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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 9

Ringing and Blocking Oscillators

The oscillator output with only L and C connected is as previously described and is indicated in Fig 9b.

If we now insert the resistor R, of such a value that the circuit is critically damped, the output is as shown in Fig 9c.

If we now insert the diode D, in addition to R, the diode limits the negative swings of anode voltage and the output is as shown in Fig 9d.

Note that if D is inserted and R removed the circuit is no longer critically damped so that a large-amplitude positive-going swing can be obtained. The diode conducts on the negative-going portions of the output waveform as before so that only a large positive-going pulse, co-incident in time with the trailing edge of the input square wave, is obtained (Fig 9e).

Normally, both R and D are inserted to prevent an excessive positive voltage swing at the anode and to ensure a greater control over the pulse duration of the output waveform.

The output from this type of ringing oscillator may be used as the trigger pulse for the modulator in a radar transmitter. If it is used as such the transmitter pulse duration depends upon the duration of the output

 

pulse from the ringing oscillator. This, in turn, may be varied by adjusting the tuning slug of the inductor L.

Blocking Oscillator

The blocking oscillator produces a series of very narrow, steep-sided pulses of large amplitude which can be used as cal pips or, more usually, as trigger pulses. The waveform at the control grid has also been used in some television receivers to provide a sawtooth timebase waveform.

The blocking oscillator is, in effect, a development of the tuned anode or tuned grid oscillator considered in Part lB of these notes (p 352). In the normal tuned grid oscillator (Fig l0a), if the feedback between anode and grid is great enough, and the CR time constant of the bias


 

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