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
®