Advantage, Disadvantage and Application of FM

Frequency Modulation Versus Amplitude Modulation

Advantages of FM

Advantage, Disadvantage and Application of FM:- In general, FM is considered to be superior to AM. Although both AM and FM signals can be used to transmit information from one place to another, FM typically offers some significant benefits over AM.

Noise Immunity.

The main benefit of FM over AM is its superior immunity to noise, made possible by the clipper limiter circuits in the receiver, which effectively strip off all the noise variations, leaving a constant-amplitude FM signal. Although clipping does not result in total recovery in all cases, FM can nevertheless tolerate a much higher noise level than AM for a given carrier amplitude. This is also true for phase-shift-induced distortion.

Capture Effect

Another major benefit of FM is that interfering signals on the same frequency are effectively rejected. Because of the amplitude limiters and the demodulating methods used by FM receivers, a phenomenon known as the capture effect takes place when two or more FM signals occur simultaneously on the same frequency. If one signal is more than twice the amplitude of the other, the stronger signal captures the channel, totally eliminating the weaker signal.

With modern receiver circuitry, a difference in signal amplitudes of only 1 dB is usually sufficient to produce the capture effect. In contrast, when two AM signals occupy the same frequency, both signals are generally heard, regardless of their relative signal strengths. When one AM signal is significantly stronger than another, naturally the stronger signal is intelligible; however, the weaker signal is not eliminated and can still be heard in the background. When the signal strengths of given AM signals are nearly the same, they will interfere with each other, making both nearly unintelligible. Although the capture effect prevents the weaker of two FM signals from being heard, when two stations are broadcasting signals of approximately the same amplitude, first one may be captured and then the other.

This can happen, e.g., when a driver moving along a highway is listening to a clear broadcast on a particular frequency. At some point, the driver may suddenly hear the other broadcast, completely losing the first, and then, just as suddenly, hear the original broadcast again. Which one dominates depends on where the car is and on the relative signal strengths of the two signals.

Transmitter Efficiency.

A third advantage of FM over AM involves efficiency. Recall that AM can be produced by both low-level and high-level modulation techniques. The most efficient is high-level modulation in which a class C amplifier is used as the final RF power stage and is modulated by a high-power modulation amplifier. The AM transmitter must produce both very high RF and modulating signal power. In addition, at very high power levels, large-modulation amplifiers are impractical.

Under such conditions, low-level modulation must be used if the AM information is to be preserved without distortion. The AM signal is generated at a lower level and then amplified with linear amplifiers to produce the final RF signal. Linear amplifiers are either class A or class B and are far less efficient than class C amplifiers. FM signals have a constant amplitude, and it is therefore not necessary to use linear amplifiers to increase their power level. In fact, FM signals are always generated at a lower level and then amplified by a series of class C amplifiers to increase their power. The result is greater use of available power because of the high level of efficiency of class C amplifiers. Even more efficient class D, E, or F amplifiers are also used in FM or PM equipment.

Disadvantages of FM

Excessive Spectrum Use.

Perhaps the greatest disadvantage of FM is that it simply uses too much spectrum space. The bandwidth of an FM signal is, in general, considerably wider than that of an AM signal transmitting similar information. Although it is possible to keep the modulation index low to minimize bandwidth, reducing the modulation index also reduces the noise immunity of an FM signal. In commercial two way FM radio systems, the maximum allowed deviation is 5 kHz, with a maximum modulating frequency of 3 kHz. This produces a deviation ratio of 5/3 = 1.67.

Deviation ratios as low as 0.25 are possible, although they result in signals that are much less desirable than wideband FM signals. Both of these deviation ratios are classified as narrowband FM. Since FM occupies so much bandwidth, it is typically used only in those portions of the spectrum where adequate bandwidth is available, i.e., at very high frequencies. In fact, it is rarely used below frequencies of 30 MHz. Most FM communication work is done at the VHF, UHF, and microwave frequencies.

Circuit Complexity.

One major disadvantage of FM in the past involved the complexity of the circuits used for frequency modulation and demodulation in comparison with the simple circuits used for amplitude modulation and demodulation. Today, this disadvantage has almost disappeared because of the use of integrated circuits. Although the ICs used in FM transmission are still complex, they require very little effort to use and their price is just as low as those of comparable AM circuits. Since the trend in electronic communication is toward higher and higher frequencies and because ICs are so cheap and easy to use, FM and PM have become by far the most widely used modulation method in electronic communication today.

FM and AM Applications

Here are some of the major applications for AM and FM

Application Type of Modulation

AM broadcast radio == AM

FM broadcast radio == FM

FM stereo multiplex sound == DSB (AM) and FM

TV sound == FM

TV picture (video) == AM, VSB

TV color signals == Quadrature DSB (AM)

Cellular telephone == FM, FSK, PSK

Cordless telephone == FM, PSK

Fax machine == FM, QAM (AM plus PSK)

Aircraft radio == AM

Marine radio == FM and SSB (AM)

Mobile and handheld radio == FM

Citizens band radio == AM and SSB (AM)

Amateur radio == FM and SSB (AM)

Computer modems == FSK, PSK, QAM (AM plus PSK)

Garage door opener == OOK

TV remote control == OOK


Family Radio service == FM

Bluetooth radio == FSK

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Raheem Kolachi

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