AP 3302 Pt. 3
Section 2
CHAPTER 9
Ringing and Blocking Oscillators
leading
and trailing edges are both the result of avalanches). Very short pulse durations,
of the order of 1 uS, are possible. The pulse duration is determined by the
inductance and self-capacitance of the pulse transformer and also by the value
of C1 for a short pulse duration, C1 should be small. The p.r.f. of the circuit
is determined by the time taken for Vg to return to cut-off. This may be controlled
by varying R1 or C1 or by taking R1 to a variable positive aiming voltage as
in the multivibrator.
It is also possible, by placing a resistor R2 in the cathode of the valve, to take the output from the cathode. The output pulses are now positive-going.
The blocking oscillator may be synchronized in much the same way as a multivibrator. If we apply a series of positive-going sync pulses to the grid such that Vg rises above the cut-off level at each sync pulse -before it would normally do so - then each output pulse is synchronized with the corresponding input pulse.
The ringing which occurs in the anode circuit may be prevented by connecting a diode across the output winding as shown in Fig 12. The diode is connected in such a way that it plays no part in the circuit action until Va tries to rise above h.t. +; it then conducts to hold the output at h.t. ±. As the first half-cycle of the ringing action cannot now take place the whole effect is eliminated and the output waveform is then as shown in Fig 12.
Triggered Blocking Oscillator
The astable, ie free-running, blocking oscillator considered in the previous paragraphs is sometimes used to produce the master timing pulses for a complete radar system, thus determining the instant of time at which each circuit in the radar operates. However, the p.r.f. of a free-running blocking oscillator is erratic due to slight variations in the circuit conditions (see Fig 13). Because of this it is usual to synchronize the operation of the blocking oscillator with pulses whose p.r.f. can be accurately controlled.
However, under other circumstances, the requirement is that the blocking oscillator produces no output until it is triggered, ie a monostable circuit may be required. The circuit and wave-forms of a triggered blocking oscillator are shown in Fig 14. R1 is now returned to a negative bias voltage which is sufficient to keep the valve cut off under normal conditions. To cut the valve on and produce one cycle of blocking oscillator operation the circuit must be triggered by applying a positive pulse to the grid, of sufficient amplitude to lift Vg above cut-off. As soon as Vg rises above cut-off the action is as described for the free-running circuit.
