Frequency Modulation : Explanation with Advantages and Disadvantages
When the frequency of carrier wave is changed in accordance with the intensity of the signal, the process is called frequency modulation. In frequency modulation, the amplitude and phase of the carrier wave remains constant. Only, the frequency of the carrier wave is changed in accordance with the signal. The frequency variation of the carrier wave depends upon the instantaneous amplitude of the signal as shown in animation. When the signal voltage is zero at A,C,E and G, the carrier frequency is unchanged. When the signal approaches its positive peaks at B and F, the carrier frequency is increased to maximum as shown by closely spaced cycles in animation. But during the negative peak of signal as at D, the carrier frequency is reduced to minimum as shown by widely spaced cycles in animation.
The louder signal causes greater frequency change in modulated carrier as indicated by increased bunching and spreading of the waves as compared with relatively weaker signal. The frequency of an FM transmitter without signal input is called the resting frequency or centre frequency (fo ) and this is the allotted frequency of the transmitter. When the signal is applied, the carrier frequency deviates up and down from
its resting value fo. The change or shift either above or below the resting frequency is called frequency deviation (?f). The total variation in frequency from the lowest to the highest is called carrier swing (CS).
Hence, Carrier swing = 2 × frequency deviation
= 2 × ?f
For the purposes of FM broadcasts, it has been internationally agreed to restrict maximum deviation to 75 kHz on each side of the centre frequency for sounds of maximum loudness.
Advantages of Frequency Modulation
(i) It gives noiseless reception. Noise is a form of amplitude
variation and a FM receiver will reject such noise signals.
(ii) The operating range is quite large.
(iii) The efficiency of transmission is very high.
Disadvantages of Frequency Modulation
(i) A much wider channel is required by FM.
(ii) FM transmitting and receiving equipments tends to be more complex.
