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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Free Running (Astable) Multivibrators


The importance of square waves for timing and for controlling the action of the circuits in a radar installation has already been mentioned. So far we have learnt the terms used to describe square waves and have seen the effects of applying square waves to various types of circuit. The next step is to discover how square waves are produced (Fig 1).

In Chapter 4 we saw how limiting circuits may be used to convert sine wave inputs to approxi-mate square wave outputs. Apart from the fact that this is a rather indirect way of obtaining a square wave, the output waveform is not sufficiently steep-sided for accurate timing and precise triggering. If the output from a squarer is being used to trigger a succeeding stage, any variation in the bias of that stage will alter the instant of triggering (Fig 2a). With a steep-sided square wave input this does not happen (Fig 2b).

The circuits used to produce steep-sided square wave outputs are known as square wave generators or pulse generators. Most of these circuits have two valves (or two transistors) which are coupled by CR combinations in such a way that the valves switch each other on and off at regular intervals, giving square waves of voltage at the output. The term "square waves" is being used here very loosely. The output may in fact be symmetrical or asymmetrical. Many different circuits are in common use as square wave generators and some of these are discussed in this chapter.

Some Important Basic Facts

Most pulse generators depend for their operation on a combination of several simple basic facts, all of which we already know. The action of some of these circuits may appear, at first, quite complicated. They appear so only because several actions may be taking place simultaneously, or with very short time intervals between them. Each action considered by itself is basically simple and if we consider each step in isolation and in the correct sequence, the action of the whole becomes quite clear.


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