Lecture 8: PM Modulation 1st semester 1439 - 2017 By: Elham Sunbu

OUTLINES: PM Modulation Angle Modulation

PM Modulation - In phase modulation (PM), the carrier signal changes its phase with the changes in the message signal amplitude. - FM and PM are types of what is called Angle modulation.

Phase Shift - A begins its cycle and reaches its peak before wave B, it is said to lead wave B. Wave  C, on the other hand, lags wave B by 30 degrees.

PM Modulation - FM and PM are interrelated; one cannot change without the other changing. - In FM, the frequency of a carrier signal deviates from its center frequency by an amount that is proportional to the message signal amplitude. - In PM, the frequency (and phase) of a carrier signal changes by an amount that is proportional to the message signal amplitude and frequency.

PM Modulation - FM requires the carrier’s frequency to deviate both above and below its center frequency. - During the process of frequency modulation, the peaks of each successive cycle in the FM modulated waveform occur at times other than they would if the carrier were un modulated. - This is actually an incidental phase shift that takes place along with the frequency shift in FM.

PM Modulation - Just the opposite action takes place in phase modulation. - Notice that the time period of each successive cycle varies in the modulated wave according to the audio-wave variation. - Since frequency is a function of time period per cycle, we can see that such a phase shift in the carrier will cause its frequency to change.

PM Modulation - In Frequency modulation, the frequency of a carrier signal deviates from its center frequency by an amount that is proportional to the message signal amplitude. - FM uses the message signal, m(t), to vary the carrier frequency within some small range about its original value.

PM and FM - PM is similar to FM. - When the instantaneous phase of a carrier is varied, the instantaneous frequency changes as well. - When the instantaneous frequency is varied, the instantaneous phase changes.

PM and FM - But PM and FM are not exactly equivalent, especially in analog applications. - When an FM receiver is used to demodulate a PM signal, or when an FM signal is intercepted by a receiver designed for PM, the audio is distorted. - This is because the relationship between phase and frequency variations is not linear ; that is, phase and frequency do not vary in direct proportion.

PM and FM - The basic difference between FM & PM lies in which property of the carrier is directly varied by modulating signal. - In FM, the frequency of carrier is varied directly. - In PM, phase of the carrier is varied directly. - The frequency change in FM is vital, but in PM it is merely incidental. - In PM, only the phase shift is important.

PM and FM - Unlike FM, PM is not widely used for transmitting radio waves. - This is because PM requires more complex receiving hardware.

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