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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Free Running (Astable) Multivibrators

This difficulty can be overcome by returning the grid resistors to h.t. + instead of to earth. The capacitors are now aiming at h.t.+. The result is that when the capacitor voltage rises through cut-off, the capacitor discharge is still at the 'steep' part of its exponential and this steep rise through cut-off gives a more decisive cut-off point which is less subject to random variations in the circuit conditions.

The circuit of a multivibrator using this modification is shown in Fig 13. The circuit contains symmetry and p.r.f. controls. The symmetry control is as previously described. The p.r.f. control varies the 'aiming voltage' of both grids by equal amounts causing both valves to reach cut-off earlier or later depending upon the sense of variation. The frequency is thus varied without affecting symmetry.

Synchronized Multivibrator

If we apply a positive-going synchronizing pulse to the grid of one valve in a multivibrator before Vg would normally reach its cut-off point we can make the valve conduct at the instant the sync pulse is applied. This is shown in Fig 14. The frequency of the sync pulses must be slightly higher than that of the free-running multivibrator and their amplitude must be sufficient to take the grid above cut-off at the desired instant. Each cycle of the multivibrator is then 'locked' or synchronized to the frequency of the sync pulses. This method is often used to ensure that various circuits in a radar system all operate at the same instant of time. Note however that the astable multivibrator is not dependent for its operation on the sync pulses. It will still free-run at its own fre-quency without them, unlike the monostable and bistable multivibrators which must be triggered.

If we apply sync pulses to both valves, as in Fig 15, each half-cycle of the multivi-brator output is synchronized. The sync pulse 'x' cuts on V1 and pulse 'y' cuts on V2.


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