We already discussed modulation and in particular- the amplitude modulation. but when it comes to practical applications it is frequency modulation, which is preferred. However, the theory and generation of FM is not as simple as AM. FM is a complex process because it involves minute frequency variations of the carrier whereas in AM we produce large-scale amplitude variations. There are some more differences between the two which we shall discuss after going in details of FM.

In frequency modulation, only the instantaneous frequency of carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal, however the amplitude of the carrier remains constant. The amount of deviation in frequency is proportional to the amplitude of the signal while the rate is determined by the frequency of modulation source. The frequency of modulated carrier increases as the signal amplitude increases but decreases as the signal amplitude decreases.

When the signal amplitude is zero, the carrier frequency is at its normal frequency. for example, when the signal approaches its positive peaks at band H, the F-M carrier frequency increases to maximum (High) as shown by closely spaced lines. On the other hand, during the negative peak, of signal as at L Band, the F-M carrier frequency reduces to minimum (Low) as shown by widely spaced lines.

(1) ADVANTAGES OF FREQUENCY MODULATION

(a) It gives high-fidelity reception.

(b) It gives gives noiseless reception. As discussed before, noise is a form of amplitude variations and a FM receiver will reject such signals.

(c) The efficiency of transmission is very high.

(d) The operating range is quite large.

                             LIMITATION OF FREQUENCY MODULATION

(a) Signal reception is very small.

(b) Implementation of FM set up cost is very high.

(c) It requires much wider channel.