6.4.13 Advantages And Disadvantages Of FM Over AM

FM has a number of advantages over AM:

An FM system has a much larger allowable range of modulating signal amplitudes because there is less restriction on the maximum value of frequency deviation than in AM where the modulation factor is limited to m=1.
The FM transmitter is more efficient than the AM transmitter since the amplitude of an FM wave is constant, as opposed to an AM transmitter where each of the stages must be able to cope with a range of power levels as the carrier and/or sidebands vary between zero and full power.
FM systems are much less susceptible to noise than AM systems since most noise will be additive and amplitude variations in the received signal will be ignored by the receiver.
The capture effect allows an FM receiver to suppress the weaker of two co-channel signals simultaneously present at its front end—conversely, an AM receiver will pass to the listener all of (the sum of) the signals present at its front end.

FM has a number of disadvantages compared to the various forms of AM:

The main disadvantage of FM is the much wider bandwidth required to achieve the signal-to-noise ratio improvement.
Since FM requires a wider bandwidth than AM, higher frequencies must be used at which communications range is normally limited to line-of-sight.
FM modulation and demodulation equipment tends to be more complex and therefore more expensive than for AM.
The FM capture effect may also be a disadvantage when a receiver is near the edge of the service area and may be captured by an unwanted signal or by noise.

Although FM and PM are often treated as distinct modulation schemes, they are closely related: frequency is the time derivative of phase. Consequently, many practical modulators and demodulators exploit this relationship, and it is common to generate FM indirectly via phase modulation (or to interpret FM in terms of phase). We now turn to phase modulation, first to complete the picture for analog angle modulation, and then because phase modulation provides the conceptual foundation for the digital modulation schemes introduced later in this chapter.