Comparison Of FM & AM Signals

Both AM and FM system are used in commercial and non-commercial applications. Such as radio broadcasting and television transmission. Each system has its own merits and demerits. In a Particular application, an AM system can be more suitable than an FM system. Thus the two are equally important from the application point of view.

Advantage of FM systems over AM Systems

The advantages of FM over AM systems are:

  • The amplitude of an FM wave remains constant. This provides the system designers an opportunity to remove the noise from the received signal. This is done in FM receivers by employing an amplitude limiter circuit so that the noise above the limiting amplitude is suppressed. Thus, the FM system is considered a noise immune system. This is not possible in AM systems because the baseband signal is carried by the amplitude variations it self and the envelope of the AM signal cannot be altered.
  • Most of the power in an FM signal is carried by the side bands. For higher values of the modulation index, mc, the major portion of the total power is contained is side bands, and the carrier signal contains less power. In contrast, in an AM system, only one third of the total power is carried by the side bands and two thirds of the total power is lost in the form of carrier power.
  • In FM systems, the power of the transmitted signal depends on the amplitude of the unmodulated carrier signal, and hence it is constant. In contrast, in AM systems, the power depends on the modulation index ma. The maximum allowable power in AM systems is 100 percent when ma is unity. Such restriction is not applicable int case of FM systems. This is because the total power in an FM system is independent of the modulation index, mf and frequency deviation fd. Therefore, the power usage is optimum in an FM system.
  • In an AM system, the only method of reducing noise is to increase the transmitted power of the signal. This operation increases the cost of the AM system. In an FM system, you can increase the frequency deviation in the carrier signal to reduce the noise. if the frequency deviation is high, then the corresponding variation in amplitude of the baseband signal can be easily retrieved. if the frequency deviation is small, noise 'can overshadow this variation and the frequency deviation cannot be translated into its corresponding amplitude variation. Thus,  by increasing frequency deviations in the FM signal, the noise effect can he reduced. There is no provision in AM system to reduce the noise effect by any method, other than increasing its transmitted power.
  • In an FM signal, the adjacent FM channels are separated by guard bands. In an FM system there is no signal transmission through the spectrum space or the guard band. Therefore, there is hardly any interference of adjacent FM channels. However, in an AM system, there is no guard band provided between the two adjacent channels. Therefore, there is always interference of AM radio stations unless the received signalis strong enough to suppress the signal of the adjacent channel.

Disadvantages of FM Systems over AM Systems

The disadvantages of FM systems over AM systems are:

  • There are an infinite number of side bands in an FM signal and therefore the theoretical bandwidth of an FM system is infinite. The bandwidth of an FM system is limited by Carson's rule, but is still much higher, especially in WBFM. In AM systems, the bandwidth is only twice the modulation frequency, which is much less than that of WBFN. This makes FM systems costlier than AM systems.
  • The equipment of FM system is more complex   than AM systems because of the complex circuitry of FM systems; this is another reason that FM systems are costlier AM systems.
  • The receiving area of an FM system is smaller than an AM system consequently FM channels are restricted to metropolitan areas while AM radio stations can be received anywhere in the world. An FM system transmits signals through line of sight propagation,   in which the distance between the transmitting and receiving antenna should not be much. in an AM system signals of short wave band stations are transmitted through atmospheric layers that reflect the radio waves over a wider area.