Sunday, May 8, 2011

Phase Modulation

In the case of PM(phase modulation) the information or AF(Audio Frequency) changes the phase of the carrier wave without changing its frequency or amplitude. Phase modulation (PM) is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Main advantages of PM are improved signal to noise ratio and less radiated power.
The carrier signal is represented by
c(t) = A cos(wct)
The modulating signal is represented by
m(t) = B sin(wmt)
Then the final modulated signal is
A cos((wc + m(t))t) = A cos((wc + B sin(wmt))t)
Because of demodulation reasons, the frequency of m(t) is always kept much smaller than that of the carrier signal.
 PM is not very widely used for radio transmissions. This is because it tends to require more complex receiving hardware and there can be ambiguity problems in determining whether, for example, the signal has changed phase by +180° or -180°. PM is used, however, in digital music synthesizers .

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