Page 99 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes FM signal is 2fm. On the other hand, when b is large, the bandwidth is determined (empirically)
as 2 fm (1 + b).

6.5.3 Phase Modulation

Phase Modulation (PM) is similar to frequency modulation. Instead of the frequency of the
carrier wave changing, the phase of the carrier wave changes. In PM the phase of the carrier is
made proportional to the instantaneous amplitude of the modulating signal. Modulating index
for PM is given as b = Dj, where Dj is the peak phase deviation in radians. As in the case of
angular modulation argument of sinusoidal is varied and therefore we will have the same
resultant signal properties for frequency and phase modulation. A distinction in this case can be
made only by direct comparison of the signal with the modulating signal wave, as shown in
Figure 6.16.

Figure 6.16: Phase Modulation

t

!
Caution Phase modulation and frequency modulation are interchangeable by selecting
the frequency response of the modulator so that its output voltage will be proportional to
integration of the modulating signal and differentiation of the modulating signal
respectively. Bandwidth and power issues are same as that of the frequency modulation.

A comparison of FM and AM

The main advantages of FM over AM are:

1. Improved signal to noise ratio (about 25dB) w.r.t. to man made interference.
2. Smaller geographical interference between neighboring stations.
3. Less radiated power.
4. Well defined service areas for given transmitter power.

The disadvantages of FM are:
1. Much more Bandwidth (as much as 20 times as much).
2. More complicated receiver and transmitter.

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