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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

Diode Step-by-step Counters

The circuit and waveforms of a basic diode step-by-step counter (sometimes referred to as a diode ‘pump’ circuit) are illustrated in Fig 9. A train of positive-going input pulses is applied to the circuit. The leading edge of the first input pulse at A is applied through C1 to D2 causing the diode to conduct. Both C1 and C2 then charge rapidly through D2 until Vc1 + Vc2 = Applied Voltage (100V). The capacitor C1 is usually small compared with C2. If C2 = 9C1 (typical figures) then one-tenth of the applied voltage appears across C2 (in this example 10V) and nine-tenths is developed across C1 (in this example 90V).

At the end of the pulse the applied voltage falls to zero and the voltage across C1 and C2 are then such as to cut D2 off. C2 therefore remains charged to + 10V. On the other hand the voltage across C1 is of the correct polarity to cut on D1 and the diode rapidly discharges C1 to zero. If D1 were not present C1 would also remain charged and there would be no further circuit action.

The leading edge of the next input pulse at B again cuts on D2, but since C2 is already charged to + 10V the effective input voltage is now the difference between the applied voltage and Vc2, ie 90V. This new voltage is again shared between C1 and C2 in the ratio one-tenth across C2 and nine-tenths across C1 (assuming C2 = 9C1). Thus C1 charges to 81V and C2 rises by 9V to + 19V.


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