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.
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