intelligence may be impressed on the carrier by varying any of its characteristics. In the preceding paragraphs the method of modulating a pulse train by varying its amplitude was discussed. Time characteristics of pulses may also be modulated with intelligence information. Two time characteristics may be affected: (1) the time duration of the pulses, referred to as PULSE-DURATION MODULATION (pdm)">
In pulse-modulated systems, as in an analog system, the intelligence may be impressed on the carrier by varying any of its characteristics. In the preceding paragraphs the method of modulating a pulse train by varying its amplitude was discussed. Time characteristics of pulses may also be modulated with intelligence information. Two time characteristics may be affected: (1) the time duration of the pulses, referred to as PULSE-DURATION MODULATION (pdm) or PULSE-WIDTH MODULATION (pwm); and (2) the time of occurrence of the pulses, referred to as PULSE-POSITION MODULATION (ppm), and a special type of PULSE-TIME MODULATION (ptm) referred to as PULSE-FREQUENCY MODULATION (pfm). Figure 2-43 shows these types of ptm in views (C),(D), and (E). Views (A) and (B) show the modulating signal and timing, respectively.
Figure 2-43A. - Pulse-time modulation (ptm). MODULATION
Figure 2-43B. - Pulse-time modulation (ptm). TIMING
Figure 2-43C. - Pulse-time modulation (ptm). PDM
Figure 2-43D. - Pulse-time modulation (ptm). PPM
Figure 2-43E. - Pulse-time modulation (ptm). PFM
PULSE-DURATION MODULATION. - Pdm and pwm are designations for a single type of modulation. The width of each pulse in a train is made proportional to the instantaneous value of the modulating signal at the instant of the pulse. Either the leading edges, the trailing edges, or both edges of the pulses may be modulated to produce the variation in pulse width. Pdm can be obtained in a number of ways, one of which is illustrated in views (A) through (D) in figure 2-44. A circuit to produce pdm is shown in figure 2-45. Adding the modulating signal [figure 2-44, view (A)] to a repetitive sawtooth [view (B)] will result in the waveform shown in view (C). This waveform is then applied to a circuit which changes state when the input signal exceeds a specific threshold level. This action produces pulses with widths that are determined by the length of time that the input waveform exceeds the threshold level. The resulting waveform is shown in view (D).
Figure 2-44A. - Pulse-duration modulation (pdm). MODULATING SIGNAL
Figure 2-44B. - Pulse-duration modulation (pdm). REPETITIVE SAWTOOTH PULSES
Figure 2-44C. - Pulse-duration modulation (pdm). MODULATING SIGNAL AND SAWTOOTH ADDED
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.
Ppm can be generated in several ways, but we will discuss one of the simplest. Figure 2-46 shows three waveforms associated with developing ppm from pdm. The pdm pulse train is applied to a differentiating circuit. (Differentiation was presented in NEETS, Module 9,
Introduction to Wave-Generation and Wave-Shaping Circuits.) This provides positive- and negative-polarity pulses that correspond to the leading and trailing edges of the pdm pulses. Considering pdm and its generation, you can see that each pulse has a leading and trailing edge. In this case the position of the leading edge is fixed, whereas the trailing edge is not, as shown in view (A) of figure 2-46. The resultant pulses after the differentiation are shown in view (B). The negative pulses are position-modulated in accordance with the modulating waveform. Both the negative and positive pulse are then applied to a rectification circuit. This application eliminates the positive, non-modulated pulses and develops a ppm pulse train, as shown in view (C).
Figure 2-46. - Pulse-position modulation (ppm).
PULSE-FREQUENCY MODULATION. - Pfm is a method of pulse modulation in which the modulating wave is used to frequency modulate a pulse-generating circuit. For example, the pulse rate may be 8,000 pulses per second (pps) when the signal voltage is 0. The pulse rate may step up to 9,000 pps for maximum positive signal voltage, and down to 7,000 pps for maximum negative signal voltage. Figure 2-47, views (A), (B), and (C) show three typical waveforms for pfm. This method of modulation is not used extensively because of complicated pfm generation methods. It requires a stable oscillator that is frequency modulated to drive a pulse generator. Since the other forms of ptm are easier to achieve, they are commonly used.
Figure 2-47A. - Pulse-frequency modulation (pfm). MODULATION
Figure 2-47B. - Pulse-frequency modulation (pfm). TIMING
Figure 2-47C. - Pulse-frequency modulation (pfm). PFM
Q.23 What characteristics of a pulse can be changed in pulse-time modulation?
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