2-46
Figure 2-44D.—Pulse-duration modulation (pdm). WIDTH MODULATED PULSES FROM CIRCUIT OF
FIGURE 2-45.
Figure 2-45.—Circuit for producing pdm.
In the circuit of figure 2-45, a series of sawtooth pulses, occurring at the sampling rate, is applied to
a one-shot multivibrator. The multivibrator has the signal voltage Essuperimposed on the bias voltage Ein.
Each pulse triggers a cycle of multivibrator operation which terminates after a time interval and varies
linearly with the voltage Es. The pulse of plate voltage produced by the multivibrator will have a leading
edge at T1. The leading edge will vary in position with the signal voltage, while the trailing edge at T2 is
fixed by the termination of the sawtooth pulse. The length of the output pulse is thus duration or width
modulated. If the sawtooth has an instantaneous buildup and a sloping trailing edge, then the leading edge
(T1) is fixed and the trailing edge (T2) varies. If the sawtooth generator produces a slope on both leading
and trailing edges, both T1 and T2 are variable in position, but the result is still pdm. Pdm is often used
because it is of a constant amplitude and is, therefore, less susceptible to noise. When compared with
ppm, pdm has the disadvantage of a varying pulse, width and, therefore, of varying power content. This
means that the transmitter must be powerful enough to handle the maximum-width pulses, although the
average power transmitted is much less than peak power. On the other hand, pdm will still work if the
synchronization between the transmitter and receiver fails; in ppm it will not, as will be seen in the next
section.
PULSE-POSITION MODULATION.—The amplitude and width of the pulse is kept constant in
the system. The position of each pulse, in relation to the position of a recurrent reference pulse, is varied
by each instantaneous sampled value of the modulating wave. Ppm has the advantage of requiring
constant transmitter power since the pulses are of constant amplitude and duration. It is widely used but
has the disadvantage of depending on transmitter-receiver synchronization.
