Radio 101: Understanding Pulse Time Modulation

by: Tymon Hytem

In Pulse time modulation the signal is sampled by pulses of constant amplitude but the instantaneous magnitude of the sample determines one of the following characteristics of the pulse that is width and position or frequency. Thus at the sampling time, width or position or frequency of pulse varies proportional to the instantaneous value of the signal. Thus the following three types of Pulse time modulation are pulse width modulation, pulse position modulation and pulse frequency modulation.

Pulse time modulation has the same advantages over pulse amplitude modulation as frequency modulation has over amplitude modulation. In each type of Pulse time modulation since the pulse amplitude is kept constant amplitude limiters may be used to provide high degree of noise immunity. In pulse width modulation each pulse has fixed amplitude and starting time but the width of each pulse is kept proportional to the magnitude of the signal at that instant. Thus pulses occur at regular intervals. As compared to the pulse position modulation the pulse width modulation has the drawback that its pulses have varying width and hence varying power content. This implies that the transmitter output power must be large enough to handle the maximum width pulses. The average power transmitted is only about half the peak power. Hence the power rating has the advantage that it still works when the synchronization between the transmitter and receiver has failed. The pulse position modulation does not work in such a condition. Pulse width modulation may be achieved by applying trigger pulses at the sampling rate to the input of a mono stable multi vibrator to initiate the output pulses and applying the signal to be sampled to control end of multi vibrator pulses and thus determine the duration or width of the output pulses.

Pulse width modulation uses an emitter coupled mono stable multi vibrator circuit. This circuit works as an excellent voltage to time converter since its gate width depends upon the voltage to which the capacitor C is charged. On varying this voltage in accordance with the signal voltage a series of rectangular pulses are obtained at the output with widths varying as required. It may be seen that the circuit performs two jobs. That is sampling the signal and converting the sample into pulse width modulated sample. The demodulation of pulse width modulation is quite simple and achieved by simply feeding the pulse width modulation voltage to an integrating circuit. The output is a signal whose amplitude at any time is proportional to the pulse width at that time. In Pulse position modulation amplitude and width of the pulses is kept constant but the position of each pulse in relation to the position of the recurrent reference pulse or synchronizing pulse is varied by each instantaneous sampled value of the modulating wave.