Tektronix 422, Instruction Manual

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Circuit Description-Type 422 AC-DC
(C)
___ ___________ Pulse
I | Duration
I I
Pulse-*-!
I duration I
Fig. 3-14. Illustration of energy-storage principle: (A) Simplified diagram of energy-storage circuit, (B) graph o f charge stored in inductor
versus source voltage, (C) idealized waveforms.
the collector of Q1120. A third winding of T l 120 provides
a negative going output pulse to the M ultivibrator stage.
This pulse is of short duration due to the action o f Q 1120.
(See waveform 1, Fig. 3-15.)
The operating frequency o f the Blocking Oscillator stage
is determined by C l 121-R1123-R1124-R1125 in the
em itter circuit of Q1120. When Q1120 conducts, its emit­
ter quickly goes positive along w ith the signal applied to the
base from T l 120. C l 121 quickly discharges to the level on
the em itter of Q1120. When Q1120 shuts o ff, its emitter
level is determined by the charge rate o f C l 121. The charge
current fo r Cl 121 is supplied through R1124-R1125 and
through R1123-D1155. As C l 121 charges toward the zero-
reference voltage level, the em itter level o f Q 1120 follows.
However, when the emitter level o f Q 1120 drops to where
it is about 0.5 volt more negative than the base level set by
diodes D1117-D1118, Q1120 is forward biased to start a
new cycle. Frequency of operation is about seven kilohertz.
The Oscillator Frequency adjustment R1125 varies the
charge time of Cl 121 and thus determines the frequency at
which the blocking oscillator operates. The signal at the
em itter o f Q 1120 is connected to the Pulse Width Control
stage through R1123 (see waveform 2, Fig. 3-15).
Multivibrator. Transistors Q 1105 and Q1115 form a
bistable m iltivibrator. The negative-going output pulses
from the Blocking Oscillator stage are connected to the
junction o f D1105 and D l l 15 through R 1118. These
pulses switch the M ultivibrator as follows: assume that
Q1105 is conducting and Q1115 is off. The current through
Q1105 raises its emitter more positive than the em itter o f
Q 1115 and also pulls the collector o f Q1105 more negative
than the collector o f Q1115. The negative-going pulses
from the Blocking Oscillator stage are connected to the
bases o f both Q1105 and Q1115 through diodes D 1105 and
D l l 15. This pulse momentarily interrupts the conduction
o f both transistors. When the pulse ends, the stored charge
on commutating capacitor C l 106 determines which transis­
to r conducts first. While Q1105 is conducting, C l 106
charges positive on the Q1105 side to the level at the
emitter o f Q1105. This charge holds the em itter of 0 1115
more negative than the emitter o f Q1105 while they are
momentarily off. Then as the Blocking Oscillator pulse
ends, both Q1105 and Q1115 attempt to come into con­
duction. However, since the emitter o f Q1115 is the more
negative, this transistor achieves the greater base emitter
bias and conducts the more heavily. The collector o f Q 1115
goes negative and pulls the base of Q 1105 negative also, to
reverse bias Q1105. Thus, Q1115 has gained control o f the
stage. The action is similar but opposite when the next
trigger pulse is received from the Blocking Oscillator stage.
The output pulses at the collectors of Q1105 and Q1115
are connected to the Steering Switch stage. Waveform 3
(Fig. 3-15) shows the M ultivibrator output from Q1105.
The output from Q1115 is similar except that it is 180° out
of phase w ith the waveform shown.
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