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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 9

Ringing and Blocking Oscillators

circuit long enough, the charge which builds up on C1 when grid current flows during the positive-going half-cycles of grid voltage is sufficient to bias the valve well beyond cut-off. Oscillations then cease until C1 has discharged sufficiently to cut the valve back on. As a result, the circuit oscillates in 'bursts' (Fig l0b). An oscillator working under these conditions is termed a 'self-quenching' or 'squegging' oscillator.

The blocking oscillator works in a somewhat different way: the inductive coupling between anode and grid is so 'tight' and the CR value large enough that the valve 'blocks off' very rapidly. The output is a single pulse of voltage only.

The blocking oscillator is a member of the relaxation oscillator family (like the multivibrator). In any relaxation oscillator, the valve conducts heavily for a given period of time and then cuts off, this being followed by a relaxation period during which the valve is returning to the state where it will again conduct.

The basic circuit of a blocking oscillator, with its associated waveforms, is shown in Fig 11. The tight coupling between anode and grid is provided by an iron-cored pulse transformer. The tuned circuit capacitance is usually omitted, the self-capacitance of the windings being sufficient.

If we assume that Vg is rising towards cut-off, then at instant A the valve conducts, the anode current rises and Va falls. The rising current in L1 induces a voltage in L2, the direction of the windings being such that Vg rises. The anode current therefore rises further, Va falls further and an avalanche occurs. Vg is driven above zero volts and Va falls to a low value.

When Vg rises above zero, grid current starts to flow, charging C1 in such a direction as to tend to make Vg negative. The voltage developed across C1 opposes the e.m.f. induced across L2 and the feedback is then insufficient to maintain the increasing anode current. At instant B, the anode current begins to fall causing Vg to fall. The anode current therefore falls further and another avalanche occurs in the reverse direction. Vg is now driven well below cut-off and Va rises to h.t.+.

With the abrupt cessation both of anode and grid currents, ringing occurs in the anode and grid circuits and damped oscillations at the ringing frequency are produced. The damping in the grid circuit is made high enough to prevent the positive-going portion of the grid ring rising above the cut-off level.

During the interval B to C the capacitor C1 is discharging through R1 towards zero volts. When Vg rises to cut-off, the valve again conducts and the action is repeated as from instant A.

In this circuit the output is taken from the anode. It consists of a series of negative-going, large-amplitude pulses with very steep edges (the


 

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