PHASE MODULATION (PM)
In phase modulation, the instantaneous amplitude of the baseband signal modifies the phase of the carrier signal keeping the amplitude and frequency constant (Figure 10.3). This modulation is used to generate frequency modulated signals. It is similar to frequency modulation except that the phase of the carrier is varied instead of varying frequency.
The carrier phase changes according to increase or decrease in the amplitude of the baseband signal. When the modulating signal goes positive, the amount of phase lead increases with the amplitude of the modulating signal. Due to this, the carrier signal is compressed or its frequency is increased.
On the other hand, the negative half cycle of the baseband signal produces a phase lag in the carrier signal. This appears to have stretched the frequency of the carrier wave. Hence similar to frequency modulated wave, phase modulated wave also comprises of compressions and rarefactions. When the signal voltage is zero (A, C and E) the carrier frequency is unchanged.
The frequency shift in carrier wave frequency exists in phase modulation as well. The frequency shift depends on (i) amplitude of the modulating signal and (ii) the frequency of the signal.
• If a square wave is used as the baseband signal, then phase reversal takes place in the modulated signal.
• FM and PM waves are completely different for square wave modulating signal.
• FM signal produced from PM signal is very stable.
• The centre frequency called resting frequency is extremely stable.
PM wave is similar to FM wave. PM generally uses a smaller bandwidth than FM. In other words, in PM, more information can be sent in a given bandwidth. Hence, phase modulation provides high transmission speed on a given bandwidth.