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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 6

Free Running (Astable) Multivibrators

Instant B. When TR2 emitter has fallen sufficiently, TR2 begins to conduct, taking the charging current from R2 which was previously passing into TR1 emitter. This fall of current in TR1 causes TR1 collector voltage to rise towards the negative supply voltage level taking TR2 base voltage with it. As TR2 base becomes more negative, TR2 conducts more heavily and a cumulative action results which causes TR2 emitter to rise towards -VC taking TR1 emitter voltage with it (via C1). TR2 collector voltage falls towards earth as TR1 collector voltage rises towards -VC.

Interval B to C. With TR2 conducting, C1 now commences to charge in the opposite direction through RV, R1 and RL2. As it does so TR1 emitter voltage falls until it reaches the point where TR1 starts to conduct. When this happens TR1 collector voltage falls, taking TR2 base voltage with it. TR2 emitter is held negative by the charge on C1 so that the fall in TR2 base voltage cuts TR2 off and TR2 collector voltage rises to -VC Thereafter the cycle is repeated as from instant A.

In this circuit, variation of RL1 varies the time constant and so determines the time for which TR1 is on. It is therefore a p.r.f control. In some circuits various values for C1 may be switched in, giving various selected values of p.r.f., with RL1 as a fine control. RV is a symmetry control which determines the mark-to-space ratio of the output. Very short pulse durations of the order of a few microseconds may be obtained with this circuit.

How Multivibrators are used

Fig 20 shows the basic schematic diagram of a primary radar installation which we built up in Section 1. The master timing unit produces a waveform which triggers off each transmitter pulse and so controls the p.r.f. of the output. The timing waveform is also applied to the indicator c.r.t. to synchronize the spot movement with the transmitter pulses.

If we require a p.r.f. of 500 pulses per second we could use a multivibrator in the timing circuit with the frequency control set to give 500 cycles of output per second. To convert the square wave output from the multivibrator into timing pulses we could pass it through a differentiating CR circuit, the output from which would be a series of pips (Fig 21). The negative-going pips could then be removed by a negative limiter.


 

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