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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Electronic Switching Circuits

Hard Valve Modulator

Fig 9 shows in outline how grid modulation of the final stage of a relatively low-power pulsed radar transmitter may be achieved.

V2 is the transmitting valve and V1 the modulator. In this circuit V1 is normally conducting and passing a heavy current. The voltage developed across RL1 by V1 current is applied as bias to the grid-cathode circuit of V2. Under normal conditions this bias is sufficient to keep V2 cut off. When the modulator V1 is cut off by a negative trigger pulse from the trigger unit the bias on V2 from RL1 is lifted for the duration of the pulse and V2 conducts heavily for this period. High power oscillations at the frequency of the tuned circuit are thus developed for the duration of the trigger pulse and these are applied as output to the aerial.

Fig 10 illustrates how anode modulation may be achieved by using a hard valve modulator. The modulator valve is placed in series with the high power r.f. oscillator across the h.t. supply. The modulator is normally held cut off by a fixed bias. The oscillator circuit has therefore no earth return and no oscillations occur until a trigger pulse from the master timing unit lifts the bias on the modulator valve. The modulator valve then cuts on for the duration of the trigger pulse and completes the circuit to the oscillator so that oscillations occur for the required period of time.

Unlike the grid-modulated transmitter of Fig 9, the anode modulator must be capable of handling the high peak current required by the oscillator. The modulator of Fig 10 must therefore be a high power valve. The main limitation of the hard valve modulator is the difficulty of producing valves capable of handling the large power required. For this reason gas-filled valves are often used in the modulator stage of high-power radar transmitters.

Thyratron Switch

The thyratron is a gas-filled triode or tetrode (hydrogen gas is often used) which is used in many radar modulators to provide the high-power, fast-acting switch to turn the transmitter valve on and off. The main properties of a thyratron are:

a.      Once the valve has started to conduct, the gas in the valve ionizes; the grid then has no further control over the anode current and the only way of switching the valve off is to reduce the anode voltage below a given level. Thus although a trigger voltage to the grid of a thyratron can initiate a pulse it cannot end it. This is a disadvantage compared with a hard valve modulator.


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