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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

We have now seen some examples of how cal pips are used. In each case the circuit which generates the cal pips must produce very narrow pulses of voltage, and the time interval between successive pulses must be very accurately known. With these requirements in mind let us now see how cal pips are produced.

Production of Cal Pips from Sine Waves

Very accurate cal pips can be produced by the arrangement shown in block form in Fig 5. The arrangement is known as a cal pip generator. The oscillator generates a sine wave at an accurate frequency. The sine wave is then squared, differentiated and applied to a limiter, where it is positively limited from a fixed negative voltage. Finally, the narrow negative-going pips from the limiter are applied to a pulse shaper which gives the output waveform shown in Fig 5; this is a series of narrow positive-going pulses of exactly the same frequency as that of the oscillator.

If we require cal pips at intervals of one radar mile, the period of the output waveform must be 10.75 uS (1/93,000 second) . The frequency of the oscillator must therefore be 93 kc/s.

Cal Pip Generator Switching

When we use cal pips on a radar display as range markers, the first pip on each trace must occur at the instant the spot begins to move, otherwise the other pips will not be correctly positioned along the trace. We must therefore trigger the cal pip generator in some way to ensure correct synchronization between the trace and the cal pips.

We normally use a pulse generator, such as a flip-flop, to produce square waves which switch on the indicator circuits at the instant the transmitter fires and switch them off when echoes have returned from maximum range. This same switching waveform is used to switch on the cal pip


 

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