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Baseband Pulse and Digital Signaling Chap. 3
3–11 PULSE TIME MODULATION: PULSE WIDTH
MODULATION AND PULSE POSITION MODULATION
Pulse time modulation (PTM) is a class of signaling techniques that encodes the sample values
of an analog signal onto the time axis of a digital signal. PTM is analogous to angle modulation,
which is described in Chapter 5. (As we have seen, PAM, PCM, and DM techniques
encode the sample values into the amplitude characteristics of the digital signal.)
The two main types of PTM are pulse width modulation (PWM) and pulse position
modulation (PPM). (See Fig. 3–43.) In PWM, which is also called pulse duration modulation
(PDM), sample values of the analog waveform are used to determine the width of the pulse
signal. Either instantaneous sampling or natural sampling can be used. Figure 3–44 shows a
technique for generating PWM signals with instantaneous sampling, and Fig. 3–45 displays
PWM with natural sampling. In PPM, the analog sample value determines the position of a
narrow pulse relative to the clocking time. Techniques for generating PPM are also shown in
the figures, and it is seen that PPM is easily obtained from PWM by using a monostable
Sampling times
Analog
t
(a) Analog Signal
PWM
t
(b) Corresponding PWM Signal
PPM
(c) Corresponding PPM Signal
t
Figure 3–43
Pulse time modulation signaling.