The circuit performs two functions it samples, and converts this sample in Pulse Width Modulation. If this voltage is varied in accordance with a signal voltage, a series of rectangular pulses are obtained,with widths varying as required. 18.9 makes an excellent voltage to time converter, since its gate width is dependent on the voltage to which the capacitor C is charged. The emitter-coupled monostable multivibrator shown in Fig. The circuit diagram for such an arrangement is shown in Fig. Pulse width modulation may be generated by applying trigger pulses (at the sampling rate) to control the starting time of pulses from a monostable miltivibrator, and feeding in the signal to be sampled to control the duration of the pulses. This is enough to not only accommodate varying widths but also to permit time division multiplexing (10 6 = 1 MHz frequency clock). If the pulse in a practical system has a recurrence rate of 8000 pulses/s the time of starting the adjoining pulses is 10 6/ 8000 = 125 μs. In this contrast, a negative pulse width is not possible, because it would make the pulse end before it began. Zero amplitude is thus the averaging signal level, which corresponds to the average pulse width of 1 ,us In this system the width corresponding to zero amplitude can be selected as 1.0 μs, assuming that the signal amplitude varies at this point between + 1 V (width = 2 μs) and - 1 V (width = 0 μs). These may be represented by pulse widths of 1.9, 1.5, 1.0 and 0.6 μs. 18.8, there may be a sequence of signal sample amplitudes of (say) 0.9, 0.5, 0 and - 0.4 V.
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