Study of PAM, PPM and PDM.
(a)Study of Pulse Amplitude Modulation using Natural & Flat top Sampling.
Most digital modulation systems are based on pulse modulation. It involves variation of a pulse parameter in accordance with the instantaneous value of the information signal. This parameter can be amplitude, width, repetitive frequency etc. Depending upon the nature of parameter varied, various modulation systems are used. Pulse amplitude modulation, pulse width modulation, pulse code modulation are few modulation systems cropping up from the pulse modulation technique. In pulse amplitude modulation (PAM) the amplitude of the pulses are varied in accordance with the modulating signal. In true sense, pulse amplitude modulation is analog in nature but it forms the basis of most digital communication and modulation systems. The pulse modulation systems require analog information to be sampled at predetermined intervals of time. Sampling is a process of taking the instantaneous value of the analog information at a predetermined time interval. A sampled signal consists of a train of pulses, where each pulse corresponds to the amplitude of the signal at the corresponding sampling time. The signal sent to line is modulated in amplitude and hence the name Pulse Amplitude Modulation (PAM).
Natural sampling:In the analogue-to-digital conversion process an analogue waveform is sampled to form a series of pulses whose amplitude is the amplitude of the sampled waveform atb the time the sample was taken. In natural sampling the pulse amplitude takes the shape of the analogue waveform for the period of the sampling pulse as shown in figure.
Flat Top sampling:-After an analogue waveform is sampled in the analogue-to-digital conversion process, the continuous analogue waveform is converted into a series of pulses whose amplitude is equal to the amplitude of the analogue signal at the start of the sampling process. Since the sampled pulses have uniform amplitude, the process is called flat top sampling as shown in figure
Circuit Diagram:-
Signal Reconstruction:-
Related O/P Waveforms:-
(b) Study of PPM using DC Input, Sine wave Input.
The Amplitude and width of the pulses is kept constant in this system, while 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. As mentioned in connection with pulse width modulation, pulse-position modulations has the advantage of requiring constant transmitter power output, but the disadvantages of depending on transmitter receiver is synchronization.
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There may be a sequence of signal sample amplitudes of (say) 0.9, 0.5, 0 and -0.4V. These can be represented by pulse widths of 1.9, 1.5, 1.0 and 0.6μs respectively. The width corresponding to zero amplitude was chosen in this system to be 1.0μs, and it has been assumed that signal amplitude at this point will vary between the limits of + 1 V (width = 2μs) and -1 V (width = 0μs). Zero amplitude is thus the average signal level, and the average pulse width of 1μs has been made to correspond to it. In this context, a negative pulse width is not possible. It would make the pulse end before it began, as it were, and thus throw out the timing in the receiver. If theb pulses in a practical system have a recurrence rate of 8000 pulses per second, the time between the commencements of adjoining pulses is 106 /8000 =125μs. This is adequate not only to accommodate the varying widths but also to permit time-division multiplexing. Pulse width modulation has the disadvantage, when compared with pulse position modulation, which will be treated next, that its pulses are of varying 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 perhaps only half of the peak power. On the other hand, puls width modulation still works if synchronization between transmitter and receiver fails, whereas pulse-position modulation does not, as will be seen.
For DC Input:
circuit-diagram:
Connection Diagram:-
For Sine wave Inputs:
Circuit Diagram:
(c) Study of PWM using different Sampling Frequency.
In pulse width modulation of pulse amplitude modulation is also often called PDM (pulse duration modulation) and less often, PLM (pulse length modulation). In this system, as shown in Figure, we have fixed amplitude and starting time of each pulse, but the width of each pulse is made proportional to the amplitude of the signal at that instant.
there may be a sequence of signal sample amplitudes of (say) 0.9, 0.5, 0 and -0.4V. These can be represented by pulse widths of 1.9, 1.5, 1.0 and 0.6μs respectively. The width corresponding to zero amplitude was chosen in this system to be 1.0μs, and it has been assumed that signal amplitude at this point will vary between the limits of + 1 V (width = 2μs) and -1 V (width = 0μs). Zero amplitude is thus the average signal level, and the average pulse width of 1μs has been made to cores to it. In this context, a negative pulse width is not possible. It would make the pulse end before it began, as it were, and thus throw out the timing in the receiver. If the pulses in a practical system have a recurrence rate of 8000 pulses per second, the time between the commencements of adjoining pulses is 106 /8000 = 125μs. This is adequate not only to accommodate the varying widths but also to permit time-division multiplexing. Pulse width modulation has the disadvantage, when compared with pulse position modulation, which will be treated next, that its pulses are of varying 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 perhaps only half of the peak power. On the other hand, pulse width modulation still works if synchronization between transmitter and receiver fails, whereas pulse-position modulation does not, as will be seen.
Connection Diagram:-
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