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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 8

Other Square Wave Generators

point due to the voltage division in the potentiometer chain RL1, R3, R4 connected between h. t. + and the bias. The grid of V2 is held at cathode potential by grid current limiting through R6 so that V2 is also conducting. V2 anode current is, however, cut off by the voltage on the suppressor grid so that the total space current of V2 flows to the screen. V2 anode voltage is thus at h.t. + and its screen voltage is at a low value.

Instant A. The negative trigger input pulse cuts off anode current in Vi and Vi anode voltage rises, taking up the suppressor grid voltage of V2. Anode current now flows in V2 and V2 anode voltage falls, taking V2 grid voltage with it (via C3). This produces the normal 'Miller step' action in V2 as explained for the phantastron. With the flow of anode current in V2, the screen current falls and V2 screen voltage rises. The fall in V2 anode voltage is also applied via C2 to V1 grid. V1 grid base is shorter than that of V2 so that the fall in V2 anode voltage is sufficient to cut off V1 at the grid and V1 anode voltage remains at a high value.

Interval A to B. The usual Miller rundown now occurs, during which V2 grid voltage rises slowly, the discharge of C3 through R6 being opposed by Miller feedback. V2 anode voltage thus falls linearly. V1 grid voltage is subject to two opposing forces: C2 is trying to discharge through R5 to lift V1 grid voltage to zero, but at the same time V2 anode voltage is falling, tending to take V1 grid voltage with it. The circuit component values are made such that the fall in V2 anode voltage has the greater effect so that V1 grid voltage falls slowly. V1 therefore remains cut off, V1 anode voltage remains at a high value and V2 suppressor grid voltage remains above the anode current cut-off level. V2 screen grid voltage therefore remains at a high value.

Interval B to C. At B, V2 anode voltage reaches its bottoming value and, with no Miller feedback, V2 grid voltage rises rapidly to zero as C3 discharges through R6. V2 anode voltage remains at its bottoming voltage however, since V1 is still cut off and V2 suppressor grid is thus still above its anode current cut-off level. Because V2 anode voltage has now stopped falling, V1 grid voltage can rise to zero as C2 discharges through R5. V1 cuts on at point C. V2 grid voltage rises more rapidly than that of V1 because it is aiming for h.t. + whereas V1 grid voltage is aiming for zero volts through a high value resistor (R5).

Instant C. V1 grid reaches cut-off and V1 conducts. V1 anode voltage therefore falls taking V2 suppressor grid voltage with it. V2 anode current therefore falls and its anode voltage rises making V1 grid voltage rise further (via C2). An avalanche therefore occurs which rapidly switches the circuit back to its initial stable state, where it remains until another trigger pulse is applied (at instant D).

A square wave output is obtained from the screen of V2 and also from the anode of V1. V2 anode provides a sawtooth timebase waveform.The pulse duration can be altered by varying the Miller rundown time with the aid of anode-catching diodes, as in the phantastron.


 

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