FREQUENCY MODULATION (FM)
The frequency of the carrier signal is modified in proportion to the instantaneous amplitude of the baseband signal in frequency modulation. Here the amplitude and the phase of the carrier signal remain constant. Increase in the amplitude of the baseband signal increases the frequency of the carrier signal and vice versa. This leads to compressions and rarefactions in the frequency spectrum of the modulated wave as shown in Figure 10.2. Louder signal leads to compressions and relatively weaker signals to rarefactions.
When the amplitude of the baseband signal is zero in Figure 10.2(a), the frequency of the modulated signal is the same as the carrier signal. The frequency of the modulated wave increases when the amplitude of the baseband signal increases in the positive direction (A, C). The increase in amplitude in the negative half cycle (B, D reduces the frequency of the modulated wave (Figure 10.2(c)).
When the frequency of the baseband signal is zero (no input signal), there is no change in the frequency of the carrier wave. It is at its normal frequency and is called as centre frequency or resting frequency. Practically this is the allotted frequency of the FM transmitter.
• Large decrease in noise. This leads to an increase in signal-noise ratio.
• The operating range is quite large.
• The transmission efficiency is very high as all the transmitted power is useful.
• FM bandwidth covers the entire frequency range which humans can hear. Due to this, FM radio has better quality compared to AM radio.
• FM requires a much wider channel.
• FM transmitters and receivers are more complex and costly.
• In FM reception, less area is covered compared to AM.