Tektronix 7A18A User manual

Tektronix, Inc.
P.O. Box 500
Beaverton, Oregon 97077
070-4329-00
Product Group 42
Tektrone<
COMMITTED TO EXCELLENCE
PLEASE CHECK FOR CHANGE INFORMATION
AT THE REAR OF THIS MANUAL.
7A18A
DUAL
TRACE
AMPLIFIER
INSTRUCTION
MANUAL
Serial Number
First Printing OCT 1982
Revised NOV 1983

Copyright © 1971, 1982 Tektronix, Inc. All rights reserved.
Contents of this publication may not be reproduced in any
form without the written permission of Tektronix, Inc.
Products of Tektronix, Inc. and its subsidiaries are covered
by U.S. and foreign patents and/or pending patents.
TEKTRONIX, TEK, SCOPE-MOBILE, and are
registered trademarks of Tektronix, Inc. TELEQUIPMENT
is a registered trademark of Tektronix U.K. Limited.
Printed in U.S.A. Specification and price change privileges
are reserved.
INSTRUMENT SERIAL NUMBERS
Each instrument has a serial number on a panel insert, tag,
or stamped on the chassis. The first number or letter
designates the country of manufacture. The last five digits
of the serial number are assigned sequentially and are
unique to each instrument. Those manufactured in the
United States have six unique digits. The country of
manufacture is identified as follows:
BO000000 Tektronix, Inc., Beaverton, Oregon, USA
100000 Tektronix Guernsey, Ltd., Channel Islands
200000 Tektronix United Kingdom, Ltd., London
300000 Sony/Tektronix, Japan
700000 Tektronix Holland, NV, Heerenveen,
The Netherlands

7A18A
TABLE OF CONTENTS
Page
Section 1 SPECIFICATION
Introduction ....... 6... eee eee ee 1-1
Electrical Characteristics .......... 1-1
7A18A and Mainframe Frequency
Response ........ 0. eee eee eee 1-1
Environmental Characteristics ...... 1-2
Physical Characteristics .......... 1-2
Section 2 OPERATING INSTRUCTIONS
Installation cixciesemieeiweswrses 2-1
Front Panel Controls and Connectors 2-1
General Operating Information
Basic Applications
THE FOLLOWING SERVICING
INSTRUC-
TIONS ARE FOR USE BY QUALIFIED PER-
SONNEL
ONLY. TO AVOID PERSONAL INJU-
RY, DO NOT PERFORM ANY SERVICING
OTHER THAN THAT CONTAINED
ATING
QUALIFIED TO DO SO.
Section 3 CIRCUIT DESCRIPTION
Introduction .............
Block Diagram Description .
Detailed Circuit Description .
Section 4 PERFORMANCE CHECK &
ADJUSTMENT
Recalibration Interval ......
Test Equipment Required ..
Part I—Performance Check
Part li—Adjustment
IN OPER-
INSTRUCTIONS UNLESS YOU ARE
Section 5
Section 6
Section 7
Section 8
Section 9
Page
MAINTENANCE
Preventive Maintenance .......... 5-1
Troubleshooting .............0055 5-1
Replacement Parts .............. 5-3
Component Replacement ......... 5-4
OPTION INFORMATION
ELECTRICAL PARTS LIST
Abbreviations and Symbols
Parts Ordering Information
DIAGRAMS AND CIRCUIT BOARD
ILLUSTRATIONS
Symbols
Symbols
and Reference Designators
Voltage and Waveform Conditions
MECHANICAL PARTS LIST
Mechanical Parts List Information
Index of Mechanical Parts Illustrations
Mechanical Parts List
Accessories
CHANGE INFORMATION
Abbreviations and symbols used in this manual are based
on or taken directly from IEEE Standards 260 “Standard
Symbols for Units”, MIL-STD-12B and other standards of
the electronics industry. Change information, if any, is locat-
ed at the rear of this manual.

7A18A
LIST OF ILLUSTRATIONS
Fig.
No.
2-1
2.2
2-3
2-4
2-5
2-6
3-1
5-1
6-1
6-2
6-3
Front-panel controls and connectors .....
Measuring the peak-to-peak voltage of a
WaVETOIM ©... ee eee eee nee
Measuring instantaneous voltage with
respect to some reference .............
Measuring phase difference betwen two
SIQNalS. 2... eee eee eee
High resolution phase measurment using
time-base magnifier ...................
Using the ADD mode for common-mode
rejection 2... eee eee
Channel 1 Input Amplifier detailed block
GIGS os cscevemismiewimseaimmimean
Channel 2 Input Amplifier detailed block
GaGa vices cmiees wa sms wea eee wow wane
U270 input combinations for DISPLAY
MODE selection ................0000.
Input/Output combinations for DISPLAY
MODE and TRIGGER SOURCE switch
SOIGCHONS sacccienimrasswacwadmem meme
Locations of adjustments used in this
procedure ........ eee eee eee eee
Electrode configuration for semiconductors
used in this instrument ................
7A18A Option 06 ............... 00 eee
Side view of 7A18A—Option 06 .........
Rear view of A2-Amplifier board for either
Option 06 or standard 7A18A ...........
8-1 and 8-2 are of circuit board assemblies and are in the
diagrams section

7A18A
OPERATORS SAFETY SUMMARY
The general safety information in this part of the summa-
ry is for both operating and servicing personnel. Specific
warnings and cautions will be found throughout the manual
where they apply, but may not appear in the summary.
Terms in This Manual
CAUTION statements identify conditions or practices
that could result in damage to the equipment or other
property.
WARNING statements identify conditions or practices
that could result in personal injury or loss of life.
Terms as Marked on Equipment
CAUTION indicates a personal injury hazard not immedi-
ately accessible as one reads the marking, or a hazard to
property including the equipment itself.
DANGER indicates a personal injury hazard immediately
accessible as one reads the marking.
Symbols In This Manual
/ This symbol indicates where applicable
cautionary or other information is to be found.
Symbols As Marked On Equipment
4 DANGER - High voltage.
ss Protective ground (earth) terminal.
A ATTENTION - refer to manual.
Power Source
This product is intended to operate from a power source
that will not apply more than 250 volts rms between the
supply conductors or between either supply conductor and
ground. A protective ground connection by way of the
grounding conductor in the power cord is essential for safe
operation.
Grounding the Product
This product is grounded through the grounding conduc-
tor of the power cord. To avoid electrical shock, plug the
power cord into a properly wired receptacle before connect-
ing to the product input or output terminals. A protective
ground connection by way of the grounding conductor in the
power cord is essential for safe operation.
Danger Arising From Loss Of Ground
Upon loss of the protective-ground connection, all acces-
sible conductive parts (including knobs and controls that
may appear to be insulating) can render an electric shock.
Use The Proper Power Cord
Use only the power cord and connector specified for your
product.
Use only a power cord that is in good condition.
Refer cord and connector changes to qualified service
personnel.
“Use The Proper Fuse
To avoid fire hazard, use only the fuse of correct type,
voltage rating and current rating as specified in the parts list
for your product.
Refer fuse replacement to qualified service personnel.
Do Not Operate In Explosive Atmospheres
To avoid explosion, do not operate this product in an
explosive atmosphere unless it has been specifically certi-
fied for such operation.

7A18A
SERVICE SAFETY SUMMARY
FOR QUALIFIED SERVICE PERSONNEL ONLY
Refer also to the preceding Operators Safety Summary
Do Not Service Alone
Do not perform intenal service or adjustment of this prod-
uct unless another person capable of rendering first aid and
resuscitation is present.
Use Care When Servicing With Power On
Dangerous voltages exist at several points in this prod-
uct. To avoid personal injury, do not touch exposed connec-
tions and components while power is on.
Disconnect power before removing protective panels,
soldering, or replacing components.
Power Source
This product is intended to operate from a power source
that will not apply more than 250 volts rms between the
supply conductors or between either supply conductor and
ground. A protective ground connection by way of the
grounding coductor in the power cord is essential for safe
operation.

7TA18A
7A18A Dual Trace Amplifier
4329-01
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Section 1—7A18A
SPECIFICATION
Introduction
switch.
Channel
2
can
be
inverted
for
differential
measure-
The
7A18A
Dual
Trace
Amplifier
plug-in
unit
is
designed
mens
ne
lal
a
ais
i
alee
all
for
use
with
TEKTRONIX
7000-Series
Oscilloscopes.
.
ae.
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.
The 7A18A is a dual-channel, medium-bandwidth amplifi-
er. Internal gain and compensation circuits are automatically
switched to correspond to the setting of the VOLTS/DIV
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.
Deflection Factor Accuracy Within 2% with GAIN adjusted at
10 mV/Div.
Uncalibrated (VARIABLE) Continuously variable between
calibrated steps; extends deflection
factor to at least 12.5 V/Div.
GAIN Permits adjustment of deflection factor for
calibrated operation with all 7000-series
oscilloscopes.
Frequency Response System
Dependent (8 div reference signal)
Upper Bandwidth DC (Direct) 75 MHz
Coupled
Lower Bandwidth AC (Capacitive) 10 Hertz or less
Coupled
With 10X Probe 1 Hertz or less
Maximum Input Voltage
DC Coupled 250 volts, (DC + Peak AC); AC
component 500 volts peak-to-peak
maximum, one kilohertz or less.
AC Coupled 500 volts, (DC + Peak AC); AC
component 500 volts peak-to-peak
maximum, one kilohertz or less.
Channel Isolation 50:1 display ratio up to 50 megahertz.
1-1

Specification—7A18A
Table 1-1 (cont)
Characteristics
Performance Requirements
Supplemental Information
Input R and C
Resistance 1 MQ +2%.
Capacitance Approximately 20.0 pF
RC Product Within + 1% between all deflection
factors.
Displayed Noise
(Tangentially Measured)
0.06 div or less at all deflection factors
Overdrive Recovery Time
0.1 ms or less to recover to within one
division after the removal of an
overdrive signal of up to +75 divisions
or —75 divisions regardless of
overdrive signal duration.
Common Mode Rejection Ratio
At least 10:1 up to 50 megahertz.
DC Drift
Drift with Time (Ambient
temperature and line voltage
constant)
0.02 division or less in any one minute,
after one hour warmup.
Drift with Temperature (line voltage
constant)
Nor more than 0.01 division per degree
C.
Time Delay between Channels
700 picoseconds or less.
Display Modes
Channel 1 only.
Dual-trace, alternate between
channels.
Added algebraically.
Dual-trace chopped between channels.
Channel 2 only.
Trigger source Selection
Channel 1 only.
Follows DISPLAY MODE selection.
Channel 2 only.
Table 1-2
ENVIRONMENTAL CHARACTERISTIC
Refer to the Specification for the associated oscilloscope.
Table 1-3
PHYSICAL
Size
Fits all 7000-series plug-in compartments.
Weight
-~2 Pounds 10 Ounces (1.4 kilograms)
1-2

Section 2—7A18A
OPERATING INSTRUCTIONS
General
To effectively use the 7A18A, the operation and capabili-
ties of the instrument must be known. 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 7A18A is calibrated and ready for use as received. It
can be installed in any compartment of Tektronix 7000-Se-
ries oscilloscopes, but is intended for principal use in vertical
plug-in compartments. To install, align the upper and lower
rails of the 7A18A with the oscilloscope tracks and fully in-
sert it. The front will be flush with the front of the oscillo-
scope when the 7A18A is fully inserted, and the latch at the
bottom-left corner of the 7A18A will be in place against the
front panel.
To remove the 7A18A, pull on the latch (which is in-
scribed with the unit identification “7A18A”) and the 7A18A
will unlatch. Continue pulling on the latch to slide the 7A18A
out of the oscillscope.
FRONT PANEL CONTROLS AND
CONNECTORS
The following descriptions apply to the controls and con-
nectors of both Input Amplifier channels when applicable.
See Fig. 2-1.
Provides signal connection to the
channel.
Input Connector
AC-GND-AC Selects signal input coupling mode.
AC—The AC component of the sig-
nal is coupled to the amplifier in-
put while the DC component is
blocked.
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.
1 { HORNTIEN
VARIABLE (CAL IN)
{DIV
_ | CH2PoLartry
if +UP :
' INVERT
250 Vpk
MAX
tama .
20pk
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4329-02
Fig. 2-1. Front-panel controls and connectors. (7A18A shown.)
DC—Both AC and DC components
of the signal are coupled to the
amplifier input.
POSITION Controls position of the trace. Posi-
tioning of the trace in the “ADD”
Display Mode is controlled by CH 1
POSITION control only.
IDENTIFY Deflects trace about 0.3 division for
trace identification. In instruments
with readout, also replaces readout
with the word “IDENTIFY”.
2-1

Operating Instructions—7A18A
VOLTS/DIV Selects calibrated deflection factors
from 5 mv/Div to 5 V/Div; ten steps
in a 1-2-5 sequence.
VARIABLE Provides continuously variable
(VOLTS/DIV) uncalibrated settings between cali-
brated steps. Extends the deflection
factor range to 12.5 V/Div or more.
When the VARIABLE control is
pushed in, it becomes a front-panel
screw-driver adjustment for calibra-
tion of deflection factor.
GAIN Adjustment
DISPLAY MODE Selects one of the following modes
of operation:
CH 1—A single-trace display of the
signal applied to Channel 1.
ALT—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 sig-
nals applied to the CH1 and
CH 2 input connectors, and the
algebraic sum is displayed on the
CRT. The CH2 POLARITY
switch allows the display to be
CH 1 + CH2 or CH1 — CH2.
Position of the trace in this dis-
play mode is controlled by a
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 indi-
cator oscilloscope.
CH 2—A single-trace display of the
signal applied to CH 2.
TRIGGER SOURCE Selects source of the trigger signal.
The trigger signals provide internal
triggering for the oscilloscope time-
base units.
CH 1—Internal triggering signal ob-
tained from signal applied to
CH 1.
MODE—Internal trigger signal
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
CH 2.
CH 2 POLARITY Provides means of inverting the
CH 2 display.
+UP—A positive-going signal at
the CH input connector de-
flects the CRT display upward.
INVERT—A positive-going signal at
the CH2 input connector de-
flects the CRT display
downward.
GENERAL OPERATING INFORMATION
Introduction
For single-trace operation, either of the two identical am-
plifier channels can be used independently by setting the
DISPLAY MODE and TRIGGER SOURCE switches to CH 1
of 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 7A18A. A 10X attenu-
ator probe offers a high input impedance and allows the
circuit under test to perform very close to normal operating
conditions.
The TEKTRONIX P6053B probe, with its readout coding
ring, was designed specifically for use with TEKTRONIX 7A-
series amplifier units equipped with readout. The readout
coding ring on the probe connects to circuit in the amplifier
unit which automatically corrects the readout displayed on
the crt to the actual deflection factor at the tip of the probe
being used. For probes to be used with amplifier units with-
out readout, see the Tektronix, Inc. catalog.
Vertical Gain Check and Adjustment
To check the gain of either channel, set the VOLTS/DIV
switch to 10 mV and connect 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. The GAIN adjustment is engaged by
pressing in the GAIN control knob and turning the knob with
a narrow-blade screwdriver (see Front Panel Controls and
Connectors). Turn the knob clockwise, then counterclock-

wise, 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 screw-
driver 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 Hz (ten hertz with a 10X probe) and
square waves with low-frequency components as these sig-
nals 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 cou-
\ pling 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.
VOLTS/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 VOLTS/DIV switch, and the
setting of the VARIABLE control. Calibration deflection fac-
tors indicated by the settings of the VOLTS/DIV switch ap-
ply 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 VOLTS/DIV
switch. With the VARIABLE control fully counterclockwise
and the VOLTS/DIV set to 5 volts/div the uncalibrated verti-
cal 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—7A18A
CH 2 POLARITY Switch
The CH 2 POLARITY 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 7A18A
whendifferential 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 posi-
tive-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 of 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 ALT position of the DISPLAY
MODE switch produces a display which alternates between
channel 1 and channel 2 with each sweep on the crt. Al-
though the ALT 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 switching becomes visually
perceptible.
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. The overall 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 different deflection factors, the resultant amplitude is diffi-
cult to determine from the crt display. In this case, the volt-
age 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 controlled 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 kHz rate (con-
trolled by mainframe). In general, the CHOP mode provides
the best display at sweep rates slower than about 0.2
millisecond/division or whenever dual-trace, non repetitive
phenomena is to be displayed.

Operating Instructions—7A18A
TRIGGER SOURCE Switch
CH 1. The CH 1 position of the TRIGGER SOURCE
switch provides a trigger signal obtained from the signal ap-
plied 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 ap-
plied 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 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
Trace Identification
When the IDENTIFY button is pressed, the trace is de-
flected about 0.3 division to identify the 7A18A trace. This
feature is particularly useful when multiple traces are dis-
played. In instruments with readout, also replaces deflection
factor readout with the word “Identify”.
BASIC APPLICATIONS
General
The following information describes the procedures and
techniques for making basic measurements with a 7A18A
and the associated Tektronix oscilloscope and time-base.
These applications are not described in detail since each
application must be adapted to the requirements of the indi-
vidual measurements. This instrument can also be used for
many appliactions not described in this manual. Contact
your local Tektronix Field Office or representative for assis-
tance 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-frequency signals below about 30 Hz use the
dc position to prevent attenuation of the signal.
4. Set the VOLTS/DIV switch to display about five divi-
sions 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 7A18A POSITION control so the lower por-
tion
of
the
waveform
coincides
with
one
of
the
graticule
lines
below
the
center
horizontal
line,
and
the
top
of
the
wave-
form
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
VARIABLE
(VOLTS/DIV)
control
is
in
the CAL IN position.
NOTE
This technique can also be used to make measure-
ments between two points on the waveform, rather
than peak-to-peak.
Position To 7
Centerline
“4 A ALA
OA ENN]
Deflection \
;
1126-40
Fig.
2-2.
Measuring
the
peak-to-peak
voltage
of
a
waveform.

—~
8. Multiply the deflection 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 attenuator
probe, and the VOLTS/DIV switch is set to 1 V.
Vi | r
Volts ertical VOLTS/DIV probe
= deflection X : X attenuation
Peak to Peak wo setting
(divisions) factor
Substituting the given values:
Volts Peak-to-Peak = 4.5 X 1 X 10
The peak-to-peak voltage is 45 V.
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 divi-
sions 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 following changes to
step 4. Set the coupling switch to DC and apply the
reference voltage to the input connector. Then posi-
tion 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 dis-
play. Set the time-base sweep rate for an optimum display
of the waveform.
Operating Instructions—7A18A
7. Measure the distance in divisions between the refer-
ence line and the point on the waveform at which the dc
level is to be measured. For example, in Fig. 2-3 the mea-
surement is between the reference line and point A.
Point A
Vertical
Distance
Reference
Line
1126-41
Fig. 2-3. Measuring instantaneous voltage with respect to some
reference.
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
VOLTS/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:
Instan- vertical VOLTS/ probe
taneous
= distance X polarity X DIV X attenuation
Voltage (divisions) setting factor
Substituting the given values:
Instantaneous 3.6 X +1X0.5V X10
Voltage
The instantaneous voltage is 18 V.
2-5

Operating Instructions—7A18A
Comparison Measurements
In some applications it may be desirable to establish arbi-
trary units of measurement other than those indicated by
the VOLTS/DIV switch. This is particularly useful when com-
paring 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 adjust-
ment is indicated by an exact number of divisions of deflec-
tion. 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 unit sweep rate to display several cycles
of the signal.
2. Set the VOLTS/DIV switch and the VARIABLE con-
trol to produce a display which is an exact number of verti-
cal 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 accu-
rately at any setting of the VOLTS/DIV switch, the ampli-
tude of the reference signal must be known. If it is not
known, it can be measured before the VARIABLE
VOLTS/DIV control is set in step 2.
4.
Divide the amplitude of ithe reference signal (volts) by
the product of the vertical deflection (divisions) established
in step 2 and the setting of the VOLTS/DIV switch. This is
the vertical conversion factor.
Vertical reference signal
Conversion = ampliltude (volts)
Factor vertical VOLTS/DIV
deflection X switch
(divisions) setting
5. To measure the amplitude of an unknown signal, dis-
connect the reference signal and connect the unknown sig-
nal to the input connector. Set the VOLTS/DIV switch to a
setting that provides sufficient vertical deflection to make an
accurate measurement. Do not readjust the VARIABLE
control.
2-6
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 V, a VOLTS/DIV setting of 5 volts and the VARIABLE
control adjusted to provide a vertical deflection of four divi-
sions. Substituting these values in the vertical conversion
factor formula (step 4):
30 V
Vertical Conversion Factor = ical Conversi aX5V
Then with a VOLTS/DIV setting of 2 V, the peak-to-peak
amplitude of an unknown signal which produces vertical
deflection of five divisions can be determined by using the
signal amplitude formula (step 6):
Signal
Amplitude ~ 2VX1.5 X5 - 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
7A18A. 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 procedure:
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 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° phase difference; if
so, take this into account in the final calculation.)
a

5. Set the VOLTS/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 con-
trols for a stable display.
7. Center the waveforms on the graticule with the
7A18A POSITION controls.
8. Adjust the time-base Variable Time/Div control 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° divi-
sion). 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 difference.
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 formula:
horizontal sweep rate
Phase Difference = difference X (degrees/division)
(divisions)
Substituting the given values:
Phase Difference = 0.3 X 45°
The phase difference is 13.5°.
High Resolution Phase Measurements
More accurate dual-trace phase measurements can be
made by increasing the sweep rate (without changing the
Variable Time/Div control). One of the easiest ways to in-
crease the sweep rate is with the time-base Magnifier
switch. Set the Magnifier to X10 and determine the magni-
fied sweep rate by dividing the sweep rate obtained previ-
ously 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
Operating Instructions—7A18A
q Channel 1 f
i Reference
\ XN
/ Channel 2
++ Lagging ++
ail Horizontal
= Difference —
1126-42
Fig. 2-4. Measuring phase difference between two signals.
Channel 1
(Reference)
Channel 2
Horizontal
Difference
1126-43
Fig. 2-5. High resolution phase measurement using time-base
magnifier.
Fig. 2-4 but with the Magnifier set to X10. With a horizontal
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 7A18A can be used to display
signals which contain undesirable components. These unde-

Operating Instructions—7A18A
sirable 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 VOLTS/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 VOLTS/DIV switch and VARI-
ABLE 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.
2-8
(A) Channel 1 Signal
eae’
+++-+++
1
Spe that +
r+++ +++-+-+++
(B) Channel 2 Signal
cob clay
(C) Resultant Display 1126-44
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.
on
\

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