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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 11

Frequency-Dividing & Counting Circuits

The circuit stays in this condition whilst C2 is discharging through R2 causing VB2 to rise towards cut-on. Any trigger pulses appearing during this time have no effect since TR1 is already conducting. When VB2 reaches cut-on the circuit quickly reverts to its stable state in which TR1 is held cut-off by the bias voltage and TR2 is conducting. It remains in this condition until the next trigger pulse appears, when the action is repeated. The count-down ratio in Fig 5 is 5:1.

Frequency Division by Blocking Oscillator

The circuit and waveforms of a blocking oscillator used as a frequency divider are shown in Fig 6. The circuit illustrated uses collector-base coupling and base timing (see p152). The free-running frequency of the blocking oscillator, set by the aiming voltage (RB1) is slightly lower than the required output frequency. The p.r.f. of the trigger pulses is made much higher than that of the free-running circuit and the conditions are then such that, in the example shown, every fifth trigger pulse raises the base voltage above cut-off earlier than normal. This gives a count-down ratio of 5:1.

Frequency Division by Thyratron

The circuit and waveforms of a thyratron frequency divider are shown in Fig 7. To cause the valve to strike, a trigger pulse must be applied to the grid. On receipt of trigger pulse 3 at time t1, the valve cuts on, the gas ionizes and a high value of anode current flows. This causes Va to fall sharply. The current flowing through the valve charges CK causing the cathode to rise exponentially. A point is reached where the current through the valve is insufficient to maintain ionization and the valve cuts off. The anode then returns to ht + and the cathode starts to fall as CK discharges through RK. Pulses 1 and 2 have no effect on the circuit action but by the time pulse 3 arrives at t2, VK has fallen sufficiently to allow the valve to ionize again and the action is repeated. In the example given the count-down ratio is 3:1 but this may be varied either by changing the time constant CK RK or by changing the amplitude of the trigger pulses applied to the grid by varying R1.

Frequency Division by Tunnel Diode

The basic circuit and waveforms of a tunnel diode used as a frequency divider are shown in Fig 8. The tunnel diode is biased to point x on the negative resistance portion of its characteristic (Fig 8b). Any slight variation of current produces a back e.m.f. across the inductance L and this back e.m.f., superimposed on the bias voltage, varies the operating point on the characteristic and causes the circuit to oscillate. Let us see how this happens.

We shall assume that the circuit is operating at point A on the characteristic and that the voltage is increasing towards the bias level. The current will tend to decrease as the negative resistance region is entered. The inductance will, however, maintain the current through the diode nearly constant and a back e.m.f. will be induced across L in the process. This back e.m.f. is superimposed on the bias voltage and, since the current is practically constant, the operating point is transferred almost instantaneously to B on the characteristic. The back e.m.f. now falls at a rate determined by the time constant of the circuit and so does the current, until point C is reached. At C the current tends to rise again and once more a back e.m.f. is induced in L which holds the diode current nearly constant. Since the current is tending to rise, the back e.m.f. now opposes the bias voltage and, with current constant, the operating point is transferred to D on the characteristic. The current in the tunnel diode now rises normally through E to point A and the action is repeated.

When used as a frequency divider, trigger pulses are applied as shown in Fig 8c. Any trigger pulses applied during the time intervals D to E and B to C are effectively suppressed because the slope resistance of the tunnel diode is very small compared with the resistance R in these regions.


 

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ęCopyright 2000 - 2002 Dick Barrett

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