In radio broadcasting, it is necessary to send audio frequency signal (eg. music, speech etc.) from a broadcasting station over great distances to a receiver. The music, speech etc., are converted into audio signals using a microphone. The energy of a wave increases with frequency. So, the audio frequency (20 - 20000 Hz) is not having large amount of energy and cannot be sent over long distances. The radiation of electrical energy is practicable only at high frequencies e.g. above 20 kHz. The high frequency signals can be sent through thousands of kilometres with comparatively small power.
Therefore, if audio signal is to be transmitted properly, the audio signal must be superimposed on high frequency wave called carrier. The resultant waves are known as modulated waves and this process is called as modulation. This high frequency wave (Radio frequency wave) is transmitted in space through antenna. At the receiver end, the audio signal is extracted from the modulated wave by the process called demodulation. The audio signal is then amplified and reproduced into sound by the loud speaker.
A high frequency radio wave is used to carry the audio signal. On adding the audio signal to carrier, any one of the characteristics namely amplitude or frequency or phase of the carrier wave is changed in accordance with the intensity of the audio signal. This process is known as modulation and may be defined as the process of changing amplitude or frequency or phase of the carrier wave in accordance with the intensity of the signal. Some of the modulation process namely,
(i) amplitude modulation,
(ii) frequency modulation and
(iii) phase modulation
Frequency modulation (FM)
When the frequency of carrier wave is changed in accordance with the intensity of the signal, the process is called frequency modulation.
In frequency modula-tion, 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 Fig a. 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 Fig c. But during the negative peak of signal as at D, the carrier frequency is reduced to minimum as shown by widely spaced cycles in Fig. c. 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.
(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.
(i) A much wider channel is required by FM.
(ii)FM transmitting and receiving equipments tends to be more complex.