AP 3302 Pt. 3
Section 2
CHAPTER 11
Frequency-Dividing & Counting Circuits
During the next pulse interval B-C, D2 again cuts off to hold Vc2 at +19V, and D1 conducts to discharge C1 to zero.
Thereafter, the action is repeated. Each input pulse increases Vc2 in steps but the increase in amplitude of successive steps is becoming progressively less. The waveform of voltage across C2 is often referred to as a ‘staircase’ waveform and has been used in some circuits. The stair-case does not rise linearly however; because of the progressively smaller steps the rise is exponential.
Note that after a large number of cycles, Vc2 would eventually reach 100V and the circuit action would then cease. In practice however C2 is discharged by another circuit before the voltage steps become so small that it is difficult to distinguish between them. When C2 is discharged the circuit returns to its initial state and the action may be repeated indefinitely.
One typical circuit for the discharge of C2 after a given number of input pulses is shown in Fig 10. The circuit is that of a triggered blocking oscillator which is normally biased beyond cut-off by the potential divider R1R2 between h.t. + and earth. The bias may be adjusted by R2 and it is set such that after a given number of input pulses, Vc2 has risen sufficiently to lift V3 grid above cut-off. Normal blocking oscillator action then takes place, the grid current of V3 discharging C2 to zero ready for the next series of input pulses. We thus obtain one output pulse after a given number of input pulses.
