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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 10

Electronic Switching Circuits

Where larger amplitude trigger pulses of controlled pulse duration are required, the output from a critically-damped ringing oscillator or from a blocking oscillator or unijunction circuit may be used. We have also seen earlier in this chapter how a tunnel diode and a transistor may be used together to produce a trigger pulse output. Most of these circuits operate at low-voltage, low-power levels.

Most radar equipments contain a ‘trigger unit’. This normally consists of a series of circuits so arranged that the initial trigger pulse is increased in amplitude and given the correct shape and time duration to trigger subsequent stages at precise instants of time.

The output from the trigger unit is of sufficient amplitude to trigger the final switching stage which, in turn, is used to turn the transmitting valve (e.g. a magnetron) on and off. The stage which controls the transmitting valve in this way is referred to as the modulator.

Modulators

The power requirements of the modulator depend to a large extent upon the type of transmitting valve used. We shall consider this in more detail in Section 5. It is sufficient at this stage to know:

a.      If the transmitting valve is a magnetron, which must be anode-modulated, the modulator must be capable of handling the full peak pulse power. This can be of the order of megawatts.

b.      If a klystron amplifier is used as the transmitting valve it can be switched on and off by power applied to a modulating electrode. In this case the modulator is required to handle only a small portion of the total output power.

c.      If a disc-seal triode or other similar grid-controlled valve is used in the transmitter, grid modulation can be used. The modulator can now be a low-power valve.


 

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