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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Frequency-Dividing & Counting Circuits

In the example shown every fifth input pulse triggers the blocking oscillator. Each output pulse thus indicates a count of five input pulses. In this particular application the counter is doing a similar job to the dividers considered earlier in this chapter. The important difference is that the input to the counter may be random pulses occurring at irregular instants of time, because C2 cannot discharge between pulses.

Although the point at which triggering of V3 occurs may be varied by R2 (and also by the ratio of C1 to C2) the circuit is not normally expected to count more than about ten pulses before C2 is discharged. The reason for this is clear when it is remembered that the steps at the top of the ‘staircase’ are very shallow. It becomes more difficult to distinguish between adjacent steps so that the triggering becomes less determinate.

There are a number of variations of this circuit. The ‘discharge’ circuit need not be a blocking oscillator; thyratron and Miller integrator circuits have also been used. A negative-going output across C2 may be obtained by reversing the polarity of the two diodes V1 and V2.

Transistor Counter

The main limitation of the diode step-by-step counter is the non-linear rise of the staircase waveform. This may be overcome by the circuit of Fig 11 in which the diode D1 is replaced by the n-p-n transistor TR1, the output voltage at Y being fed back direct to the transistor base. The operation of the circuit is basically similar to that of the diode counter except that TR1 conducts during the periods between pulses because of the positive voltage at its base from C2. With TR1 conducting, C1 discharges until Vc1 equals -Vc2 (See p 200 on ‘bootstrapping’). The result is that D2 has practically no reverse bias to overcome so that the whole input voltage is available during each pulse to charge C1 and C2 - unlike the diode counter where the effective input is the difference between the applied voltage and Vc2. Thus, in Fig 11, the voltage steps of are all equal and the staircase waveform becomes linear.

PRF Meter

The circuit of a transistor counter used as a p.r.f. meter is shown in Fig 12a. This circuit is similar to the step-by-step counter except that C2 is now shunted by R1. The result is that during each interval between pulses C2 can discharge via R1. Vc2 will rise in the manner described for the step-by-step counter and R1 will discharge C2 in the intervals. This continues until the charge gained via D2 during the pulse is exactly equal to that lost via R1 in the interval between pulses (Fig 12b). If the p.r.f. increases, the charge gained by C2 increases; Vc2 therefore rises until a new (higher) balance is obtained. The mean voltage across C2 is therefore proportional to the p.r.f. The ripple component of the output is smoothed by R2C3 and the resulting d.c. voltage is applied as the input to the base of the n-p-n transistor TR2. As the p.r.f. of the input increases, Vc2 rises causing TR2 base voltage to rise. This increases the current through TR2 and causes a higher reading in the meter which is calibrated to give a direct reading of the p.r.f. R3 varies the bias to TR2 base for calibration purposes. To obtain an output voltage proportional to the p.r.f. of the input, a collector load may be inserted in TR2 and the output voltage taken from the collector.

Bistable Counters

Various other circuits may be used as counting circuits. Apart from those discussed in this chapter the one most frequently used, especially in computers, is the bistable multivibrator. The Eccles-Jordan bistable is a natural ‘scale-of-two’ (binary) counter because to go through one cycle of operation two trigger input pulses are required. Thus each output pulse indicates two input pulses.


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