Tektronix 067-0521-00, Instruction Manual

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Circuit Description— 0 6 7 -0 5 2 1 -0 0
Fig. 3 -6 . Timing of chopped blanking pulses from 0 6 7 -0 5 2 1 -0 0 to oscilloscope crt cathode with TEST FUNCTION switch at CHOPPED.
segment apears on the crt. Then when V95A cuts off, D89
stops conducting and the lower trace appears on the crt. If
a signal is applied to the input at this time, it w ill be dis­
played by the lower trace for the duration of the trace seg­
ment. Refer to the discussion of Alternate operation for a
more detailed description of this switching signal.
A second output is produced in the plate circuits of the
multivibrator and coupled out through C l00 and Cl 02. Each
time the multivibrator switches states, the voltage in the
plate circuit of one tube drops fast as the tube turns on,
stays at the lower voltage level until the other tube turns on,
then rises slowly to its maximum value as the capacitance
charges through a relatively high impedance path. The wave­
forms produced in both plate circuits of the switching mul­
tivibrator are coupled through Cl 00 and Cl 02 to the cath­
ode circuit of V102B where they are combined and applied
through Cl 05 and pin 16 of the interconnecting plug to the
oscilloscope blanking circuit. The frequency of the composite
output chopped blanking signal is twice the repetition rate
of the switching multivibrator. See Fig. 3-6. This output
signal is inverted and amplified in the oscilloscope and
applied to the crt cathode to blank the crt beam during the
switching portions of the signal sent to the vertical amplifier
through pin 1 of the interconnecting plug.
PULSE GENERATOR CIRCUIT
General Operation
The Pulse Generator circuit consists of rate generator
multivibrator Q45/Q 55, constant-voltage transistor Q33,
current-switching transistors, Q 64/Q 74, and disconnect diodes
D64, D65, D74 and D75. Fig. 3-7 is a simplified schematic
of the circuit. When the TEST FUNCTION switch is set to the
+ PULSE or — PULSE position, the rate generator operates
as an astable circuit. One complete m ultivibrator cycle turns
the current-switching transistors on and off. As these tran­
sistors are turned on, they shunt across the disconnect diodes
which then cut off very fast, producing the fast-rise step-
function output (see Fig. 3-8). As the current-switching tran­
sistors are turned o ff again, the disconnect diodes conduct
again and the output pulse ends (see Fig. 3-9).
Detailed Operation
For the following discussion, assume that the TEST FUNC­
TIO N switch is set to the + PULSE position and the A M ­
PLITUDE control is turned fully clockwise to the maximum
amplitude position. The TEST FUNCTION switch connects the
+ 100-volt supply to the collector circuit of Q33, and ap­
plies about - f 75 volts to the base of the transistor through
D30 and R30. The transistor is thus turned on with emitter
current provided through D35 and R39 from ground. When
the circuit is correctly adjusted, the voltage a t the emitter
of Q33 is approximately + 7 5 volts, providing all positive
supply voltages for the Pulse Generator circuit. The + 6 0
volts existing at the anode of D35 provides the supply vo lt­
age for the base circuits of the rate generator multivibrator
and for one end of the amplitude control network. DRIVE
BAL R40 is adjusted so the push-pull output voltages are
equal.
As soon as supply voltages are applied to the circuit, the
rate generator begins its astable m ultivibrator operation.
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3 -7