Tektronix 7A18 User manual

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INSTRUCTION
MAN UAL
Tektronix, Inc, @ P. O. Box 500 @ Beaverton, Oregon 97005 @ Phone [phone] @ Cables: Tektronix
070-1126-01 1271

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SECTION 1
SECTION 2
SECTION 3
SECTION 4
SECTION 5
©1
7A18/7A18N
TABLE OF CONTENTS
SPECIFICATION
Introduction
Electrical Characteristics
7A18 And Mainframe Frequency
Response
Environmental Characteristics
Physical Characteristics
OPERATING INSTRUCTIONS
Installation
Front Panel Controls and Connectors
General Operating Information
Basic Applications
CIRCUIT DESCRIPTION
Introduction
Block Diagram Description
Detailed Circuit Description
MAINTENANCE
Preventive Maintenance
Troubleshooting
Replacement Parts
Component Replacement
CALIBRATION
Recalibration Interval
Test Equipment Required
Part | — Performance Check
Part I] — Adjustment
Page
1
4 SECTIONG
SECTION 7
SECTION 8
3-1
3-1
3-1
4-1
4-1
OPTION INFORMATION
ELECTRICAL PARTS LIST
Abbreviations and Symbols
Parts Ordering Information
DIAGRAMS AND CIRCUIT BOARD
ILLUSTRATIONS
Symbols and Reference Designators
Voltage and Waveform Conditions
MECHANICAL PARTS LIST
Mechanical Parts List Information
Index of Mechanical Parts Illustrations
Mechanical Parts List
Accessories
4-3 CHANGE INFORMATION
4-4
Abbreviations and symbols used in this manual are based on
5-1 or taken directly from IEEE Standard 260 “Standard
5-1 Symbols for Units’, MIL-STD-12B and other standards of
5-3 the electronics industry. Change information, if any, is
5-7 located at the rear of this manual.
REV. JUNE 1974

7A18/7A18N
Fig.
1-1.
7A18
and
7A18N
Amplifier.
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7A18/7A18N
SECTION 1
SPECIFICATION
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 encoding capabilities 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 7A18 is a dual-channel,
medium-bandwidth amplifier. Internal gain and compensa-
tion circuits are automatically switched to correspond to
the setting of the VOLTS/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
2, 5 sequence.
5 mV/Div to 5 V/Div; ten steps ina 1,
Deflection Factor Accuracy
10 mV/Div.
Within 2% with GAIN adjusted at
Uncalibrated
(VARIABLE)
least 12.5 V/Div.
Continuously variable between calibrated
steps; extends deflection factor to at
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) Coupled See Table A
Lower Bandwidth 10 Hertz or less
AC (Capacitive) Coupled
With 10X Probe 1 Hertz or less
1-1

Specification—7A18/7A18N
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 Isolation 50:1 display ratio up to 50
megahertz.
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)
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
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)
No 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.
1-2

TABLE 1-1 (cont)
Specification—7A18/7A18N
Characteristic Performance Requirement
Supplemental Information
Trigger source Selection
Channel 1 only.
Follows DISPLAY MODE selection.
Channel 2 only.
TABLE A
7A18 AND MAINFRAME
FREQUENCY RESPONSE
With 7700 Series
75 MHz
With 7500 Series
60 MHz
With 7400 Series
50 MHz
TABLE 1-2
ENVIRONMENTAL CHARACTERISTIC
Refer to the Specification for the associated oscilloscope.
oy
TABLE 1-3
PHYSICAL
Size
Fits all 7000-series plug-in compartments.
Weight
2 Pounds 10 Ounces (1.4 kilograms)
1-3

NOTES

7A18/7A18N
SECTION 2
OPERATING INSTRUCTIONS
Change
information,
if
any,
affecting
this
section
will
be
found
at
the
rear
of
the
manual.
General
To effectively use the 7A18, the operation and capabili-
ties of the instrument must be known. This section des-
cribes front-panel control functions, general information on
signal input connections, and other subjects that pertain to
various measurement applications.
Installation
The 7A18 is calibrated and ready for use as received. It
can be installed in any compartment of Tektronix 7000-
Series oscilloscopes, but is intended for principal use in
vertical plug-in compartments. To install, align the upper
and lower rails of the 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-left corner of the 7A18 will be in place
against the front panel.
To remove the 7A18, pull on the latch (which is
inscribed with the unit identification ““7A18” or “7A18N”)
and the 7A18 will unlatch. Continue pulling on the latch to
slide the 7A18 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.
Provides signal connection to the
channel.
Input Connector
AC-GND-DC Selects signal input coupling mode.
AC—The AC component of the sig-
nal is coupled to the amplifier
input 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,
Fig. 2-1. Front-panel controls and connectors. (7A18 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 0.2 division for
(7A 18 only) trace identification. In instruments
with readout, also replaces readout
with the word “IDENTIFY”.
2-1

Operating Instructions—7A18/7A18N
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 un-
(VOLTS/DIV) 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.
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 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.
The trigger signals provide internal
triggering for the oscilloscope time-
base units.
CH 1—Internal triggering signal
obtained from signal applied to
CH 1.
MODE-~—Internal trigger signal auto-
matically follows DISPLAY
MODE selection. In ADD or
2-2
CHOP display modes, the trigger
signal is the algebraic sum of CH
1 and CH 2 trigger.
CH 2-—Internal trigger signal
obtained 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 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 SOURCE switches to CH
1 or CH 2 and connecting the signal to be observed to the
appropriate input. In the discussions to follow, single-trace
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 7A18. A 10X 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 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 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. For probes to be used with amplifier
units without 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 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
©1

—,
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.
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 deflec-
tion factors indicated by the settings of the VOLTS/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 VOLTS/DIV
switch. With the VARIABLE control fully counterclock-
wise and the VOLTS/DIV set to 5 volts/div the uncali-
brated 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.
REV. D JUL 1977
Operating Instructions—7A18/7A18N
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 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 ALT position of the DISPLAY
MODE switch produces a display which alternates between
channel 1 and channel 2 with each sweep on the CRT.
Although the ALT mode can be used at all sweep rates, the
CHOP mode provides a more satisfactory display at sweep
rates below about 0.2 millisecond/division. At slow sweep
rates alternate mode switching 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. 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 difficult to determine from the CRT display. In this case,
the voltage amplitude of the resultant display can be de-
termined 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 kilo-
hertz rate (controlled by mainframe). In general the CHOP
mode provides the best display at sweep rates slower than
2-3

Operating Instructions—7A18/7A18N
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 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 (7A18 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 readout, also replaces deflec-
tion factor readout with the word “IDENTIFY”.
BASIC APPLICATIONS
General
The following information describes the procedures and
techniques for making basic measurements with a 7A18 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 this manual. Contact your
local Tektronix Field Office or representative for assistance
in making specific measurements with this instrument.
Peak-to-Peak Voltage Measurements (AC)
To make peak-to-peak voltage measurements, use the
following procedure:
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 hertz use
the DC position to prevent attenuation of the signal.
4. Set the VOLTS/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 graticule
lines below the center horizontal line, and the top of the
waveform is within the viewing area. With the time-base
Position control, move the display so one of the upper
peaks lies near the center vertical line (see Fig. 2-2).
7. Measure the divisions of vertical deflection peak-to-
peak, Check that the 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
Centerline
A A iA
N\
VN R\ |
Vertical Ye ae | | pe
a:
—
|
VO
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 atten-
uator probe, and the VOLTS/DIV switch is set to 1 V.
vertical probe
p
ve
k
=
deflection
X
VOLTS/DIV
X
attenuation
man
ee
(divisions)
ag
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 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.
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—7A18/7A18N
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
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 +1 X 0.5V X 10
Voltage
The instantaneous voltage is 18 volts.
Point A
Vertical
Distance
Reference
Line
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 VOLTS/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 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
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
not known, it can be measured before the VARIABLE
VOLTS/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 VOLTS/DIV switch. This is
the vertical conversion factor.
Vertical reference signal
Conversion = amplitude (volts)
Factor vertical VOLTS/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 VOLTS/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 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):
Vertical 30 V-
Conversion =4X5V = 1.5
Factor
Then with a VOLTS/DIV setting of 2 V, the
peak-to-peak amplitude of an unknown signal which
produces a vertical deflection of five divisions can be
determined by using the signal amplitude formula (step 6):
Signal =- oy X 1.5 X 5 = 15volts
Amplitude
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-
cedure:
1. Set the CH 1 and CH 2 coupling switches to the same
position, depending on the type of coupling desired.
2. Set the DISPLAY MODE to ALT or CHOP. In
general, CHOP is more suitable for low frequencies and
ALT is more suitable for high frequencies. Set the
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. |f 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 cal-
culation.)
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
controls for a stable display.
7. Center the waveforms on the graticule with the 7A18
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 formula:
horizontal
Operating Instructions—7A18/7A18N
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 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 X10. With a hori-
zontal difference of 3 divisions, the phase difference is:
horizontal magnified
Phase Difference = difference X sweep rate
(divisions) (degrees/division)
Substituting the given values:
Phase Difference = 3 X 4.5°
sweep rate
Phase Difference = difference X (degrees/division)
(divisions) The phase difference is 13.5°.
4
Channel
1
fA
Channel
1
NI
Aetorer
\
(Reference)
\
t
KX
Channel
2
Lagging ++
t Y IS
t Channel 2
t —> Horizontal
Difference —
f
bas
Horizontal
Difference
Fig. 2-4. Measuring phase difference between two signals.
©
Fig.
2-5.
High
resolution
phase
measurement
using
time-base
magni-
fier.
2-7

Operating Instructions—7A18/7A18N
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 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
VARIABLE 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
<
7
44
ph
(A) Channel 1 Signal
Shih dt hl ddd eke
ttt ttt tt tt
(B) Channel 2 Signal
(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.
REV. D JUL 1977
—

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 7A18 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 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 7A18 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 VOLTS/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
SOURCE 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 VARIABLE 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 17 and CH 2 Attenuator circuits are identical.
To minimize duplication, only CH 17 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 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 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—7A18/7A18N
Input Attenuator
The effective overall deflection factor of the 7A18 is
determined by the setting of the VOLTS/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 which contain the necessary resistances
and capacitors. Each attenuator is replaceable as a unit.
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.
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 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.
CHANNEL 1
INPUT AMPLIFIER
General
The Channel 1 Input 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 Q210A 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 15 volts.
3-2
Q210B provides a constant current source for Q210A.
Q210A and Q210B are encapsulated in the same case so
that
Q210B temperature-compensates the circuit.
Paraphase Cascode Amplifier
Paraphase amplifier Q220-0320, in conjunction with
Q225-0325, forms a cascode amplifier. Q220-0320 convert
the single-ended input signal to a differential output signal.
Diodes CR220-CR221 hold the voltage level at the base of
Q220 close to ground to limit the voltage swing to about
+0.6 volt. Common-base connected Q225-0325 provide
isolation between the paraphase amplifier and the GAIN
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. Adjust-
ment 1, R321, varies the base level of O320 to provide the
same voltage levels at the collectors of Q225 and Q325.
This prevents a zero-volt reference trace from changing
position when varying the 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
Q250-0350 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 Q250 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.
REV. D JUL 1977

Circuit Description—7A18/7A18N
INPUT PARAPHASE
SAUREE CASCODE
FOLLOWER AMPLIFIER
FROM —
AND
INPUT
VARIABLE
ATTEN | CONTROLS
SECOND
CASCODE EMITTER
AMPLIFIER FOLLOWER
> TO
SIGNAL
CHANNEL SWITCH
> S_ U270
‘\
POSITION
To
TRIGGER
CHANNEL SWITCH
IDENTIFY
FROM id
$13
(7A18 only)
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 Input Amplifier circuit. A
schematic of this circuit is shown on Diagram 3 in the
Diagrams section.
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
Q525 are biased off and 0426-0526 are turned on to
provide signal inversion.
PARAPHASE
INPUT CASCODE
SOURCE AMPLIFIER
FOLLOWER
GAIN
fapor 0520 0426 AND
pad VARIABLE
CONTROLS
\
\ CAL IN
CH 2 POLARITY
SECOND
CASCODE
EMITTER
AMPLIFIER eoL_LOWER
- TO
SIGNAL
Ocao CHANNEL
SWITCH
>
-———>-
_—-u270
\
N
POSITION Ls sotto
TRIGGER
CHANNEL
IDENTIFY SWITCH
FROM vere
$23
(7A18 only)
Fig. 3-2. Channel 2 Input Amplifier detailed block diagram.
REV. D, DEC. 1976

Circuit Description—7A18/7A18N
Second Cascode Amplifier
The Second Cascode Amplifier for Channel 2 consists of
0440, 0540, 0450, and O550. Position of the trace is set
by the POSITION control, R21 or by network R455-R555
as determined by the DISPLAY MODE switch. In any DIS-
PLAY MODE switch position other than ADD, +50 volts is
applied to the center arm of the POSITION control through
R32. The POSITION control varies the current drive to the
emitters of 0450-0550. 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. When the DISPLAY MODE switch is in the
ADD position, +50 volts is applied to the junction of resis-
tors R455-R555 through R32 to balance the current drive
to
the emitters of 0450-0550. This results in a fixed zero
volts (approximately) difference between the collectors.
Since +50 volts is not applied to the POSITION control in
the ADD position of the DISPLAY MODE switch, the
control setting has no effect on the circuit operation.
CHANNEL SWITCHES
General
The Channel Switches circuit provides Signal and Trigger
outputs to the oscilloscope via the Interface Connector as
determined by the DISPLAY MODE and TRIGGER
SOURCE switches. A schematic of this circuit is given on
Diagram 4 in the Diagrams section.
Signal Channel Switch
The Signal Channel Switch stage consists of integrated
circuit U270 and its external components. This stage selects
one, or mixes two input analog signals in response to inputs
from the DISPLAY MODE switch. The Signal Channel
Switch stage determines which input (CH 1 or CH 2) pro-
vides the signal to the oscilloscope as controlled by the
DISPLAY MODE switch setting. Resistors R276-R277 and
R376-R377 set the current gain for each channel. Networks
C274-R274-C275-R275 and C374-R374-C375-R375 pro-
vide high-frequency compensation for each channel. C275
and C375 in these networks are high-frequency compen-
sation adjustments.
Fig. 3-3 shows the U270 input combinations for each
position of the DISPLAY MODE switch. When the level at
pin 14 is LO the output of U270 is determined by the level
at pin 4. With the level at pin 14 HI and the level at pin 4
LO, the signals from both channel 1 and channel 2 are
passed to the Signal Output stage. This condition occurs
only
when the DISPLAY MODE switch is set to ADD. In
this operating mode the signal output is the algebraic sum
of channel 1 and channel 2 signals and the resultant signal
determines the mainframe deflection.
3-4
DISPLAY MODE U270
SELECTED Pin 4 Pin 14
CH 1 LO LO
ALT we LO
ADD LO HI
CHOP = LO
CH 2 HI LO
*Level is switched between the HI-level and LO-level at an approxi-
mate 0.5 megahertz rate.
**Level is switched between the Hl-level and LO-level at a rate
determined by the setting of the time-base unit sweep rate.
Fig. 3-3. U270 input combinations for DISPLAY MODE selection.
Trigger Channel Switching
The Trigger Channel Switch U470 is identical to the
Signal Channel Switch. This stage determines which input
(CH 1 or CH 2) provides the trigger signal for internal
triggering of the time-base unit. The selection of the trigger
signal is controlled by inputs from the TRIGGER SOURCE
switch. Resistors R476-R477 and R576-R577 set the cur-
rent gain for each channel. Networks
C474-R474-C475-R475 and C574-R574-C575-R575 pro-
vide high-frequency compensation for each channel.
An input/output table for this stage is shown in Fig. 3-4.
When the level at pin 14 is LO, the output of U470 is
determined by the level at pin 4. With the level at pin 14 HI
and the level at pin 4 LO, the channel 1 and channel 2
triggers are added algebraically.
Signal and Trigger Output
The Signal Output stage, 0280-0380, and the Trigger
Output stage, 0480-0580, are similar. Each stage consists
of a pair of common-base connected transistors which pro-
vide the DC level shifting necessary to drive the mainframe
circuits,
DISPLAY MODE AND
TRIGGER SWITCHING
General
The Display Mode and Trigger Switching circuit deter-
mines which input signal (Channel 1 or Channel 2) provides
the Signal and Trigger outputs to the mainframe as selected
by the DISPLAY MODE and TRIGGER SOURCE switches.
This circuit also provides plug-in mode information to the
mainframe chop blanking circuit, and readout control infor-
mation for proper CRT display.
—~