Radar System Engineering

502 THE RECEIVING SYSTEM—INDICATORS [SEC, 138
The Blocking Oscillator.—The blocking oscillator is an inductively
coupled regenerative amplifier used inthe generation of short pulses. In
the waveform diagrams of Fig. 1319, consider the moment h. The grid
is beyond cutoff and rising exponentially toward the bias potential.
The plate is at B+ potential. At time t,the tube begins to conduct, the
plate begins to fall, by virtue of the inductive coupling the grid is pulled
upward, and a violently regenerative action sets in which ultimately
pulls the grid far positive, drawing much current from the cathode.
Eventually, however, the rate of increase of plate current falls off and the
current to the grid pulls the latter downward. Regeneration takes place
I +
II
Plate
1
I
1 1
II
Point z ~
u
Cathode
u u
Ii
II
1
I n n n
t. t,
Sm;::;:g
AMAMAMA
Grid when
counting down
FIG. 13. 19.—The blocking oscillator.
in the opposite direction, the grid is driven negative, and the process
repeats. The period of the oscillation is primarily determined by RC, but
the other circuit parameters do enter. Characteristics of the tube itself
enter to such an extent that the device cannot be considered as a precision
oscillator, although it can be synchronized by one of higher frequency.
The blockhg oscillator can be used as a “scaling” or counting-down
circuit for a continuous pulse train of definite frequency by making the
natural period slightly longer than the expected one and applying signals
of the proper height as shown in the bottom diagram of Fig. 13”19. The
counting ratio is dependable only up to a value of 5 or 10 but synchronization
occurs to many times this value. Somewhat higher ratios can be
used by inserting a resonant circuit in the cathode.
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