Tektronix 3A74 User manual

INSTRUCTION
MI
ANI
UAL
Serial
Number
35
0
Tektronix, Inc.
S.W.
Millikan
Way
@
P.O.
Box
500
@
Beaverton,
Oregon
@
Phone
Mi
4-0161
@
Cables:
Tektronix
070-347
863

5
be]
a
~
—_

Type 3A74

”
-_
TYPE
3A74
AMPLIFIER
FOUR-TRACE
1
MEG,
VOLTS/DIV.
POSITION
oc
BAL.
MODE
47 pf
'
2
1
NORM,
1
MEG.
VOLTS/DIV.
POSITION
‘Ss
DC
BAL.
“
MopE
1
2
NORM,
VOLTS/DIV.
pe
BAL.
POSITION
~~
MODE
1
NORM.
©
ADJ.
2
a
os
VA
VOLTS/DIV.
POSITION
DC
BAL.
“MODE
NORM.
ADJ.
@
n(®)
&
ian
PORTLAND.
OREGON.
U.S.A.
'
SERIAL
On Se ae
KTRONIX. INC
Type
3A74
\
A)

SECTION 1
CHARACTERISTICS
General Information
The Type 3A74 Four-Trace Amplifier Plug-In Unit is de-
signed for use with all Type 560-Series Oscilloscopes* except
the Type 560 itself. The Type 3A74 contains four independent
amplifier channels. Each channel can be used separately to
produce a single display or electronically switched to pro-
duce multi-trace displays. These features reduce cable switch-
ing to a minimum.
Each amplifier channel has its own input coupling,
attenuator, variable gain, position and mode controls which
permit individual adjustment of each channel for best
measurement conditions.
In multi-trace operation there is a choice of two modes
of operation—chopped or alternate. In the chopped mode
(free-running electronic switching), an internal oscillator
switches the channels sequentially at a free-running rate of
about 500ke. In the alternate mode (triggered electronic
switching) the oscilloscope time-base generator internally
switches the channels at the end of each sweep. Switching
occurs during retrace intervals.
Bandpass
De to 2 mc.
Risetime
0.17 microsecond.
Calibrated Deflection Factors
Nine calibrated steps provided for each channel: 0.02,
0.05, 0.1, 0.2, 0.5, 1, 2, 5, and 10 volts/div. When the
gain is set accurately for the 0.02 volt/div. step, the other
steps
will
be
accurate
within
3%
of
their
respective
panel
readings.
Uncalibrated Deflection Factors
Variable controls for each channel permit continuous
uncalibrated adjustments from 0.02 volt/div. up to approxi-
mately 25 volts/div.
Input Impedance
1 megohm +1% paralleled by 47 pf +3%.
*Type 561 Oscilloscopes below serial number 580 must be modified
first to obtain proper multi-trace operation from the Type 3A74. If
your oscilloscope needs to be modified, ask your Tektronix Field
Engineer for Modification Kit No. 040-267. This modification is
not needed in any of the Type RM561 Oscilloscopes because they
are all compatible.
@l
Maximum Allowable Signal Voltage at
Input Connectors
=£600 volts de or peak.
Input Coupling
Selectable AC or DC coupling. In the AC position a
coupling capacitor is inserted, limiting the low-frequency
response to about 2 cps at 3 db down. With a 10X attenuator
probe low-frequency response is extended to 0.2 cps; with
a 100X probe, response is 2 cps.
Operating Modes
Channel 1, 2, 3, or 4 only.
Chopped—Sequential electronic switching of two or more
channels at approximately a 500-ke rate. The input for each
channel is chopped into “on” and “‘off’’ segments. Thus the
display of each channel is a series of 2-ysec “on” segments.
Alternate—Triggered electronic switching sequentially from
channel to channel during retrace intervals.
Polarity Inversion
Polarity switch for each channel permits inverting the
displayed waveform.
Construction
Aluminum-alloy chassis.
Finish
Photo-etched anodized aluminum front panel.
Net Weight
6 lbs. 4 oz.
Included Accessories
Tektronix
Qty. Part Number Description
4 103-033 Adapter, single binding post fitted
with a BNC plug.
2 070-347 Type 3A74 Instruction Manuals.

NOTES

SECTION 2
OPERATING INSTRUCTIONS
NOTE
The Type 3A74 can be used in either the X-axis
or Y-axis plug-in opening of Tektronix Type 561-
Series, 567, or RM567 oscilloscopes. In the fol-
lowing discussion assume that the Type 3A74 is
inserted in the left-hand (Y-axis) opening which
will provide vertical deflection with crt beam
blanking of the channel switching transient. If the
the Type 3A74 is used with a Type 565 or RM565
Dual-Beam Oscilloscope, assume the unit is inserted
in the left-hand vertical opening.
FUNCTIONS OF FRONT-PANEL CONTROLS
AND CONNECTORS
Table 2-1 describes the functions of the front-panel con-
trols. Since the four channels are identical, the functions of
channel 1 control and the inputs connector are described in
the table. Functions of the controls for the other channels are
identical. Fig. 2-1 shows the front-panel layout.
TABLE 2-1
INPUT CONNECTOR
BNC Type jack for coupling input voltages to the chan-
nel 1 amplifier.
AC-GND-DC
Three-position miniature toggle switch provides a
choice of ac or de coupling, or grounding the channel
] input circuit without grounding the input connector.
VOLTS/DIV.
Nine-position switch to select the calibrated vertical
deflection factors.
VAR. GAIN
Provides continuously variable attenuation between the
calibrated deflection factors, and extends the attenua-
tion range to approximately 25 volts/div. This control
has 360° rotation and it has a detent position for the
CAL. (calibrated) position.
DC BAL.
Screwdriver-adjustable potentiometer to balance the
amplifier de levels so the trace does not shift position
as the VAR. GAIN control is varied.
GAIN ADJ.
Screwdriver-adjustable potentiometer to precisely adjust
the deflection factor of the channel. (.02 VOLTS/DIV.
is the deflection factor normally calibrated.)
POSITION
Potentiometer to provide vertical positioning of the
trace.
MODE
Three-position switch provides the following choices:
‘a normal display, an inverted display, or no display at
all. In the NORM. position the display has the same
polarity as the input signal. In the INV. position the
polarity of the signal is inverted. In the OFF position
the channel is turned off; during muiti-trace operation
the channel is excluded from the switching cycle.
TRIGGER
A push-pull switch which, when pulled, permits internal
triggering on channel 1 signal only, regardless of the
channel 1 MODE switch settings. When the switch is
pushed in with two or more channels ‘‘on", the time-
base triggers from the composite time-shared vertical
signal. With only one channel on, the channel dis-
played provides the triggering signal.
Plug-In Securing Knob
Permits locking the Type 3A74 in the plug-in com-
partment. (This knob is located at bottom center of the
plug-in unit.)
CALIB.
Master gain control for adjusting the common gain of
all channels. Compensates for differences in ert deflec-
tion sensitivities.
NOTE
For plug-in units S/N 156 and up, including S/N
133 unit (modified out of sequence) CALIB. con-
trol shaft has a shorter length. The shorter shaft
permits the front-panel mounting bushing to be
used as a ground-connection banana jack.
CHOP.-ALT.
Two-position toggle switch to select either alternate or
chopped mode of operation.
FIRST TIME OPERATION
To place the Type 3A74 into operation for the first time,
rotate the plug-in securing knob counterclockwise to move
the latch to its fully counterclockwise unlatched position.
Insert the unit in the left-hand plug-in opening of a Tek-
tronix Type 560-Series oscilloscope (except the Type 560).
Rotate the plug-in securing knob clockwise until hand tight.
1. Turn on the oscilloscope power. Allow about 2 to 3
minutes warm-up time and free run the time base at 1
millisec/div. Set the front-panel controls of the Type 3A74
to these settings.
AC-GND-DC (all channels} DC
VOLTS/DIV. (all channels) 02
VAR. GAIN (all channels) CAL.
2-1

Operating Instructions — Type 3A74
..
VOLTS/DIV.
oo ar
tmeo. VOLTS/DIV. oc eau, POSITION
os, 4 . MODE | nw,
Fig, 2-1. Front-pane!l view of the Type 3A74 showing grouping of controls and connectors for each channel. Nomenclature, as used in the
text, Is also shown,
2-2

Operating Instructions — Type 3A74
POSITION (all channels) Centered
+ MODE (channel 1) NORM.
+ MODE {channel 2, 3 and 4) OFF
t TRIGGER Pushed in
Ch.
1
Trace
+-
1
CHOP.-ALT.
ALT.
Ch. 2 Trace + 2. Using the graticule centerline as a reference, vertically
-° i ae ee ee ee ee ee position the trace about two divisions above the centerline
HEE HH HH HH BEE HH EHH with the channel 1 POSITION control. (If you are using
Ch 3 trace | OT OE TP a Type 565 Dual-Beam Oscilloscope, use the upper beam
| - + Pape graticule centerline as the reference.)
. T t . .
Chef Trace + 3. Place channel 2 MODE switch to NORM. and posi-
as tion the channel 2 trace about one division above the
+ graticule center with the channel 2 POSITION control.
+ 4. Set channel 3 MODE switch to NORM. and position
its trace near the graticule centerline with the channel 3
(a) Chopping rate per channel is ~~ 125 kc; sweep rate POSITION control
is 20 usec/div. :
5. Place channel 4 MODE switch to NORM. and position
its trace one division below the graticule center with the
sweep the time base switches to the next channel in succes-
sion during the retrace interval.
~ 2 + .
~ usec ae + channel 4 POSITION control. This makes a total of four
+ traces on the crt. For each sweep cycle one channel is
“ = 6 usec > displayed and the others are shut off. At the end of each
t 6. To view the alternate-trace switching cycle in slow
HEHEHE HHH motion, decrease the sweep rate to 0.1 sec/div.
7. To observe chopped mode of operation, set the CHOP.-
E ALT. switch to the CHOP. position. If your oscilloscope has
tC a CRT Cathode Selector (Z-axis} switch mounted on the
rear panel, set this switch to the Chopped Blanking posi-
tion to blank (shut off} the crt beam while the Type 3A74
switches between channels.
davadagtiu
tapi
(b) Chopping rate per channel is = 125 ke; sweep rate 8. Set the oscilloscope triggering controls for + internal
is 1 psec/div. triggered-sweep operation. Notice that all four traces seem
to start simultaneously and continue on across the crt
screen.
9. Increase the sweep rate to 20 usec/div. Adjust the
oscilloscope Trigger Level control to obtain a stable display.
Notice that each trace is composed of many short-duration
bits or segments (see Fig. 2-2a).
popu
tpigy
Terry
+t
tt
-L Ch. 1 Trace
10. To see the chopped-mode switching action, increase
the sweep rate to 1 wsec/div and increase the crt intensity.
In chopped-mode operation all four channels are elec-
tronically switched sequentially at a rate of approximately
500 ke. Observe that each channel is “‘on’’ for about 2 usec
and then is turned off for 6 psec while the other three chan-
nels are each “‘on" for 2 usec (see Fig. 2-2b]. The sampling
rate of each channel is about 125kce. (500 ke divided by
the number of channels in use.) Switching time between
channels is approximately 0.3 psec.
2
TTT
TTT
TT
{c) Chopping rate per channel is ~ 250 kc; sweep rate 11. Now set channels 2 and 4 MODE switches to the
is 1 wsec/div. OFF position. Readjust the Triggering Level control to obtain
a stable display. Notice that the Type 3A74 switches
between channels 1 and 3 only (see Fig 2-2c). Each channel
conducts for about 2 usec and then is cut off while the
other channel conducts for an equal time. The sampling
Fig. 2-2. Chopped-mode operation. rate for each channel is now about 250 kc.
®
2-3

Operating Instructions — Type 3A74
GENERAL OPERATION
Any of the four amplifier channels can be used independ-
ently by setting the appropriate MODE switch to the
NORM. or to the INV. position and connecting the signal
to be observed to the appropriate input. The following
discussion applies equally to each channel.
Signal Connections
The signal to be displayed is applied to the appropriate
input connector on the front panel of the Type 3A74. For
best results here are some precautions you should observe
when making the connections.
1. It is often possible to make signal connections to
the Type 3A74 with short-length, unshielded test leads*.
This is particularly true for high-level, low-frequency signals.
When such leads are used, you must also use a ground con-
nection between the Type 3A74 or oscilloscope chassis
ground and the chassis of the equipment under test. Posi-
tion the leads away from any stray electric or magnetic
field source to avoid obtaining erroneous displays.
2. In many low-level applications, however, unshielded
leads are unsatisfactory for making signal connections be-
cause of unavoidable pickup resulting from radiating
fields. To prevent unwanted signal pickup, use shielded
{coaxial} cables. Be sure that the ground conductors of the
*To connect test leads to the Type 3A74, use the single binding
post adapters (Tektronix Part No. 103-033} supplied with the
cables are connected to the chassis of both the oscilloscope
and the signal source.
3. As nearly as possible, simulate actual operating con-
ditions in the equipment under test. For example, the equip-
ment should work into a load impedance equal to that
which it will see in actual use.
4. Consider the effect of loading upon the equipment
under test due to the input circuit of the Type 3A74. The
input circuit can be represented by a resistance of 1 meg-
ohm (+1%} paralleled by a capacitance of 47 picofarads
(42%). With a few feet of shielded cable, the capacitance
is increased considerably. Where the effects of these re-
sistive and capacitive loads will affect the operation of the
device under test or distort the signal, use a probe in the
manner described next.
Use of Probes
An attenuator probe lessens both capacitive and resis-
tive circuit loading and at the same time reduces plug-in
sensitivity. The attenuation introduced by the probe extends
the vertical deflection factor of the Type 3A74 so that high-
amplitude signal voltages can be conveniently displayed on
the crt. When applying signals to the probe, consider the
input voltage rating of the probe. When making the ampli-
tude measurement, be sure to multiply the observed ampli-
tude by the probe attenuation.
unit.
+ + tii, \ + i
- + f — + +
‘ ‘ F f & f
INCORRECT ADJUSTED CORRECTLY INCORRECT
(a) Effect of compensation with a 60-cycie calibrator waveform.
+ L L — | —L + — L QL VL NS 4 VL
ev_ieot
lee
iot
io
—=
=
+
r
¢ ¢ ¢
veseleeerleseeleetsetraetesetreelrteel lospeteeeterelrertreteeeteteelrsteae! — beteedtteetHeeleeeeleeelie
HEHEHE H+ HHH HEE HHH + :
QL QL $+ YN JL —L — + — — rr ct tr rr olet
¢
¢ £
INCORRECT ADJUSTED CORRECTLY INCORRECT
{b) Effect of compensation with a 1-ke calibrator waveform.
Fig. 2-3. Probe compensation waveforms.
2-4
®

To assure the accuracy of pulse or high frequency mea-
surements, check the probe compensation. Once the probe
is compensated for one channel, you can use the same
probe on the other channels without recompensating it.
The input circuits of the Type 3A74 have been adjusted at
the factory so their time constants match each other very
closely.
To adjust the probe input-compensation capacitor, set
the oscilloscope Calibrator control for an output signal of
suitable amplitude. For the Type 567 Oscilloscope use the
calibrator jack that provides the proper output amplitude.
Touch the probe tip to the appropriate calibrator output
connector and adjust the oscilloscope controls to display
several cycles of the waveform. Adjust the probe variable
capacitor for best square-wave response as shown in Fig.
2-3 ‘Adjusted Correctly’ waveforms.
NOTE
if you use a square-wave source other than the
oscilloscope calibrator for compensating the
probe, do not use a repetition rate higher than
5 ke. At higher repetition rates, the waveform
amplitude appears to change as the probe is
compensated, and you will not be able to com-
pensate the probe properly. If the probe remains
improperly compensated, transient and frequency
response of the system will be poor and your
measurements will be inaccurate.
AC-GND-DC Switch
To display both the ac and dc components of an applied
signal, set the AC-GND-DC switch to DC; to display only
the ac component of a signal, set the AC-GND-DC switch
to AC.
In the AC position of the AC-GND-DC switch, the de
component of the signal is blocked by a coupling capacitor
in the input circuit of the channel. The time constant of
the input circuit is 0.1 second and the low-frequency
response of the system is —3db at about 2 cps. Thus, when
displaying a 60-cycle calibrator square wave, you can
expect to see some drop in duration response (droop) of
the square waveshape. If a 10X attenuator probe is used
with the Type 3A74, the system low-frequency response will
be extended to about 0.2 cps.
Placing the AC-GND-DC switch in the GND position
grounds the input circuit of the Type 3A74 to provide a de
zero trace reference. At the same time the switch internally
disconnects, but does not ground, the input connector or
the applied signal.
VOLTS/DIV. Switch and VAR. GAIN Control
The amount of vertical deflection produced by a signal
is determined by the signal amplitude, the attenuation
factor (if any) of the probe, the setting of the VOLTS/DIV.
switch, and the setting of the VAR. GAIN control. Cali-
brated deflection factors indicated by the settings of the
VOLTS/DIV.
switch
apply
only
when
the
VAR.
GAIN
con-
trol is set to the detented CAL. position. Errors in display
measurements
may
result
if
the
setting
of
this
control
is
unintentionally moved away from its CAL. position.
The range of the VAR. GAIN control is approximately
2.5:1
to
provide
continuously
variable
(uncalibrated)
vertical-
@
Operating Instructions — Type 3A74
deflection factors between calibrated settings of the VOLTS/
DIV. switch. The VAR. GAIN control can be rotated in
either direction from its detented CAL. position since there
is no mechanical stop.
Voltage measurements can be made directly from the crt
by noting the calibrated VOLTS/DIV. switch setting for
the applicable channel and the amount of vertical deflec-
tion in major divisions. Then multiply the number of major
divisions by the setting of the VOLTS/DIV. switch and the
attenuation factor, if any, of the probe. An example is
given in the Applications section of this manual.
MODE Switch
To display a single signal with the Type 3A74, apply the
signal to one of the input connectors and set the appropri-
ate MODE switch to the NORM. or INV. position. Set the
MODE switches for the other three channels to OFF. Use
the NORM position to display the signal with the same
polarity as the applied signal. Use the INV. position to
invert the signal display on the crt. The normal-inverted
feature can be particularly useful when comparing two or
more signals during multi-trace operation.
To display two or more signals in multi-trace operation,
connect the signals to the input connectors and set the
appropriate MODE switches to NORM. or INV., depending on
whether you want to display the signals in a normal or
inverted position. At any time one or more signals can be
removed from the display without disconnecting the signal
from the input connector(s). To do this, simply set the
appropriate MODE switch to OFF. To remove the signal
from a channel and still get a trace on the ert for that
channel, set the AC-GND-DC switch to GND instead of
setting the MODE switch to OFF.
CHOP.-ALT. Switch
1. General Information
When you are using the Type 3A74 for single-trace
operation (i.e., the MODE switches of three channels are
set to OFF), the unit acts as a single-channel amplifier.
No switching to other channels occurs, regardless of the
setting of the CHOP.-ALT. switch. However, when more
than one channel is used, then the setting of the CHOP.-
ALT. switch is important because it controls the mode of
operation which produces the multi-trace displays.
In general, use the CHOP. position (chopped-mode opera-
tion) with lower sweep rates for displaying non-repetitive
and low-repetition-rate signals. Non-repetitive signals are
single-shot or randomly occurring signals. Use the ALT.
position (alternate-mode operation} with the higher sweep
rates for displaying high repetition-rate signals.
NOTE
If your oscilloscope has a CRT Cathode Selector
(Z-axis) switch mounted on the rear panel, set
this switch to the Chopped Blanking position
during chopped-mode multi-trace operation.
Leave the switch in the Chopped Blanking posi-
tion at all times except for Z-axis modulation
applications,
2-5

Operating Instructions — Type 3A74
2. Specific Information
This information is provided as an operational guide
concerning the use of chopped and alternate modes of
operation.
(a) Displaying non-repetitive signals. To compare time
and phase relationships of non-repetitive signals, use
chopped-mode operation. The suggested fastest sweep
rate that you can use and still obtain good resolution is
summarized in Table 2-2.
TABLE 2-2
Chopped-Mode Resolution
Number of Recommended Approx. Number of
Channels in Maximum Segments in Each
Use Sweep Rate Trace
2 10 psec/div. 25
3 20 psec/div. 35
4 20 psec/div. 25
If sweep rates faster than those shown in Table 2-2 are
used, resolution decreases and you may prefer to use
single-trace operation.
(b) Displaying repetitive signals. The most likely dividing
point between the use of chopped-mode operation at low
sweep rates and alternate-mode operation at high sweep
rates is 0.2 millisec/div. At this sweep rate, or when using
higher sweep rates, alternate-mode operation produces
an uninterrupted display when viewing repetitive signals
500 cps or higher. That is, the alternate-mode switching
cycle is sufficiently fast to produce an apparently steady
display.
If the lower sweep rates are used for viewing signals
500 cps or lower, the alternate-mode switching cycle
becomes more noticeable and you may prefer to use
the chopped-mode of operation to produce steadier
displays.
Multi-Trace Triggering
1. Comparing time-related signals. To obtain a stable
display and to see true time or phase relationship between
signals which bear a fixed time relationship to each other,
apply the reference signal to channel 1. Use the channel 1
controls in a conventional manner. Pull the TRIGGER switch
outward and the time base will internally trigger on channel
1 only. Apply the signals to be compared to the other chan-
nels. Set the CHOP.-ALT. switch to a position best suited
for your application.
With the TRIGGER switch in its outward position, trigger-
ing on channel 1 only is independent of the channel 1
MODE switch setting. That is, you can set channel 1 MODE
switch to OFF anytime you want to remove channel 1
waveform from the screen and exclude it from the switching
cycle. Triggering will not be affected. Signals in the re-
maining channels will continue to be displayed unaffected
and referenced to channel 1], regardless of the MODE
switch setting.
2. Displaying signals unrelated in time. It is often pos-
sible to obtain a stable display of signals unrelated in
time. This is accomplished by setting the CHOP.-ALT.
2-6
switch to ALT., pushing the TRIGGER switch to its inward
position, and using ac-coupled internal triggering of the
time base. Then, with a proper setting of the time-base
Triggering Level control, the time base can trigger on the
signal in each channel to assure proper electronic switch-
ing of the channels.
If you should experience some difficulty in finding a
proper setting for the Triggering Level control, consider
these factors which affect triggering: the setting of the
Type 3A74 POSITION controls; the ac-coupled setting (AC,
AC Slow, AC Fast, AUTO.—depending on which positions
are available) of the time-base Triggering Coupling switch.
The Type 3A74 POSITION controls affect the de com-
ponent of the composite trigger from channel to channel,
and the different ac-coupled settings of the time-base Trig-
gering Coupling switch affect the waveshape of the trigger
as it is ac-coupled into the time-base triggering circuits.
Consequently, in addition to adjusting the Triggering Level
control, you should also try vertically positioning the dis-
played waveforms to different points on the ert and try
the different ac-coupled settings of the Triggering Coupling
switch to find the best triggering conditions.
Calibrated Gain Adjustment
Any time you move the Type 3A74 from one oscilloscope
plug-in opening to another you must adjust the common
gain of the unit to compensate for differences in crt de-
flection sensitivities.
1. Set the front-panel controls of the Type 3A74 to these
settings:
AC-GND-DC (channel 1) DC
VOLTS/DIV. (channel 1) 02
VAR. GAIN (channel 1) CAL.
POSITION (channel 1) Centered
MODE (channel 1) NORM.
MODE (channel 2, 3 and 4} OFF
TRIGGER Either position
CHOP.-ALT.
2. Set the time-base sweep rate and triggering controls
for a 0.1-millisec/div. free-running sweep.
Either position
3. Apply a 0.1-volt signal from the oscilloscope Cali-
brator to the channel 1 connector. (Use a short test lead
through a suitable connector adapter to make the direct
connection. Use a 0.05-volt Calibrator signal if you are
using a Type 567 Oscilloscope.)
4. Adjust the CALIB. control to obtain a deflection of
exactly five major divisions. (2.5 major divisions if you are
using a Type 567 Oscilloscope.)
NOTE
If the CALIB. control does not have sufficient
range, set channel 1 GAIN ADJ. near midrange.
(A setting a few degrees on either side of mid-
range will give the CALIB. control enough range.}
Then set the CALIB. control as described in step
4 of this procedure. Next, proceed to the Channel
Gain Adjustments procedure and set the GAIN
ADJ. for channels 2, 3, and 4.

Channel Gain Adjustments
From time to time you should check the gain of each
channel to assure that the vertical deflection factor cor-
responds to the .02 VOLTS/DIV. switch panel marking. To
check the gain of each channel, proceed as follows:
1. Check the gain of channel! 1 by performing the pre-
vious Calibrated Gain Adjustment procedure. If you have
done this, leave the controls as they were in that procedure
and go to the next step.
2. Remove the signal from channel 1 and apply it to
channel 2.
3.
Set channel 1 MODE switch to OFF and channel 2
MODE switch to NORM.
4. Set the remaining channel 2 front-panel controls so
they correspond to the settings used for channel! 1.
5. Rotate channel 2 GAIN ADJ. to obtain a deflection of
exactly five major divisions (2.5 major divisions if you are
using a Type 567 Oscilloscope.)
6. Set the GAIN ADJ. for channels 3 and 4 in the same
manner as for channel 2.
7. To double-check the exactness of your calibration,
apply the calibrator signal to all four channels. Set all
MODE switches to NORM. and superimpose the displays
of all four channels. Check the displays of all four channels
to see that they match in deflection and that the deflection
is the same as described in step 5 of this procedure.
NOTE
Once all gain adjustments have been set properly
and the CALIB. control has adequate range, you
need only to adjust the CALIB. control when you
Operating Instructions — Type 3A74
move the Type 3A74 from one plug-in opening to
another.
DC Balance Adjustments
If the de balance of a channel is not properly adjusted,
the crt dc-reference-level position of the trace will shift
as the VAR. GAIN control of that channel is rotated.
Though the adjustment procedure for channel 1 is de-
scribed here, the same basic procedure is used for the
other channels. To properly set the de balance, proceed
as follows:
1. Set channel 1 AC-GND-DC switch to GND and the
VAR. GAIN control to CAL.
2. Set channel 1 MODE switch to NORM. and position a
0.1-millisec/div. free-running trace to the center of the
graticule. The centerline serves as a convenient reference
point.
3. Rotate channel 1 VAR. GAIN control to the maximum
attenuation position (a few degrees clockwise past the CAL.
position). Note the direction and distance the trace moved.
4. Rotate channel | VAR. GAIN control back to its CAL.
position.
5. As a preliminary adjustment, slowly adjust the DC
BAL. control to move the trace in the same direction as the
trace moved in step 3. Continue rotating the DC BAL.
control to a point which positions the trace 1.5 times the
distance noted in step 3.
6.
As a final adjustment, rotate the VAR. GAIN control
back and forth several times between the CAL. and maxi-
mum attenuation settings. At the same time, adjust the DC
BAL. control! until there is no trace shift.
2-7

NOTES

SECTION 3
APPLICATIONS
The following applications describe how to make basic
voltage, phase, and time-delay measurements. These same
techniques can be applied and extended to a wide
variety of applications.
AC Voltage Measurement
To measure the ac peak-to-peak voltage or amplitude of
a waveform, the AC-GND-DC switch should be set to the
AC position. In this position only the ac components of the
signal are displayed on the ert. (However, when the ac
component of the signal is very low in frequency, use the
DC position of the switch.)
Peak-to-peak voltage measurements of a signal applied
to one channel are made as follows:
1. Set the appropriate VOLTS/DIV. switch such that the
expected voltage to be applied to the input connector is
not more than about six times the setting. If these conditions
are fulfilled, the expected voltage will produce a vertical
deflection of not more than 6 major divisions. Make sure
the VAR. GAIN control is set to the CAL. position.
2. Apply a repetitive signal to the appropriate channel,
preferably through a coaxial cable or an attenuator probe.
3. Set the appropriate MODE switch to NORM. and the
other MODE switches to OFF.
4. Set the triggering controls and the sweep rate of
the time base for a stable display of several cycles of the
waveform.
5. Use the positioning controls to position the waveform
to @ convenient point on the ert so that the voltage meas-
urement can be determined easily. For example, position
the waveform so that the negative peaks of the waveform
coincide with one of the lower graticule lines and one
f\
[|
tT
Peak-to-Peak |
Vertical i Ld
Deflection \
|
LJ
(b)
+
pepe
tua
pe
diya
tae
td
TA
Fig. 3-1. Measuring peak-to-peak voltage.
®
of the positive peaks lies behind the vertical centerline of
the graticule.
6. With the aid of the graticule, measure the vertical
deflection in divisions from the negative peak to the posi-
tive peak.
In measuring signal amplitudes, it is important fo remem-
ber that the width of the trace may be an appreciable part
of the overall measurement. For this reason, you should
consistently make all measurements from one side of the
trace. This is particularly true when measuring signals of
small amplitude. Notice in Fig. 3-1 that points a and b
correspond to the bottom side of the trace. The measure-
ment would be just as accurate if points a and b cor-
responded to the top side or center of the trace.
7. Multiply this distance by the setting of the VOLTS/
DIV. switch and the attenuation factor, if any, of the probe.
(Remember that the VAR. GAIN control must be set to the
CAL. position.)
As an example, assume you measure a vertical distance
of 4.6 divisions, the VOLTS/DIV. switch setting is .2, and
you are using a 10X attenuator probe.
Vertical VOLTS/DIV. Probe
erncal Switch x Attenuation = volts (p-p}
Deflection :
setting Factor
46 X 2 X 10 = 9.2 volts (p-p)
Instantaneous Voltage Measurements
To measure the dc level at a given point on a waveform,
the following procedure using channel 1 as an example is
given.
1. Set the channel 1 VOLTS/DIV. switch such that the
expected voltage to be applied to the input connector is not
more than about six times the setting, to obtain not more
than about 6 major divisions of vertical deflection. Be sure
the VAR. GAIN control is set to the CAL. position.
2. Set the triggering and time-base controls so that the
sweep free runs at the desired rate.
3. Set the channel 1 AC-GND-DC switch to GND {or
ground the probe tip), and vertically position the trace so
that it lies along one of the horizontal graticule lines. This
line will be used as a ground (zero) reference line. The
reference line you choose will depend upon the polarity
and dc level of the signal to be measured. Do not move the
POSITION control of the Type 3A74 after the reference line
has been established.
As an alternate method, you can set the oscilloscope
for automatic triggering and use the trace of an unused
channel as a visible reference. To establish a ground
reference using channel 2 {for example}, set channel 2
AC-GND-DC switch to GND. Check that channel 1 AC-
GND-DC switch is set to GND also. Superimpose the
traces. After establishing the reference, do not move the
channel 1 and 2 POSITION controls.
3-1

Applications — Type 3A74
4. Set the channel 1 AC-GND-DC switch to DC.
5. Apply the signal, preferably through a coaxial cable
or an attenuator probe, to the channel 1 input connector.
6. Set the triggering controls of the time base for a
stable display.
7. Measure the vertical distance in divisions from the
ground (zero) reference line established in step 3 to the
point on the waveform that you wish to measure.
If the channel 1 MODE switch is set to NORM. and the
point on the waveform is above the reference line, the
polarity is indicated to be positive (+). If the point is
below the line, the polarity is negative (—). If the MODE
switch is set to INV., the indicated polarities will be
reversed.
8. Multiply this distance measured by the setting of the
VOLTS/DIV. switch and the attenuation factor, if any, of
the probe. This is the instantaneous de level of the point
measured.
THT
TT
TTT
b
iy
D
Too
—
LJ
c [
-———P
E
J
HLH
tH
at
[ ‘f
l
4
pepe trp ee tpg py de py
TIT TTT TTT TTT Tt
Vertical
Deflection
(+3.6 div.)
8
Reference
(Ground)
TOTTI
er
i
dea
pdrr
pi
tips
ty
Fig. 3-2. Measuring a voltage with respect to some reference.
As an example, assume the MODE switch is set to NORM.
You measure a vertical deflection of 3.6 divisions above
ground reference line (see Fig. 3-2}, the VOLTS/DIV. switch
setting is 5, and you are using a 10X attenuator probe.
Instantaneous
Voltage . VOLTS/DIV. Probe
:
_
Vertical
:
.
with Respect to = - X Switch X Attenuation
Deflection :
Reference Setting Factor
Instantaneous
Voltage _ : _
With Respect to 4+3.6 div. X 5 & 10 +180 volts
Reference
9. If you want to recheck the (zero) reference or estab-
lish a new reference without disconnecting the applied
signal, set the channel 1 AC-GND-DC switch to GND.
If you want to establish a reference other than zero, set
3-2
the channel 1 AC-GND-DC switch to DC, touch the probe
to the desired reference voltage, and position a free-running
trace along one of the horizontal graticule lines.
Voltage Comparison Measurements
In some applications you could establish a set of deflec-
tion factors other than those indicated by the VOLTS/DIV.
switch. This is useful for comparing signals which are
exact multiples of a given voltage amplitude. The follow-
ing procedure describes how to get deflection factors for
channel 1. The same procedure can be used for the other
channels.
1. Apply a reference signal of known amplitude to chan-
nel 1 and, with the channel 1 VOLTS/DIV. switch and
VAR. GAIN control, adjust the amplitude of the display for
an exact number of graticule divisions. Do not move the
VAR. GAIN control after you have obtained the desired
deflection.
2. Divide the amplitude of the reference signal {in voits)
by the product of the deflection in divisions (established in
step 1) and the VOLTS/DIV. switch setting. The result is the
Deflection Conversion Factor.
Deflec.
C
Factor
=
Reference
signal
amplitude
in
volts
etlec.
Conv.
Factor
=“
éflec.
in
Div.)
(VOLTS/DIV.
Setting)
3. To calculate the True Deflection Factor at any setting
of the VOLTS/DIV. switch, multiply the VOLTS/DIV. switch
setting by the Deflection Conversion Factor obtained in
step. 2.
True Deflec. Factor = (VOLTS/DIV. Setting) (Deflec. Conv.
Factor)
The true deflection factor obtained for any setting of
the channel 1 VOLTS/DIV. switch applies only to this
channel, and only as long as the VAR. GAIN control is
not moved from the position to which it was set in step 1.
As an example, suppose the amplitude of the reference
signal applied to channel 1 is 30 volts, and the VOLTS/DIV.
switch setting is 5. The VAR. GAIN control is adjusted to
decrease the amplitude of the display to exactly 4 divi-
sions.
_ 3000
Deflec. Conv. Factor = ia) (5) = 15
True Deflec. Factor = (5)(1.5) = 7.5 volts/div.
4. To determine the peak-to-peak amplitude of a signal
to be compared, disconnect the reference signal and apply
the signal to be compared to channel 1. Set the VOLTS/DIV.
switch to a setting that will display the signal with ad-
equate deflection so that a measurement can be made.
5. Measure the vertical distance in divisions and deter-
mine the amplitude by using this formula:
. . _ Deflec. Conv. Deflec. V VOLTS/DIV.
Signal Amplitude = Factor * in div. * Setting
As an example, suppose the signal to be compared caused
a deflection of 6 divisions at a VOLTS/DIV. setting of 10.
The VAR. GAIN control was not moved from the setting
used in the previous example. Substituting these values for
®

the Deflection Conversion Factor in the Signal Amplitude
formula,
Signal
Amplitude
_ Z
(in
volts)
=
(1.5)
(6)
(10)
=
90
volts
Phase-Shift Measurements
Phase-shift comparisons of two to four sine-wave signals
of the same frequency can be made using the multi-trace
feature of the Type 3A74. To make the comparisons, pro-
ceed as follows:
1. Set the AC-GND-DC switches to identical settings—
either all AC or all DC, depending on the type of coupling
desired.
2. Set the MODE switches to NORM. and pull the TRIG-
GER switch outward.
3. Place the CHOP.-ALT. switch to the position desired.
In general, the ALT. position is more suitable for high-
frequency signals and the CHOP. position is more suitable
for low-frequency signals.
4. Apply the reference signal to channel 1; apply the
signals to be compared to the other channels. Use equal
delay coax cables or probes.
Chan, 2 Chan. 3
(Leading) Chan. 1 ‘Legging) chan. 4
(Reference) (Lagging)
Z
LLZAKLN: a
™S™”_
oN
~—
—<
Trt
Leb
=
LL
—
ae
NY
>
TUTTI
TTT
a i
>
wow
ee
ms
\
TVET
TTT
a
——
‘aon!
TTT
tI. ie ~- 1
—|ch. 3fe
patch. 44
(pe 9 diy, (360°)
360°
Applications — Type 3A74
5. Set the VOLTS/DIV. switches for a suitable amplitude
display.
6. Set the time-base sweep rate and triggering controls
to obtain a stable display of less than one cycle of the
waveforms.
7. Adjust the VAR. GAIN control for each channel so
that the signal amplitudes are equal and fill the screen
vertically. Reset the VOLTS/DIV. switches if there are set-
tings available that will aid in obtaining the equal ampli-
tude signal displays. Equal amplitudes are not essential
but they help to make comparisons easier. Carefully center
the signals vertically on the screen; that is, for equal
distances above and below the center graticule line.
8. Rotate the Variable Time/Div. control of the time base
counterclockwise until one cycle of the reference signal
occupies exactly 9 divisions horizontally. Use the Slope
switches and the Triggering Level control of the time base
to trigger on the reference waveform at any point you
desire. Each division on the screen now represents 40° of
one cycle at this time-base setting (see Fig. 3-3).
9. Measure the horizontal distance between the reference
waveform and each of the other waveforms. Note the
distance for each channel and whether they are leading
or lagging. To make each phase comparison measurement
easier, switch the nonapplicable waveforms “off by set-
ting the appropriate MODE switches to the OFF position
until you need to display them.
10. For each distance measured, multiply the distance
by 40° per division to obtain the amount of phase shift
compared to the reference waveform.
Chan, 1
Ch, 2 (Reference)
Lyi4
a
+
N
\
\
Ch. 2 Distance
poppe
22020
RRR
R
ORR
RRUSe
Ee
BERR
eee
el
Phase Shift — Ch. 2 Distance in Div. Times 8°
D div. = = 40°
egrees per div 9 div. 0
Phase Shift = Horizontal Distance in Div. Times 40°
Fig. 3-3. M ring the ph shift between electrical waveforms.
®
Fig. 3-4. Computing the phase shift when the oscilloscope sweep
rate is increased 5X. Accurate phase-shift measurements within a
range of 80° can be made using this method.
3-3

Applications — Type 3A74
For more precise measurements, you could increase
equally the vertical sensitivity of all channels used and the
sweep rate established in steps 6, 7, and 8, but do not
change the setting of the time-base Variable Time/Div.
control. However, when you increase the sweep rate, you
must take this into consideration in your calculations.
For example, if you increase the sweep rate by a factor
of 5, and then measure the distances between waveforms,
each division will represent 8° (40° + 5) of a cycle. By
doing this, you can measure phase shift up to 80° more
accurately. When preparing to make the measurement,
horizontally position the waveforms to points where the
graticule markings aid in determining the exact distance.
{To be certain that the waveform is centered vertically,
check the DC. BAL. adjustment before vertical sensitivity
is increased.) Fig. 3-4, for example, shows how the phase
shift of channel 2 waveform can be computed using this
method. Other phase-shift measurements can be determined
using this same basic procedure.
Time-Delay Measurements
The calibrated sweeps of Tektronix oscilloscopes cause
any horizontal distance on the screen to represent a definite
known interval of time. Using this feature in combination
with the multi-trace feature of the Type 3A74, you can
measure the time lapse or delay between events displayed
on the crt. This is done by using the following method:
1. Set the AC-GND-DC switch to AC or DC depending on
the type of coupling desired.
2. Apply the reference signal to channel 1; apply the
signals which have more delay than the reference signal to
the other channels. Use coaxial cables or probes having
equal delay.
3. Pull the TRIGGER switch to its outward position and
set the CHOP.-ALT. switch to the position desired.
4, Set the MODE switches to NORM.
5. Set the VOLTS/DIV. switches and the VAR. GAIN
controls to obtain suitable vertical deflections. Set the
time-base sweep rate and triggering controls to obtain a
stable display at a sweep rate that will allow you to measure
the distance between waveforms accurately. Be sure the
Variable Time/Div. control is set to the Calibrated position.
6. If some of the waveforms are negative-going and
others are positive-going, you may prefer to set the appro-
priate MODE switches to the INV. position to make all wave-
forms go in the same direction on the display.
7. With the aid of the graticule markings, measure
the horizontal distance between the reference waveform and
each of the other waveforms. For most measurements the
distance is usually measured between 50% amplitude points
on the rising portion of the waveform. To make the measure-
ments easier, switch the waveforms not being measured “off
by setting the appropriate MODE switches to the OFF posi-
tion.
8. Multiply the distance measured for each channel
by the setting of the time-base Time/Div. switch to obtain
the apparent time interval.
3-4
9. To obtain the actual time interval, divide the apparent
time inteval by 5 if 5X sweep magnification is used, by
10 if 10X sweep magnification (Type 565 Oscilloscope) is
used, and 1 if sweep magnification is not used.
(Time/Div. Switch Setting) (Distance in Div.)
Sweep Magnification
Time Delay =
For example, assume the Time/Div. switch setting is 2
uSec, the 5X magnification is in use, and you measure a
horizontal distance of 3 divisions between the leading edge
of the reference waveform and the leading edge of the wave-
form displayed on another channel. In this example, 3 divi-
sions multiplied by 2 microseconds per division gives you
an apparent time delay of 6 microseconds. The apparent time
delay divided by 5 gives you an actual time delay of 1.2
microseconds.
Four-Trace X-Y Displays
It is possible to display up to four different sets of
sampled X-Y parameters on the oscilloscope screen through
the use of two Type 3A74 Plug-In Units.
To display four sets of X-Y parameters at the same time,
proceed as follows:
1. Insert the Type 3A74’s into the vertical (Y-axis) and
horizontal (X-axis) openings of the oscilloscope.
2. Set the CHOP.-ALT. switch of the Y-axis unit to CHOP.
and the X-axis unit to ALT. Best transient blanking response
is obtained by setting these switches to the positions as stated.
3. Connect @ pair of (X-Y} signals to each channel. For
example, connect one signal of a pair to channel 1 of the
Y-axis plug-in. Connect the other signal of the pair to
channel 1 of the X-axis plug-in. Proceed in this manner to
connect each pair of signals to the other channels of both
units.
NOTE
Channel 4 in both units synchronize together
through pin 18 of the interconnecting plugs. There-
fore, it is important that you always leave channel
4 “on”’ in both units by setting their MODE switches
to either NORM. or INV., as desired. In addition,
always use the same number of channels between
units to assure proper synchronization between
pairs of signals.
4. Set the VOLTS/DIV. switches and the POSITION con-
trols for the desired displays. If the displays do not properly
synchronize, set the channel 4 MODE switch of the left-hand
or Y-axis plug-in to OFF momentarily and then back to the
“on” position.
If you are applying sine-wave signals of the same fre-
quency to each channel, the resulting displays will appear
as a straight line, ellipse, a circle, or other variations of
the Lissajous pattern for phase differences between 0° and
360°. Fig. 3-5 shows an example of possible displays that
can be obtained. To calculate the sine of the phase dif-
ference between a pair of signals, use this method:
{a) Center one of the X-Y displays horizontally and
vertically on the crt. Position the others off the crt.
®

Applications —- Type 3A74
IN
\
‘
N
>»
\
\
swe! aie ee - pe
erry Tt , NEES TOT TFT \
_ XN
\
\LY
Fig. 3-5. X-Y displays obtained by using four pairs of channels
simultaneously.
(b) Increase the size of the display by increasing the in-
put signal amplitudes or by increasing the vertical and
horizontal sensitivities of the plug-in units. Center the
display as described in the previous step.
(c) Measure the distances A and B on the display as shown
in Fig. 3-6. A/B is equal to the sine of the phase dif-
ference between the two signals (see Table 3-1).
EDN Ty
AE 3
A | it
HH safle HH ses +H ha HHHflnad
Sheep
ee
rt
papa
typ
ap
TUR
T
Tet
t
L
|
\
4.
Fig.3-6. X-Y method of calculating phase difference (0) of two
sine waves.
(d) After making the calculation for one display, position
it off the crt and position one of the other dis-
plays on the crt and calculate the phase difference.
Continue in this manner for the remaining displays.
3-5

Applications — Type 3A74
ANGLE
(in degrees)
OCN
A
OR
WHO
SIN
.0000
0175
0349
.0523
.0698
.0872
1045
1219
1392
1564
1736
1908
.2079
.2250
2419
.2588
2756
2924
3090
3256
3420
3584
3746
3907
4067
4226
4384
4540
4695
4848
Natural Sine Functions
TABLE 3-1
ANGLE
(in degrees) SIN
5000
5150
5299
446
5592
.5736
.5878
.6018
6157
6293
6428
656]
6691
.6820
6947
7071
193
7314
743)
7547
7660
J7T7\
.7880
7986
.8090
8192
8290
8387
.8480
8572
ANGLE
(in degrees)