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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 9

Ringing and Blocking Oscillators

generator at the instant the trace begins and switch it off when the trace ends. The arrangement is shown in Fig 6.

Ringing Oscillator

The oscillator most commonly used in a cal pip generator is the ringing oscillator, the circuit and waveforms of which are shown in Fig 7. As explained in the previous paragraph, the oscillator is being alternately switched on and off by the same square wave input that is being used to trigger the timebase circuits. When the valve is suddenly switched on at A, current flows into the tuned circuit, making it ring (see p 81). The conducting valve, however, is effectively in parallel with the tuned circuit and very quickly damps the oscillations. At B when the valve is cut off the supply current to the tuned circuit ceases abruptly, the magnetic field around the inductor collapses, inducing a back e.m.f. into the circuit, and the tuned circuit again rings. This time however, with the valve cut off, there is very little damping of the tuned circuit and the oscillations decay slowly. The frequency of oscillations is determined by the resonant frequency of the tuned circuit and this may be varied by adjustment of the pre-set capacitor C2. This, in turn, varies the time interval between successive cal pips. For five-mile cal pips, the interval between each pip is 5 x 10.75 uS = 53.75 uS = 1/18.600 second and the frequency must be adjusted to 18.6kc/s.     '

When the resonant circuit is placed in the anode of the valve, as in Fig 7, the first half-cycle of ringing, when the valve is cut off, is positive-going. Under certain circumstances the first half-cycle of ringing is required to be negative-going. This is obtained by placing the tuned circuit in the cathode of the valve.


 

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