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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Square Waves applied to CR circuits

Response of CR Circuits to Asymmetrical Square Waves

The rectangular waveform shown in Fig 13a is one of the more common waveforms found in radar. When such a waveform is applied to a CR circuit the terms short, medium or long CR refer only to the particular portion of the waveform under consideration. In a CR circuit, in which C is 50pF and R is 1 MW, the time constant is:- CR = 50 x 10-12 x 106 = 50 us.

If the asymmetrical square wave shown in Fig 13a is applied to this CR circuit, the circuit acts as a long CR to the short pulse AB but as a short CR to the period BC between pulses. The waveform appearing across R would then be as shown in Fig 13b.

Fig 14 shows the steady-state waveforms of a CR circuit in which the values of C and R are such that the circuit acts as a long CR to both parts of each cycle of an asymmetric square wave (two inputs shown). The following points should be noted:-

a. C can charge and discharge only by a small amount in each pulse period.

b. As the pulse periods are unequal a larger charging current during a short pulse period is

needed to balance a smaller discharging current during the long pulse period.

c. The circuit becomes stable when the shaded areas in Fig 14 are equal. The waveform is then balanced even if it does not appear to be so. The mean value of VR is now zero and that of VC is equal to the mean value of the input.

d. If the symmetry of the input waveform changes, the top and bottom levels of the VR waveform change. This may be seen by comparing the waveforms for the two different inputs in Fig 14.


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