HP 212A - Pulse Length; And Blocking Oacillatos

3-4 PULSE POSITION MY AND BLOCKING OSCILLATOR
The pulse position multivibrator tube V105AB is also a one shot rnultivi-
brator which is triggered by the pulse from the repetition rate multivi-
brator. A positive pulse 2 to ll2 microseconds in length is produced by
this multivibrator. The duration of this pulse is controlled by the vari-
able resistor R132.
The blocking oscillator tube V 106B forms the pulse which is supplied to
the pulse length multivibrator and the delay line.
Following the block-
ing oscillator is a cathode follower tube VlO7B which couples the block-
ing oscillator to the delay line,
In series with the grid of this tube is a
crystal rectifier CR104 and a resistor R152 which prevent reflections
from the delay line being fed to the blocking oscillator circuit due to the
cathode to grid capacity of V107B.
3-5 PULSE FORMING THYRATRON Vb
Tube V2, which is normally connected as a diode,
and the charging choke
LZ are used to charge the pulse forming line so that plate voltage is ap-
plied to the pulse forming thyratron Vb. If for any reason the thyratron
draws excessive current, then the relay REL-1 applies a large negative
voltage to the grid of the tube V2 thereby stopping the flow of current to V6.
When the pulse forming line is charged, a pulse from the blocking oscil-
lator and delay line will trigger the thyratron tube. Once the thyratron
tube is fired, it will continue to carry current, irrespective of the grid
voltage, until the conduction is quenched by means of the pulse forming
line.
The pulse forming line causes the plate voltage to fall to zero after
10 rnicroseconda due to a reverse polarity transient which travels down to
the open end of the line and is reflected back to the starting point.
This
action limits the tube conduction and forms a 10 microsecand pulse across
the cathode resistor R13.
The damping diode tube V3 is used to damp out any negative overshoots.
3-6
PULSE LENGTH MV AND BLOCKING OSCILLATOR
The positive pulse from the blocking oscillator Vl ObB is inverted by tube
V 107A.
The negative pulse from V107A triggers the pulse length multivi-
brator consisting of V108A and V108B.
The pulse length is varied by the
PULSE LENGTH control (R142).
The trailing edge of the negative pulse from the multivibrator triggers the
blocking oscillator V109A. The output of the blocking oscillator is passed
on to the thyratron V16 through the cathode follower tube V109B.