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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Other Square Wave Generators

Instant A. The positive trigger causes Vg1 to rise. The total space current therefore rises and the increased screen current Is causes Vg2 to fall due to the increased voltage drop across R3. Since C2 cannot discharge instantly this fall in Vg2 drives Vg3 negative via C2 and Ia falls, causing Va to rise. Since Ia has fallen, Is must have risen by the same amount (since Ia + Is = space current) and this causes Vg2 to fall further. There are here all the conditions for an avalanche so that as soon as the positive trigger is applied, Ia cuts off and all the space current goes to the screen. Vg2 therefore falls to a low value and Va rises to h.t.+.

Instant B. When the trigger input ceases and Vg1 returns to zero the total space current falls. This causes the screen current Is to fall so that Vg2 and Vg3 both rise. However, so long as the discharge current of C2 keeps Vg3 beyond the anode current cut-off point, no other change occurs.

Interval B to C. The circuit is now in its unstable state and remains there whilst Vg3 returns towards zero with the discharge of C2 through R2 (see Fig 2).

Instant C. Vg3 rises above the anode current cut-off point and Ia flows, causing Va to fall. With the rise in Ia, Is falls and Vg2 rises. The rise in Vg2 is transferred through C2 to the suppressor grid and Vg3 rises further. Ia therefore rises further, Is falls still more and an avalanche results which continues until Vg3 is sufficiently positive to cause the flow of suppressor grid current. Va falls to its low working value and Vg2 rises.

Interval C to D. C2 now recharges rapidly through Rs (see Fig 2). As it does so, Vg3 returns to zero and Vg2 returns to its original level. The circuit is now in its original stable state and remains there until the next trigger pulse is applied.

The square wave output is taken from the anode. It is locked to the trigger pulses in time and its pulse duration is determined by the time taken for the suppressor grid voltage to rise to the anode current cut-off level, i.e. by the time constant C2R2. By varying R2, the pulse duration is varied. The diode D is inserted to limit the positive rise of Vg3 at instant C.

This circuit may also be triggered by applying a negative pulse to the suppressor grid. At instant A the negative pulse reduces Ia, causing Is to rise. Vg2 and hence Vg3 fall so that Ia falls further. An avalanche results and the circuit action is then as previously described.


This circuit is developed from the Miller timebase circuit which we shall consider in detail in a later chapter. The sawtooth waveform produced at the anode of a phantastron may be used as a timebase waveform. More usually, however, it is the square wave output from either the screen grid or the cathode which is required. The basic circuit of a phantastron and its associated waveforms are shown in Fig 3. In this circuit R1 is the grid resistor to limit the grid to zero volts when grid current flows, and RL. is large enough to cause bottoming at a certain low value of Va. The most distinguishing feature of this circuit is the 'Miller' capacitor C1 connected between anode and control grid. Note also that neither Rs nor RK is decoupled; rapid voltage changes can therefore occur at the screen grid and the cathode.

In the stable state, before the application of a trigger pulse to the suppressor grid, the valve is conducting normally. The grid voltage Vg1 is held at zero volts relative to its cathode by grid current limiting via R1. The cathode voltage VK (and hence Vg1) is therefore some volts positive to earth because of the voltage drop produced across RK by the space current. The suppressor grid is connected to earth through R2 and so is negative with respect to its cathode. The value of RK is made large enough to ensure that the suppressor grid is sufficiently negative to its cathode to cut off the anode current. The total space current of the valve therefore flows to the screen grid and the screen grid voltage Vg2 is low. With no anode current, Va is at h.t. + and the capacitor C1 is charged.


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Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

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