Tektronix 067-0521-00, Instruction Manual

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Circuit Description— 0 6 7 -0 5 2 1 -0 0
Fig. 3 -3 . Simplified schematic diagram showing input signal path during the two states of the dual-trace switching multivibrator, with TEST
FUNCTION switch set to .ALTERNATE or CHOPPED.
The voltage drop across D89 sets the dc level of the output
applied to C22 at approximately 0.2 volt, causing the dis­
played trace to start about 2 cm above zero signal dc level.
The major portion of any signal applied to the EXT INPUT
connector is shunted to ground through D89 (see Fig. 3-3), so
essentially no waveform is displayed. The initial + 0 .2 volt
signal dc level rapidly drops toward the dc voltage applied
through R22 due to the ac-coupling through C22. A t sweep
rates slower than about 50msec/cm, the trace reashes the
dc voltage very near the beginning of the sweep.
A negative-going sync pulse produced by the oscilloscope
at the end of each sweep is received by the Dual-Trace
Switching circuit through pin 16 of the inter-connecting plug
and is applied through Cl 05 to the cathode of V102B and
through D102 to the cathode of V102A. The pulse causes
V102A to conduct monentarily, and the pulse appears at the
grid of V95A through C91 and R91. V95B, however, is
already cut off and the pulse has no effect on V95B. The
same pulse applied to the cathode of V102B, which is con­
ducting, appears at the grid of V95A through C97 and R97
and cuts it off, switching the m ultivibrator to its other bistable
state. After switching, V95A and V102B are cut off, and
V95B and 102A are conducting. W ith no current through
V95A, bias current is removed from D89 and D90, cutting
them off. The dc output level applied to C22 drops from 0.2
volt to approximately ground, set by R88/R89. Any signal
now applied to the EXT INPUT connector is attenuated by
the 1000:1 attenuator at the input, and applied through C22
to the vertical am plifier of the oscilloscope. The lower trace
with the waveform is then displayed on the crt.
A t the end o f the sweep, another sync pulse arrives through
pin 16 and Cl 05 and is directed to the cathode of V102B
and through D102 to the cathode of V102A. The pulse causes
V102B to conduct momentarily, and since V102A is already
conducting, the pulse appears a t both multivibrator grids.
V95B then reverts to its original state (cut off), and V95A
again conducts. W ith V95A conducting, current through
D89 and D90 shunts any input signal to ground and sets the
dc output level at 0.2 volt, starting another cycle of
operation.
When using an oscilloscope capable of alternating
between time-base circuits, an alternate-trace slave pulse is
applied through pin 7 of the inter-connecting plug and C l01
to the cathode of V102A (see Fig. 3-4). The slave pulse is a
negative gate from the sweep switching circuit of the oscillo­
scope, time-related to the sync pulses in such a way as to
lock the upper trace of the Alternate display to the Time Base
B sweep of the oscilloscope. (The Trace Separation control
on a Type 547 should be set to zero). This operation sim­
ulates the locking of channel 2 on a dual-trace plug-in pre­
am plifier to the Time Base B sweep, and Channel 1 to the
Time Base A sweep.
The negative slave pulse starts at the end of Time Base A
sweep as a voltage at pin 7 drops from about + 4 5 volts to
zero, and ends at the end of Time Base B sweep as the volt­
age returns to + 4 5 volts. The pulse appears as a differenti­
ated signal at the cathode of V102A. The positive portion of
the differentiated signal does not appear at the cathode of
3 -4
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