Tektronix 7A18 User manual

Tektronix, Inc
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P .
O . Box 500 " Beaverton,
Oregon 97005 "
Phone
644-0161
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Cables : Tektronix
070-1126-01
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OPTION INFORMATION
REV . JUNE
1974
CHANGE
INFORMATION
7A18/7A18N
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 manual .
SECTION 1
SPECIFICATION Page
Introduction
1-1
Electrical Characteristics 1-1
7A18 And Mainframe Frequency
Response 1-3
Environmental Characteristics 1-3
Physical Characteristics 1-3
SECTION 2 OPERATING INSTRUCTIONS
Installation 2-1
Front
Panel Controls and Connectors 2-1
General Operating
Information 2-2
Basic Applications 2-4
SECTION 3 CIRCUIT DESCRIPTION
Introduction 3-1
Block Diagram Description
3-1
Detailed Circuit Description
3-1
SECTION4 MAINTENANCE
Preventive Maintenance 4-1
Troubleshooting
4-1
Replacement
Parts 4-3
Component
Replacement 4-4
SECTION 5 CALIBRATION
Recalibration Interval 5-1
Test Equipment Required
5-1
Part
I - .-
Performance Check 5-3
Part II -- Adjustment
5-7
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
SECTION8 MECHANICAL
PAR'I'S LIST
Mechanical Parts
List Information
Index
of Mechanical Parts Illustrations
Mechanical Parts
List
Accessories

7A18/7A18N
Fig . 1-1 .
7A18 and 7A18N
Amplifier .

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
Deflection Factor
GAIN
Deflection Factor Accuracy
Characteristic
Calibrated Range
Uncalibrated
(VARIABLE)
Frequency Response
System Dependent
(8 div reference signal)
Upper Bandwidth
DC (Direct) Coupled
Lower Bandwidth
AC (Capacitive) Coupled
With
10X Probe
Change information, if
any, affecting this section will be found
at the rear of the manual .
Performance Requirement
5 mV/Div to 5 V/Div ; ten
steps in a 1,
2, 5 sequence .
Within 2% with GAIN adjusted
at
0 . mV / D i v
. .
._ ._
. . .__ . ._ .
Continuously
variable between calibrated
steps ;
extends deflection factor to at
least 12 .5
V/Div .
See Table A
10 Hertz or less
1 Hertz
or less
TABLE
1-1
ELECTRICAL
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,
series
oscilloscopes .
7A18/7A18N
Permits adjustment
of deflection factor
for calibrated
operation with all 7000-

Specification-7A18/7A18N
TABLE
1-1 (cont)
Characteristic
Maximum
InputVoltage
DC Coupled
AC Coupled
Channel
Isolation
Input
R and C
Resistance
Capacitance
RC Product
m . .
Displayed Noise
(Tangentially Measured)
Overdrive Recovery Time
Common
Mode Rejection
DC
Drift
Display Modes
Drift with Time (ambient
temperature and line voltage
constant)
Drift with Temperature
(line voltage constant)
Time Delay between Channels
Performance Requirement
50 :1 display
ratio up to 50
megahertz
.
1 MQ±2%~
- Approximately 20 .0 pf
At least 10 :1 up
to 50 megahertz .
Channel
1 only .
Dual-trace, alternate
between channels .
Added algebraically .
Dual-trace chopped
between channels .
Channel
2 only .
Supplemental, Information
250
volts, (DC+ Peak AC) ; AC com-
ponent500 volts peak-to-peak maxi-
mum, one kilohertz or less .
500 volts, (DC
+
Peak AC) ; AC compo-
nent 500 volts peak-to-peak
maxi-
mum, one kilohertz or
less .
Within
:,_
1% between
all deflection
-"
factors .
300
microvolts or less at 5 mV/Div in
7000-Series
Oscilloscope .
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
.
0.02 division
or less in anyone minute,
after one
hour warmup .
No more than
0 .01 division per degree C .
700
picoseconds or less .

Cha,~~o,puo
Trigger source Selection
Size
Weight
With 7700 SeHes
75 MHz
Performance Requirement
:7-
Channel l only .
Follows DISPLAY MODE selection .
TABLE 1-1 (cont)
TABLEA
7A18 AND
MAINFRAME
FREOUENCY RESPONSE
60 MHz
TABLE
1-2
ENVIRONMENTAL
CHARACTERISTIC
Refer to
We Specification for
the associated oscilloscope .
TABLE
1-3
PHYD!CAL
Fits all 7000-series
plug-in compartments .
2 Pounds 10 Ounces
(1 .4 kilograms)
Specification-7A18/7Al8N
With 7400S=,ip
50 MHz

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
SECTION 2
OPERATING INSTRUCTIONS
Change
information, if any, affecting this section will be found at the rear of the manual .
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 7A18 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 .
Input Connector

Provides signal connection to the
channel .
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
.
Fig .
2-1 . Front-panel controls and connectors . (7A18shown .)
POSITION
GND-Grounds the amplifier input
while maintaining the same load
for the input signal . Provides a
charge path for the AC coupling

IDENTIFY
capacitor to precharge the input
circuit before switching the
input to AC .
(7A18 only)
7A18/7A18N
DC-Both AC and DC components
of the signal are coupled to the
amplifier input .
Controls position of the
trace . Posi-
tioning of the trace
in the "ADD"
Display Mode is controlled
by CH 1
POSITION control
only
.
Deflects trace about 0 .2 division for
trace identification . In instruments
with readout, also replaces readout
with the word "IDENTIFY" .

Operating Instructions-7A18/7A181\1
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
(VOL1 - S/DIV)

calibrated settings between cali-
brated steps . Extends the deflection
factor range to 12 .5 volts/division
or more .
GAIN Adjustment

When the VARIABLE control is
pushed in, it becomes a front-panel
screw-driver adjustment for cali-
bration of deflection factor .
DISPLAY MODE

Selects
one
of the following modes
of operation :
2-2
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 .
-T- he 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 .
Thetwo 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- I nternal

triggering

signal
obtained from signal applied to
CH 1 .
MODE--Internal trigger signal auto-
matically 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
obtained from signal applied
to
CH 2 .
CH 2 POLARITY

Provides means of inverting
the CH
2 display .
Introduction
Signal Connections
+
-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 OPERATINGINFORMATION
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 .
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
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 . 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

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 . APR
1974
CH 2
POLARITY Switch
DISPLAY MODE Switch
Operating Instructions---7A18/7A18N
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 .
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, 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 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 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 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 :
CH 1

Channel l
CH 2

Channel 2
ADD

Algebraic sum of channel 1 and channel 2
CHOP

Algebraic sum of channel 1 and channel 2
ALT

Alternates between channel 1 and channel 2
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" .
General
Peak-to-Peak
Voltage Measurements (AC)
To make peak-to-peak voltage measurements, use the
following procedure :
2-4
MODE

TRIGGER SIGNAL SOURCE
BASIC APPLICATIONS
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 .
1 . Apply the signal to either
input connector .
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 belowabout 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 .
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
Volts

-
deflection
X
VOLTS/D IV
X attenuation
Peak to Peak

setting
(divisions)

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 .
NO TE
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
T riggering controls for a stable
display . Set the
time-base sweep rate for an
optimum
display of the waveform .
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 :
Operating
Instructions---7A18/7A18N
Instan-
vertical

VOLTS/
probe
taneous
= distance X polarity X

DIV

X
attenuation
Voltage
(divisions)

setting
factor
Substituting the given values
:
Instantaneous

3 .6
X f1
X 0 .5 V X 10
Voltage
The instantaneous voltage is 18
volts .
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 - fS/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 v ~ 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

_3 _0 V
Conversion - 4 X 5
V = 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

-
2
VX

1 .5 X 5 =

15
volts
Amplitude
Dual-Trace Phase Difference
Measurements
Phase comparison
between two signals of the same fre-
cluency 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 . 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 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

sweep
rate
Phase
Difference = difference X
(degrees/division)
(divisions)
Fig . 2-4 . Measuring phase difference
betweentwo signals .
Operating
Instructions--7A18/7A181\1
Substituting the given values
:
Phase
Difference - 0 .3 X 45 °
The
phase difference is 13 .5 ° .
HighResolution
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"
The
phase difference is 13 .5 ° .
Fig . 2-5 . High resolution
phase measurement using
time-base
magni-
fier .
2- 7

Operating Instructions--7A18/7A18N
Common Mode Rejection
T - he 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 POLARI I -
Y 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 .
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 Channel
1 .
(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 .

Introduction
Change
information, if any, affecting this section will be found at the rear
of
this manual .
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
numberon each block
refers to the schematic on which the
complete circuit
is found .
The signal to be displayed on the
CRT is applied to the
input connector . The signal passes
through the input
coupling switch, where the appropriate
coupling is selected,
to the
attenuators . The VOLTS/DIV switch
selects the
correct
amount of attenuation and the signal is passed to
the input amplifier
.
The 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
General
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
The Attenuator circuit
determines the input coupling
and the 7A18
deflection factor .
NO
TE
7A18/7A18N
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
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 .

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 .
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
CHANNEL 1
INPUT AMPLIFIER
The Input Source
Follower 0210A provides a high input
impedance with a low impedance drive for the following
stage . R210 limits the current drive to the gate of Q210A .
Dual-diode CR210
provides circuit protection by limiting
the voltage swing at
the gate of 0210A to about ± (positive
or negative) 15 volts . Q210B provides a constant current
source for 0210A . 0210A and Q210B are encapsulated in
the same case so that 0210B temperature-compensates the
circuit .
Paraphase Cascode Amplifier
Paraphase amplifier
Q220-0320, in conjunction with
0225-0325, forms a cascode amplifier . 022 . .0-0320 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
±0 .6 volt . Common-base
connected 0225-0325 provide
isolation between the paraphase amplifier and the GAIN-
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 Q225 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 IDEN
-
TIFY switch
currents .
Position of the
trace is determined by the setting of the
POSITION
control, R11 . This control changes the current
drive to Q250-0350 .
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 Q260-0360 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 .

INPUT
SOURCE
FOLLOWER
PARAPHASE
CASCODE
AMPLIFIER
EMITTER
FOLLOWER
L, .~ . .
TO
TRIGGER
CHANNEL SWITCH
U470
1126-08
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 .
CHANNEL 2 INPUT AMPLIFIER
REV . D, DEC . 1976
Fig .
3-1 . Channel 1 Input Amplifier detailed block diagram .
Paraphase Cascode Amplifier
Fig . 3-2 . Channel 2 Input Amplifier detailed
block diagram .
Circuit Description-7A18/7A181\1
The Paraphase Cascode Amplifier for
Channel 2 consists
of Q420, 0520, Q425, 0525, Q426, and Q526
. 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, Q425 and 0525 are
biased on and the signal is passed to the Second Cascode
Amplifier stage as for the Channel 1 InputAmplifier . With
the
CH
2 POLARITY switch set to INVERT, Q425 and
0525 are biased off and
Q426-0526 are turned on to
provide signal inversion .
3-3
INPUT
SOURCE
FOLLOWER
PARAPHASE
CASCODE
AMPLIFIER
SECOND
CASCODE
AMPLIFIER
EMITTER
FOLLOWER
FROM
Q41 OA
0420 GAIN 0440
TO
SIGNAL
INPUT
0520Q426 AND 0540 0460
CHANNEL
ATTEN
0410B 0425 0526 VARIABLE
-I
0450
0560
SWITCH
0525 CONTROLS 0550
U270
DC
AL IN
w
TO
BAL
CH 2 POLARITY
TRIGGER
CHANNEL
IDENTIFY
SWITCH
FROM
U470
S23
(7A18 only)

Circuit Description- 7A18OA18N
Second Cascode Amplifier
The Second Cascode Amplifier for Channel 2 consists of
0440, 0540,Q450, and 0550 . Position of the trace is set
by the
POSITION control, R21 or by network 19455-8555
as determinedby the
DISPLAY MODE switch . In any DIS-
PLAY MODE switch position other than ADD, I-50 volts is
applied to the center arm of the POSITION control through
1932 . The POSITION control varies the current drive to the
emitters of Q450-0550 . 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 19455-19555 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 .
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
CHANNEL
SWITCHES
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 19276-19277 and
19376-19377 set the current gain for each channel . Networks
C274-19274-C275-19275 and C374-19374-C375-19375 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 .
SELECTED
CH 1
ALT
CHOP
---~
*Level is switched between the HI-level and LO-level at an
approxi-
mate 0 .5 megahertz rate .
**Level is switched between the HI-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 TRIGGERSOURCE
switch . Resistors 19476-19477 and 19576-19577 set the cur-
rentgain for each channel . Networks
C474-19474-C475-19475 and C574-19574-C575-19575 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
determinedby 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, Q280-0380, 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 .
General
DISPLAY MODE AND
TRIGGER SWITCHING
The Display Mode and Trigger Switching
circuit deter-
mines which input signal (Channel 1 or Channel 2) provides
the Signal and T - rigger 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 .

DISPLAY MODE Switch
The DISPLAY MODE switch provides logic
level outputs
" TRIGGER SOURCE Switch
The TRIGGER SOURCE switch provides
logic level out-
puts to the Trigger Channel
Switch (U470, Channel
Switches diagram 4) . A table of the outputs for
each switch
position is shown in Fig . 3-4 .
General
Connectors
TRIGGER SOURCE switch
selections .
REV . D, JAN
.
1975
Fig . 3-4 . Input/Outpu
t combinations for DISPLAY MODE and
CONNECTORS AND READOUT
The Connectors and
Readout circuit consists of the
power supply and signal distribution from the Interface
Connector
and the Readout Encoding circuit . A schematic
of this
circuit is shown on Diagram 6 in the Diagrams
section .
All the connections made to the
mainframe by the 7A18
are shown on the Connectors portion
of Diagram 6 . Also
shown are the power supply decoupling components
.
Readout Encoding (7A18
only)
Circuit Description-7A18/7A18N
1 - he Readout Encoding circuit consists
of switching
resistors and probe sensing stage Q620 . This circuit encodes
the Channel 1 and 2, Row and Column output lines for
readout of deflection factor, uncalibrated deflection factor
(VARIABLE) information, and signal inversion (channel 2
only) . Data is encoded on these output lines by switching
resistors between them and the time-slot input lines, or by
adding
current through 0620 .
R647-CR647 are switched between time-slot three
(TS-3) and Column output line when the CAL IN switch is
in the uncal position . This results in the symbol > (greater
than) being
displayed preceding the deflection factor read-
out . R648 (Channel 2 only) is switched
between TS-2 and
the Column output line when the CH 2 POLARITY switch
is in the INVERT position . - This results in the symbol I
(inverted) being displayed preceding the deflection
factor
readout .
Switching resistors are used to indicate the
setting of the
VOLTS/DIV
switch to the mainframe readout
system . I - he
VOLTS/DIV
switch is a cam-type switch . The dots on the
contact-closure chart
(see Diagram 6) indicate when the
associated contacts are
closed . R633, R634, and R635
select the number
1, 2, or 5 depending on the resistor
combination that is
switched in . R637 selects the m (milli-)
prefix and R639 selects
the symbol V (volts) in the 5 mV
through .5 V (500 mV)
positions of the VOLTS/DIV
switch . R638 selects
the symbol V in the 1, 2, and 5 V
positions . R630, R631, and the
output of the probe sensing
stage (Q620) select the
decimal point (number of zeroes)
again depending on the
resistor combination switched in by
the VOLTS/DIV switch .
Probe sensing
stage Q620 identifies the attenuation
factor of the
probe connected to the input connector by
sensing the
amount of current flowing from the current
sink through the probe
coding resistance . The output of
this circuit corrects the
mainframe readout system to in-
clude the probe attenuation factor .
The third contact of the
input connector provides the input
to the probe sensing
stage from the probe coding
resistance (coded probes
only ;
see Operating Instructions) . The third contact
is also
used for
the IDENTIFY input . The coding resistor forms a
voltage divider
with R621 through CR621 to the --15 V
supply . The resultant
voltage sets the bias on Q620 and
determines, along with emitter
resistor R622, the collector
current . When the -15 volt
time-slot pulse is applied to
Interface Connector B33, Q620 is
interrogated and its
collector current is added to the
column current output
through Interface Connector A37 .
3-
5
to the Signal Channel Switch stage (U270,
Channel
Switches diagram 4) . A table of the outputs for each
position of the DISPLAY MODE switch is shown in Fig .
3-3 .