Tektronix 067-0521-00, Instruction Manual

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Circuit Description— 0 6 7 -0 5 2 1 -0 0
through R53, and the voltage in the emitter circuit becomes
more negative. As the base-emitter junction of Q55 be­
comes forward biased, this transistor turns on and causes
the m ultivibrator to switch states. As Q55 turns on and Q45
turns off, the base-emitter junctions of Q64 and Q74 become
forward biased, turning on these transistors and shunting
current away from the disconnect diodes. The fast switching
characteristics of the disconnect diodes enable them to cut
o ff very fast when the current source is removed, producing
the fast-rise (3 nsec) output step as both output voltages go
to +67.5 volts. As the differential voltage between pins 1
and 3 changes to zero volts, the fast rise of the pulse is
displayed on the crt.
As soon as current through Q55 has reached a maximum
and Q45 is cut off, the capacitors in the emitter circuit of
Q45 begin to charge, allowing the emitter voltage to go
negative. When the base-emitter junction of Q45 becomes
forward biased, the multivibrator switches states again. The
positive-going voltage at the collector of Q55 decreases the
conducting of Q74 and turns o ff Q64, restoring the current
source to the disconnect diodes. As current rises through the
output resistors, the output voltages to the oscilloscope return
to + 67.2 and +67.8 volts, and the displayed +pulse returns
to its baseline. The next pulse is started when Q55 turns
on again.
The switching rate of the rate generator m ultivibrator is
determined by the supply voltages and the resistor-capacitor
combinations in the base and emitter circuits. The front-
panel REPETITION RATE switch allows the capacitance in
the emitter circuits to be changed, controlling the charging
rate in the emitter circuits and thus the switching rate of
the multivibrator. W ith C49 and C50 connected in the cir­
cuit (LOW), the repetition rate of the output pulse is approxi­
mately 5 kc; with C48 in the circuit (MED), the rate is 120kc
and with C47 in the circuit (HIGH), the repetition rate is
600 kc.
The output of the Pulse Generator circuit simulates a fast-
rise input pulse from a 50-ohm system. The impedance pre­
sented to the oscilloscope vertical am plifier is determined
prim arily by the values of R66 and R76. The center voltage
at the output (+67.5 volts) is adjusted by PULSE DC LEVEL
control R30 during calibration. Vertical positioning of the
pulse display on the crt is controlled by dc voltages added
to the output through R69 and R79.
Operation of the Pulse Generator circuit is exactly the
same in — PULSE as in +PULSE, but the polarity of the out­
put is reversed by the TEST FUNCTION switch.
3 -1 0 <Di