Advantages of FM over AM
(i) FM receivers may be fitted with amplitude limiters to remove the amplitude variations caused by noise. This makes FM reception more immune to noise than AM reception.
(ii) It is possible to reduce noise still further by increasing the frequency-deviation. This is a feature which AM does not have because it is not possible to exceed 100 per cent modulation without causing severe distortion.
(iii) Standard Frequency Allocations provide a guard band between commercial FM stations. Due to this, there is less adjacent-channel interference than in AM.
(iv) FM broadcasts operate in the upper VHF and UHF frequency ranges at which there happens to be less noise than in the MF and HF ranges occupied by AM broadcasts.
(v) The amplitude of the FM wave is constant. It is thus independent of the modulation depth, whereas in AM, modulation depth governs the transmitted power. This permits the use of low-level modulation in FM transmitter and use of efficient class C amplifiers in all stages following the modulator. Further since all amplifiers handle constant power, the average power handled equals the peak power. In AM transmitter the maximum power is four times the average power. Finally in FM, all the transmitted power is useful where as in AM, most of the power is carrier power which does not contain any information.
Disadvantages of FM over AM
(i) A much wider channel typically 200 kHz is required in FM as against only 10 kHz in AM broadcast. This forms serious limitation of FM.
(ii) FM transmitting and receiving equipment particularly used for modulation and demodulation tend to be more complex and hence more costly.