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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Clamping Circuits

The voltage VT is obtained from a voltage divider network, and the square wave input for the blanking pulse is applied to the grid through the usual CR coupling network. However only a.c. voltages appear across R and the square wave input will vary Vg above and below the bias level VT by equal amounts (Fig 12c). The brightness of the trace will therefore increase and the electron beam may not be blanked out between traces. This may be avoided by using a negative clamping circuit to clamp the input to the bias level VT (Fig 12d). The use of a clamping circuit has another advantage. If the symmetry of the square wave input changes, e.g. due to a change in the range scale, the mean value of the input will tend to vary causing a variation in the brightness of the trace. Clamping prevents this.

c. Deflection modulation. In an electrostatic c.r.t. the vertical position of the trace in a type A display, in the absence of a signal input, is determined by the shift voltage applied to the Y deflecting plates of the c.r.t. If only a few signals are being received the mean level of the voltage applied to the Y plates increases slightly (Fig 1 3a). If many signals are received the level rises still further. The result is that the trace tends to 'jitter' up and down as the number of signals being received varies. This can be prevented by negatively clamping the input waveform to the shift voltage (Fig 13b). Fig l3c shows the basic circuit.

d. Intensity modulation. This is the type of modulation used in a p.p.i. display. In this case, in the absence of clamping, variation in the number of signals applied to the c.r.t. grid varies the overall brightness of the display because of the variation in the mean level of the input (Fig 14a).


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