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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Square Waves applied to CR circuits

Differentiating Circuit

There are many occasions in radar when we need to convert a square wave input to a series of positive and negative-going pips of voltage. These pips may be used for timing purposes. They may be obtained by applying a square wave to a short CR circuit and taking the output across the resistor. A CR circuit used for this purpose is called a differentiating circuit (Fig 8). If the pulse duration of the input square wave is 200 us the CR time constant must be 20 us or less. Thus if the capacitor C is l00pF the resistor R must be 200kW or less.

Note from Fig 8 that the positive-going pips are coincident in time with the rising edges of the square wave input, and the negative-going pips with the falling edges. In other words the pips are linked in time to the p.r.f. of the square wave input and this time linkage is important. We shall see later that either the positive-going pips or the negative-going pips may be eliminated by a limiter circuit, leaving us with pips all in one direction which may be used for triggering other circuits at precise instants of time.

Medium CR Circuit

Medium CR circuits are those in which the time constant is comparable with the pulse duration of each part of one cycle of the square wave input. Let us assume that a synimetrical square wave of voltage, of amplitude l00V and pulse duration 100 us, is applied to a CR circuit where C = lOOpF and R = 1MW (Fig 9a).

Time constant CR = 100 x 10-12 x 106 100 us.

This is equal to the pulse duration so that the combination is a medium CR to this particular input.

The resultant waveforms of VC and VR are shown in Fig 9b and are derived as follows

a. A to B. The input rises from zero to + 100V. Because C cannot change its charge instantaneously, VC remains at zero volts and VR rises instantly to +100V.

b. B to C. C commences to charge and VC rises exponentially, reaching 63 per cent of 100V, i.e. + 63V, in a time of CR (100 us). VR falls to +37V in the same time, maintaining the relationship VC + VR = V = 100V.

c. C to D. The input falls by 100V to zero and, since VC is + 63V and cannot change immediately, VR also falls by 100V from + 37V to - 63V.

d. D to F. C discharges exponentially and in time CR (100 us) VC falls by 63 per cent of 63V, i.e. VC falls from + 63V to 63 - (63/100 x 63) = + 23.5V. VR rises from - 63V to - 23.5V in the same time (VC + VR = 0 at this instant).

e. E to F. The input rises by 100V from zero so that VR rises immediately by 100V from - 23.5V to + 76.5V. VC remains at +23.5V.


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