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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Limiting Circuits


A typical triode squarer circuit and its associated waveforms are illustrated in Fig 11. During the positive half-cycle of the input, grid current limiting maintains Vgk practically at zero volts and the output voltage Va is at some value determined by the circuit constants (say + 50V). On the negative-going half-cycle of the input, when Vgk falls below cut-off, the anode voltage Va rises to h.t. + (say + 300V) and remains there until Vgk again rises above cut-off. We can see from Fig 11 that the output waveform is not perfectly square: it varies slightly while the valve is conducting and takes a short but definite time to rise and fall. The waveform may be improved by applying the squarer output to a diode limiter, arranged to clip the curved portion of the output waveform. The frequency of the output is exactly the same as that of the sine wave input and the wave-form is normally sufficiently square to be converted into triggering pulses which can then be used to synchronize other stages in the equipment.


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