Saturday, 22 June 2013

Phase Modulation (PM)


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Phase Modulation uses at least two analog signals. The first signal is a carrier, and the other signal modifies the carrier signal to convey information. In Phase Modulation, the shape of the carrier’s signal curve is made to change at given points with respect to time.


  
            Figure shows the process of phase modulation. Both signals are sine waves that have the same fixed frequency and amplitude. They are however offset from each other. The two cross the amplitude reference line at different times and therefore, have different phases.
           
            The difference in phase between the two sine waves is a phase angle. As seen in the above figure, the two signals are offset by one-half cycle or 180 degrees out of phase. The resulting compound phase modulated signal is shown in figure 2.





Advantages


·        Phase modulation provides the signal modulation that allows computers to communicate at higher data rates through telephone system.

Disadvantages


·        Phase modulation requires two signals with a phase difference between them. A reference pattern and a signal pattern are both required.

Uses of PM


·        This techniques is used to convey colour information in colour television broadcasts.
·        Medium speed modems use Phase Modulation techniques to convert digital signals into phase modulated (PM) signals. This process of Phase Shift Keying (PSK) allows a modem to modulate and demodulate a phase modulation signal between phases that represent the digital signals received from or transmitted to a computer or terminal.