks and
fully insert it. The front will be flush with the front of the
oscilloscope when the 7A 18 is fully inserted, and the latch
at the bottom ·l eft corner of the 7A 18 will be in place
aga inst the front panel.
To remove the 7A 18, pull on the latch (which is
inscribed with the unit identification "7 A 18" or "7 A1 8N")
and the 7A 18 wi ll un latch. Continue pulling on the latch to
slide the 7A 18
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
AC·GND·DC
@
Provides sign al connection to t he
channel.
Selects signal input coupling mode.
AC-The AC component of the si g·
n al is coupled to the amplifier
input whi le the DC component
is blocked.
GND -Grounds the amplifier input
while maintaining the same load
for the input signal. Pr ovides a
charge path for the AC coupli ng
capacitor to precharge t he input
circu i t before switching the
input to AC.
eM2 ...ot..AR11"Y
•• u.
J:! ,.....,
-
........
~H2
~.
DUAl T1lAC£ AMf"UFtEfII
F;9. 2·1. Front ·punel co ntr ols lind co nn &<:lors. 17A 18 snown .)
POSITION
IDENTIFY
(7 A 18 o nly )
DC -80 th AC and DC components
of the signal are coupled to the
amplifier input.
Controls positio n of the trace. Posi-
tioning of the tra ce in the "ADD"
Display Mode is co ntr olled by CH 1
POSITION control only.
Deflects trace about 0 .2 division for
trace id entification. In instruments
wi th readout, also replaces readout
w
it h the word "IDENTIFY".
2·'

Operating Instructions-7 A 18/7 A 18N
VOL TS/DIV
VARIABLE
(VOL TS/DIV)
GAIN Adjustment
DISPLAY MODE
Selects calibrated deflection factors
from 5 mV/Div to 5 V/Div; ten
steps in a 1-2-5 sequence_
Provides continuously variable u n-
calibrated settings between cali-
brated steps. Extends the deflection
factor range to 12.5 volts/division
or more.
When the VARIABLE control is
pushed in, it becomes a front-panel
screw-driver adjustment for cali-
bration of deflection factor.
Selects one of the following modes
of operation:
CH l-A single-trace display of the
signal applied to Channell.
AL T -A dual-trace display of the
signal applied to both channels.
The channels are alternately dis-
played, and switching occurs at
the end of each time-base
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. The CH 2 POLARITY
switch allows the display to be
CH 1 + CH 2 or CH 1 - CH 2.
Position of the trace in this dis-
play mode is controlled by CH 1
POSITION control only.
CHOP-A dual-trace display of the
signals applied to both channels.
The two channels time-share the
sweep as determined by the in-
dicator oscilloscope.
CH 2-A single-trace display of the
signal applied to CH 2.
TRIGGER SOURCE Selects source of the trigger signal.
2-2
The trigger signals provide internal
triggering for the oscilloscope time-
base units.
CH 1 -I nternal triggering signal
obtained from signal applied to
CH 1.
MODE-Internal trigger signal auto-
matically follows DISPLAY
MODE selection. In ADD or
CH 2 POLARITY
CHOP display modes, the trigger
signal
is the algebraic su m of CH
1 and CH 2 trigger.
CH 2-1 nternal trigger signal
obtained from signal applied to
CH 2.
Provides means of inverting the CH
2 display.
+UP-A positive-going signal at the
CH 2 input connector deflects
the CRT display upward.
INVERT -A positive-going signal at
the CH 2 input connector de-
flects the CRT display down-
ward.
GENERAL OPERATING INFORMATION
Introduction
For single-trace operation, either of the two identical
amplifier channels can be used independently by setting the
DISPLAY MODE and TRIGGER SOU RCE 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
operations using CH 1 only apply equally to CH 2 only.
Signal Connections
In general, probes offer the most convenient means of
connecting a signal to the input of the 7A 18. A lOX atten-
uator probe offers a high input impedance and allows the
circuit under test to perform very close to normal operating
conditions.
The Tektronix P6053A probe, with its readout cod;ng
ring, was designed specifically for use with Tektronix 7A-
series amplifier units equipped with readout. The readout
coding ring on the probe connects to a circuit in the ampli-
fier unit which automatically corrects the readout displayed
on the CRT to the actual deflection factor at the tip of the
probe being used. F or probes to be used with amplifier
units without readout, see the Tektronix, I nco catalog.
Vertical Gain Check and Adjustment
To check the gain of either channel, set the VOL TS/DIV
switch to 10 mV and connect 40 mV, 1 kHz signal from the
oscilloscope calibrator to the input connector of the chan-
nel being checked. The vertical deflection should be exactly
four divisions. If not, adjust the front-panel GAIN for
exactly four divisions of deflection. The GAIN adjustment
is engaged by pressing in the GAIN control knob and turn-
ing the knob with a narrow-blade screwdriver (see Front
@T

Panel Controls and Connectors). Turn the knob clockwise,
then counterclockwise, until the GAIN control is engaged.
When
the GAIN control is engaged, the vertical deflection
will change as the knob is turned. Turn the GAIN control
knob with the screwdriver until the deflection is set to
exactly four divisions, then remove the screwdriver.
Input Coupling
The Channel 1 and Channel 2 coupling (AC-GND-DC)
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 must be used to display the
DC component of the signal. It must also be used to display
AC signals below about 30 hertz (ten hertz with a 10X
probe) and square waves with low-frequency components as
these signals are attenuated in the AC position.
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 with large DC inputs. To
use this feature, first set the coupling to GND. Connect the
probe to the circuit and wait about two seconds for the
coupling capacitor to charge. Then set the coupling to AC.
The GND position provides 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.
VOL TS/DIV and VARIABLE Controls
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 VOL TS/DIV switch, and
the setting of the VAR IABLE control. Calibration deflec-
tion factors indicated by the settings of the VOL TS/DIV
switch apply only when the VARIABLE control is in the
calibrated (CAL IN) position.
The VARIABLE control provides variable, uncalibrated
settings between the calibrated steps of the VOL TS/DIV
switch. With the VARIABLE control fully counterclock-
wise and the VOL TS/DIV set to 5 volts/div the uncali-
bra ted vertical deflection factor is extended to at least 12.5
volts/division. By applying a calibrated voltage source to
the input connector, any specific deflection factor can be
set within the range of the VARIABLE control.
Operating Instructions-7 A 18/7 A 18N
CH 2 POLARITY Switch
The CH 2 POLAR ITY switch may be used to invert the
displayed waveform of the signal applied to the CH 2 input.
This is particularly useful in added operation of the 7A18
when differential measurements are to be made. The CH 2
POLARITY switch has two positions, +UP and INVERT. In
the +UP position, the displayed waveform will have the
same polarity as the applied signal and a positive DC voltage
will
move the CRT trace up. In the INVERT position, a
positive-going
waveform at the CH 2 input will be displayed
on the CRT in inverted form and a positive DC voltage will
move the trace down.
DISPLAY MODE Switch
For single-trace operation, apply the signal either to the
CH 1 input or the CH 2 input and set the DISPLAY MODE
switch to the corresponding position: CH 1 or CH 2.
To display a signal in one channel independently when a
signal
is also applied to the other channel, simply select the
desired channel by setting the DISPLAY MODE switch to
the appropriate CH 1 or CH 2 position.
Alternate Mode. The AL T position of the 0 ISPLAY
MODE
switch produces a display which alternates between
channel 1 and channel 2 with each sweep on the CRT.
Although the AL T mode can be used at all sweep rates, the
CHOP mode provides a more satisfactory display at sweep
rates
below about 0.2 millisecond/division. At slow sweep
rates alternate mode svvitching becomes visually per-
ceptible.
Add Mode. The ADD position of the DISPLAY MODE
switch can be used to display the sum or difference of two
signals, for common-mode rejection to remove an undesired
signal, or for DC offset (applying a DC voltage to one
channel to offset the DC component of a signal on the
other channel). The overall deflection factor in the ADD
mode with both VOL TS/DIV switches set to the same
position is the deflection factor indicated by either
VOL TS/DIV switch. However, if the CH 1 and CH 2
VOL TS/DIV switches are set to different deflection factors,
the resultant amplitude is difficult to determine from the
CRT display. In th is case, the voltage amplitude of the
resultant display can be determined accurately only if the
amplitude of the signal applied to one channel is known. In
the ADD mode, positioning of the trace is contro~led by the
channel 1 POSITION control only.
Chop Mode. The CHOP pOSitIOn of the DISPLAY
MODE
switch produces a display which is electronically
switched between channels at approximately a 500 kilo-
hertz rate (controlled by mainframe). In general the CHOP
mode provides the best display at sweep rates slower than
REV. APR 1974 2-3

Operating Instructions-7 A 18/7 A18N
about 0.2 millisecond/division or whenever dual-trace, non·
repetitive phenomena is to be displayed.
TRIGGER SOURCE Switch
CH 1. 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.
CH 2. 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.
MODE. In this position of the TRIGGER SOURCE
switch, the trigger signal for the time-base unit is dependent
on the setting of the DISPLAY MODE switch. The trigger
source for each position of the DISPLAY MODE switch is
as follows:
MODE
TRIGGER SIGNAL SOURCE
CH 1 Channel 1
CH 2 Channel 2
ADD
CHOP
ALT
Algebraic sum of channel 1 and channel 2
Algebraic sum of channel 1 and channel 2
Alternates between channel 1 and channel 2
Trace Identification (7A 18 only)
When the IDENTIFY button is pressed, the trace is de·
flected about 0.2 division to identify the 7A18 trace. This
feature is particularly useful when multiple traces are dis·
played. In instruments with read out, also replaces deflec·
tion factor readout with the word "I DENT I FY".
BASIC APPLICATIONS
General
The following information describes the procedures and
techniques for making basic measurements with a 7A 18 and
the associated Tektronix oscilloscope and time-base. These
applications are not described in detail since each applica-
tion must be adapted to the requirements of the individual
measurements. This instrument can also be used for many
applications not described in th is manual. Contact your
local Tektronix Field Office or representative for assistance
in making specific measurements with this instrument.
Peak-to-Peak Voltage Measurements (AC)
To make peak-to-peak voltage measurements, use the
following procedure:
1. Apply the signal to either input connector.
2-4
2. Set the DISPLAY MODE and TRIGGER SOURCE
switches to display the channel used.
3. Set the coupling switch to AC.
NOTE
For low-frequencv signals below about 30 hertz use
the
DC position to prevent attenuation of the signal.
4. Set the VOL TS/DIV switch to display about five
divisions of the waveform vertically.
5. Set the time-base Triggering controls for a stable dis-
play. Set the time-base unit to a sweep rate which displays
several cycles of the waveform.
6. Turn the 7A18 POSITION control so the lower por·
tion of the waveform coincides with one of the graticu Ie
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 V AR lAB LE (VO L TS/D IV) control is
in the CA LIN position.
NOTE
This technique can also be used to make measure-
ments between two points on the waveform, rather
than peak to peak.
T
Vertical
Deflection
Position To
Centerline
1l-_.~1IL-+_+---~-+-~L-t------+~-+---
Fig. 2-2. Measuring the peak-to·peak voltage of a waveform.
©

8. Multiply the deflection measured in step 7 by the
VOL TS/DIV switch setting. Include the attenuation factor
of the probe if used.
EXAM PL E: Assu me that the peak to peak vertical de·
flection is 4.5 divisions (see Fig. 2-2) using a lOX atten-
uator probe, and the VO L TSID IV switch is set to 1 V.
Volts
Peak to Peak
vertical
deflection X
(divisions)
probe
VOL TSIDIV X attenuation
setti ng 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:
1. Connect the signal to either input connector.
2. Set the DISPLAY MODE and TRIGGER SOURCE
switches to display the channel used.
3. Set the VOLTS/DIV switch to display about five
divisions of the waveform.
4. Set the coupling switch to GND and position the
trace to the bottom graticule line or other reference line. If
the voltage is negative with respect to ground, position the
trace to the top graticule line. Do not move the POSITION
control after this reference line has been established.
NOTE
To measure a voltage level with respect to a voltage
other than ground, make the fol/owing changes to
step 4. Set the coupling switch to DC and applv the
reference voltage to the input connector. Then
position the trace to the reference line.
5. Set the coupling switch to DC. The ground reference
line can be checked at any time by switching to the GND
position.
6. Set the time-base Triggering controls for a stable
display. Set the time-base sweep rate for an optimum
display of the waveform.
©
Operating Instructions-7 A 18/7 A18N
7. 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. 2-3 the
measurement is between the reference line and point A.
8. Establish the polarity of the waveform. With the CH 2
POLARITY switch in the +UP position, any point above
the reference line is positive.
9. Multiply the distance measured in step 7 by the
VOL TS/DIV 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 setting
of 0.5 V.
Using the formula:
I nstan-
taneous
Voltage
vertical VO L TSI
distance X polarity X DIV
(divisions) setting
probe
X attenuation
factor
Substituting the given values:
Instantaneous
Voltage
3.6 X +1 X 0.5 V X 10
The instantaneous voltage is 18 volts.
I I J
Point A <
,~,
1 I
I
f----4
.,
'-- ---
--LTi
;
~
I ,
f+++
..
Vertical j
Distance f-H-i
.+.
-L.J
! !
Reference
i
Line
_ ..
-"
I
I ,
j
Fig. 2-3. Measuring instantaneous voltage with respect to some refer-
ence.
2-5

Operating Instructions-7A18/7A18N
Comparison Measurements
In some applications it may be desirable to establish
arbitrary units of measurement other than those indicated
by the VOL TS/DIV switch. This is particularly useful when
comparing unknown signals to a reference amplitude. One
use for the comparison-measurement technique is to
facilitate calibration of equipment where the desired
amplitude does not produce an exact number of divisions
of deflection. 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 u nits of measure for
comparison measu rem ents.
To establ ish an arb itrary 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 unit 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 VOL TS/DIV switch, the
amplitude of the reference signal must be known. If it is
not known, it can be measured before the VARIABLE
VO L TS/DIV control is set in step 2.
4.
Divide the amplitude of the reference signal (volts) by
the product of the vertical deflection (divisions) established
in step 2 and
the setting of the VOL TS/DIV switch. This is
the vertical conversion factor.
Vertical
Conversion c
Factor
reference signal
amplitude (volts)
vertical VOL TS/DIV
deflection X switch
(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 VOL TS/DIV switch
to a setting that provides sufficient vertical deflection to
make an accurate measurement. Do not readjust the
VARIABLE control.
6. Measure the vertical deflection in divisions and
calculate the amplitude of the unknown signal using the
following formula.
2-6
Signal
Amplitude
VOL TS/DIV
setting
vertical
X conversion
factor
vertical
X deflection
(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
divisions. Substituting these values
conversion factor formula (step 4):
Vertical 30 V
Conversion
~ 4 X 5 V
Factor
deflection of four
in the vertical
1.5
Then with a VOL TS/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
Amplitude
2 V X 1.5 X 5 15 volts
Dual-Trace Phase Difference Measurements
Phase comparison between two signals of the same fre-
quency can be made using the dual-trace feature of the
7A18. 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-
cedu re:
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 AL T or CHOP. In
general, CHOP is more suitable for low frequencies and
AL T is more suitable for high frequencies. Set the
TRIGGER 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
or probes which have similar time delay characteristics to
connect the signals to the input connectors.
4. If the signals are of opposite polarity, set the CH 2
POLARITY switch to invert the channel 2 display.
(Signals may be of opposite polarity due to 180
0 phase
difference; if so, take this into account in the final cal-
culation.)
5. Set the VOL TS/DIV switches and the VARIABLE
controls of the two channels so the displays are equal and
about five divisions in amplitude.
©

6. Set the time-base unit to a sweep rate which displays
about one cycle of the waveforms. Set the Triggering
controls for a stable display.
7. Center the waveform s on the graticule with the 7A 18
POSITION controls.
8. Adjust the time-base Variable Time/Div control until
one cycle of the reference signal occupies exactly eight
horizontal 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 waveform.
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 divi-
sion with a sweep rate of 45° /division as shown in Fig. 2-4.
Using the formu la:
Phase Difference
horizontal
difference
(divisions)
sweep rate
X (degrees/division)
Fig. 2-4. Measuring phase difference between two signals.
©
Operating Instructions-7A1817A18N
Substituting the given values:
Phase D ifference ~ 0.3 X 45 °
The phase difference is 13.5 0.
High Resolution Phase Measurements
More accurate dual-trace phase measurements can be
made by increasing the sweep rate (without changing the
Variable Time/D iv control). One of the easiest ways to in-
crease the sweep rate is with the time-base Magnifier switch.
Set the Magnifier to X 10 and determine the magnified
sweep rate by dividing the sweep rate obtained previously
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 Xl O. With a hori-
zontal difference of 3 divisions, the phase difference is:
Phase Difference
horizontal
difference X
(divisions)
Substituting the given values:
magnified
sweep rate
(degrees/d iv ision)
Phase Difference = 3 X 4.5°
The phase difference is 13.5°.
Channel 1
(R
eferencel
"'=+-~ft--+--+---+--+-- Channel 2 +
Horizontal
Difference
Fig. 2-5. High resolution phase measurement using time-base magni.
fier.
2-7

Operating Instructions-7 A 18/7 A 18N
Common Mode Rejection
The ADD feature of the 7A18 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:
1. Set the DISPLAY MODE switch to ALT or CHOP
and the TRIGGER SOURCE switch to MODE.
2. Connect the signal containing both the desired and
undesired information to the CH 1 input connector.
3. Connect a signal similar to the unwanted portion of
the CH 1 signal to the CH 2 input connector. For example,
in Fig. 2-6 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 VOL TS/DIV switches so the signals are about
equal in amplitude.
6. Set the DISPLAY MODE switch to ADD. Set the CH
2
POLARITY switch to INVERT so the common-mode
signals are of opposite polarity.
7. Adjust the Channel 2 VOL TS/DIV switch and
VARIABLE control for maximum cancellation of the
common-mode signal. The signal which remains should be
only the desired portion of the Channell signal.
EXAMPLE: An example of this mode of operation is
shown in Fig. 2·6. The signal applied to Channell 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 is
used. A DC voltage of the proper polarity applied to
Channel 2 can be used to cancel out the DC portion of the
signal applied to Channell.
2-8
-----1--
-J
~4
~
~
~-.-
~
, I
r-
r--
" !
--
r
~, ~,
..
....
r-'
I "
'"
~
"-
,
r
"-
I +
_J.
~
r
--
-I
(A) Channell Signal
----,
,- .
-j
I
~.~. j
I !
-}
I
:/
",.
N : II
17
~
I i II
"'-
'"
/t
I
I
I
i
(B) Channel 2 Signal
!--
I
[ I
I I
!
I
-!at -
!at •
!at •
.. .
lilt •
....
....
...
r- I
Ir
..
...
-- - --J
! I
L..-..
I i
(C) Resultant Display
Fig. 2·6. Using the ADD mode for common-mode rejection. (A)
Channel 1 signal contains desired information along with line-
frequency component. (B) Channel 2 contains line frequency only.
(C) Resultant CRT display using common-mode rejection.
©

7A18/7A18N
SECTION 3
CIRCUIT DESCRIPTION
Change information, if any, affecting this section will be found at the rear of this manual.
Introduction
This section of the manual contains a description of the
circuitry used in the 7A 18 dual-trace amplifier. The descrip-
tion 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 indi-
vidual stages.
Complete schematics of each circu it are given in the
Diagrams section. Refer
to these schematics throughout the
following circuit description for electrical values and
relationsh ip.
BLOCK DIAGRAM
The following discussion is provided to aid in under-
standing the overall concept of the 7A 18 before the indivi-
dual 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
represents a major circuit within the instrument. The
number on 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 VOL TS/DIV switch selects the
correct amount of attenuation and the signal is passed to
the input amplifier.
The Channel 1 Input Amplifier circuit provides gain
setting, variable gain
control, and trace positioning. The
Channel 2 Input Amplifier provides signal polarity inversion
in
addition to gain setting, variable gain control, and trace
positioning. The outputs of these circuits are applied push-
pull to the Signal and Trigger Channel Switches.
The Channel
Switches select the proper signal and trigger
as determined by the DISPLAY MODE and TRIGGER
©
SOU RCE switches. The signal and trigger outputs are
provided to the oscilloscope via the Interface Connector.
The Readout Encoding circuit (7A18 only) provides
readout logic for the oscilloscope readout system. Data is
supplied to the mainframe readout system identifying the
polarity, deflection factor, the uncalibrated symbol (when
the
VAR IABLE control is in the outward position), and the
plug-in mode. 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".
DETAILED CIRCUIT DESCRIPTION
ATTENUATOR
General
The Attenuator circuit determines the input coupling
and the 7A18 deflection factor.
NOTE
The CH 1 and CH 2 Attenuator circuits are identical.
To minimize duplication, only CH 1 is described in
detail throughout this discussion.
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
showing 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. This cap-
acitor prevents the DC component of the signal from
passing to the amp lifier. 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 C 10 to be precharged 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.
3-1

Circuit Description-7A1817A18N
Input Attenuator
The effective overall deflection factor of the 7A18 is
determined by the setting of the VOL TS/DIV switch,
S100B. The basic deflection factor is five millivolts per divi-
sion of CRT deflection. To increase the basic deflection
factor to the values indicated on the front panel, precision
attenuators are switched into the circuit. These attenuators
are hybrid devices wh ich contain the necessary resistances
and
capacitors. Each attenuator is replaceable as a unit.
S1 OOB 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.
For switch positIOns above five millivolts, the atten-
uators are switched into the circuit singly or in pairs to
produce the 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
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 VOL TS/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
CHANNEL 1
INPUT AMPLIFIER
The Channel 1 I nput Amplifier converts the single-ended
signal applied
to the Channel 1 input connector to a dif-
ferential (push-pull) output. Fig. 3-1 shows a detailed block
diagram
of the Channel 1 Input Amplifier. 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 0210A.
Dual-diode CR210 provides circuit protection by limiting
the voltage swing at the gate of 0210A to about ± (positive
or negative) 15 volts. 0210B provides a constant cu rrent
3-2
source for 0210A. 0210A and 0210B are encapsulated in
the same case so that 0210B temperature-compensates the
circuit.
Paraphase Cascode Amplifier
Paraphase amplifier 0220-0320, in conjunction with
0225-0325, forms a cascode amplifier. 0220-0320 convert
the single-ended input signal to a differential output signal.
Diodes
CR220-CR221 hold the voltage level at the base of
0220 close to ground to limit the voltage swing to about
±O.6 volt. Common-base connected 0225-0325 provide
isolation
between the paraphase amplifier and the GAl N-
VARIABLE controls. The gain of the Channel 1 Input
Amplifier is set in this stage by front-panel GAIN control
R237A with the CAL IN switch pressed in. When the CAL
IN switch is in the outward (uncalibrated) position and
turned fully counterclockwise to minimum resistance, the
gain of the amplifier is reduced by a factor of at least 2.5.
Adjustment 1 R321 varies the base level of 0320 to provide
the same voltage levels at the collectors of 0225 and 0325.
This prevents a zero-volt reference trace from changing
position when varying the GAIN or VARIABLE controls.
Second Cascode Amplifier
The Second Cascode Amplifier stage provides a signal
gain
of approximately two. This stage includes the POSI-
TION
control and, in the 7A18 only, trace IDENTIFY
circuitry. The emitters of common·base connected
0250-0350 provide a low-impedance point for injection of
the POSITION control and IDENTIFY switch currents.
Position
of the trace is determined by the setting of the
POSITION control, R11. This control changes the current
drive to 0250·0350. Since the emitters are a very low·
impedance point in the circuit, there is negligible voltage
change
at these points. However, the change in current
from the POSITION control produces a resultant DC
voltage difference at the collectors to change the position
of the trace. Trace identification is accomplished by
inserting resistor R357 from ground through CR357 to the
junction of R11-R256. This results in a slight increase in
the emitter current of 0250 to cause the trace to move.
This aids
in identifying the channel 1 trace when multiple
traces are displayed.
The network C246-C345-C245-R246-R345-R245 pro-
vides high frequency compensation. R245-C245 in this net-
work provide high-frequency response adjustment for this
stage.
Emitter Follower
Emitter Follower stage 0260·0360 provides a low out-
put impedance to drive the Signal and Trigger Channel
Switches,
U270-U470. This stage also provides isolation
between the Second Cascode Amplifier and U270-U470.
©
----------------------------_._---,-,_ ... _'"

FROM
INPUT
ATTEN
INPUT
SOURCE
FOLLOWER
DC
BAL
PARAPHASE
CASCODE
AMPLIFIER
GAIN
AND
VARIABLE
CONTROLS
Circuit Description-7 A 18/7 A18N
SECOND
CASCODE
AMPLIFIER
EMITTER
FOLLOWER
I POSITION I
IDENTIFY
FROM
S13
(7A18 only)
TO
SIGNAL
CHANNEL SWITCH
U270
TO
TRIGGER
CHANNEL SWITCH
U470
1126-08
Fig. 3·1. Channel 1 Input Amplifier detailed block diagram.
CHANNEL 2 INPUT AMPLIFIER
General
The Channel 2 Input Amplifier circuit is basically the
same as the Channel 1 Input Amplifier circuit. Only the
differences between the two circuits are described here.
Portions
of this circuit not described in the following des-
cription
operate in the same manner as for the Channel 1
Input Amplifier circuit (corresponding circuit numbers
assigned
in the 400 - 599 range). Fig. 3-2 shows a detailed
block diagram
of the Channel 2 I nput Amplifier circuit. A
schematic
of this circuit is shown on Diagram 3 in the
Diagrams section.
FROM
INPUT
ATTEN
PARAPHASE
CASCODE
AMPLIFIER
\ \ \
I CH 2 POLARITY I
GAIN
AND
VARIABLE
CONTROLS
I CA~ IN I
Paraphase Cascode Amplifier
The Paraphase Cascode Amplifier for Channel 2 consists
of 0420, 0520,0425,0525,0426, and 0526. In addition
to the functions described under Channel 1 Input Ampli-
fier,
the Channel 2 Paraphase Cascode Amplifier stage
provides a means
of inverting the displayed signal. With the
CH 2 POLARITY switch set to +UP, 0425 and 0525 are
biased on and
the signal is passed to the Second Cascode
Amplifier stage as for the Channel 1 Input Amplifier. With
the CH 2 POLARITY switch set to INVERT, 0425 and
0525 are biased off and 0426-0526 are turned on to
provide signal inversion.
SECOND
CASCODE
AMPLIFIER
IDENTIFY
FROM
S23
(7A18 only)
EMITTER
FOLLOWER
TO
SIGNAL
CHANNEL
SWITCH
U270
TO
TRIGGER
CHANNEL
SWITCH
U470
Fig. 3-2. Channel 2 Input Amplifier detailed block diagram.
REV. D, DEC. 1976 3-3