Introduction:
(1) Pulse Amplitude modulation.(PAM)
(2) Pulse Time modulation. (PTM)
Figure shown below explains the principle of PAM. A baseband signal f(t) is shown in (a) part, and carrier pulse train fc(t) is shown in the (b) part. The frequency of the carrier pulse train is decided by the sampling theorem. According to the sampling theorem, if a modulating signal is band limited to X Hertz (i.e., there are no frequency components beyond X Hz in the frequency spectrum of the modulating signal), the sampling frequency must be atleast 2X Hertz and, hence, the frequency of the carrier pulse train must also be atleast 2X Hz. A pulse amplitude modulated signal fm(t) is shown in the figure in (c) part. It can be seen that amplitude of the pulses depends on the value of f(t) during the time of the pulse. The PAM signal fm(t) is known as discrete time signal, as this signal is discrete on time axis and continuous on amplitude axis.
In the above figure, the baseband signal f(t) is shown to have only a positive polarity. In practice, however, we can have a baseband signal with a positive as well as negative polarity. But, in such a case, the modulated pulses will also be of positive as well as negative polarities. As the transmission of such a bipolar pulses is inconvenient; a clamping circuit is used so that we always have a base band signal with only the positive polarity.
In
analog modulation systems, some parameter of sinusoidal carrier is
varied in accordance with the instantaneous value of the modulating
signal. In pulse modulation systems, the carrier is no longer a
continuous signal but consists of a pulse train, some parameter of which
is varied in accordance with the instantaneous value of the modulating
signal.
Types of Pulse Modulation Systems:
There are two types of pulse modulation systems:(1) Pulse Amplitude modulation.(PAM)
(2) Pulse Time modulation. (PTM)
Pulse Amplitude Modulation
In PAM, the amplitude of the pulses of the carrier pulse train is varied in accordance with the modulating signal; whereas in PTM, the timing of the pulses of the carrier pulse train is varied.Figure shown below explains the principle of PAM. A baseband signal f(t) is shown in (a) part, and carrier pulse train fc(t) is shown in the (b) part. The frequency of the carrier pulse train is decided by the sampling theorem. According to the sampling theorem, if a modulating signal is band limited to X Hertz (i.e., there are no frequency components beyond X Hz in the frequency spectrum of the modulating signal), the sampling frequency must be atleast 2X Hertz and, hence, the frequency of the carrier pulse train must also be atleast 2X Hz. A pulse amplitude modulated signal fm(t) is shown in the figure in (c) part. It can be seen that amplitude of the pulses depends on the value of f(t) during the time of the pulse. The PAM signal fm(t) is known as discrete time signal, as this signal is discrete on time axis and continuous on amplitude axis.
In the above figure, the baseband signal f(t) is shown to have only a positive polarity. In practice, however, we can have a baseband signal with a positive as well as negative polarity. But, in such a case, the modulated pulses will also be of positive as well as negative polarities. As the transmission of such a bipolar pulses is inconvenient; a clamping circuit is used so that we always have a base band signal with only the positive polarity.
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