6.4.7 Narrowband FM And Wideband FM
There are two main forms of FM, corresponding to very small or very large values of the deviation ratio, D: narrowband FM and wideband FM respectively.
For D<<1, narrowband FM is produced. Narrowband FM is similar to AM since, if D<<1 then only two sidefrequencies are present, and the transmission bandwidth, BW, is:
as is to be expected for an AM-type signal. Not surprisingly, narrowband FM with D<<1 has little advantage over AM and is rarely used for transmission, although it is often employed as an intermediate step in the generation of wideband FM.
However, narrowband FM with a modulation index slightly below unity is widely used in communications. More than just two sidefrequencies are therefore present and the transmission retains the advantages of FM over AM, without using a large amount of bandwidth. With a modulation index of approximately 1, and a maximum modulating frequency of 3 kHz for voice, the maximum frequency deviation is approximately 5 kHz.
The appropriate bandwidth depends on the application: a larger deviation yields better noise immunity, but wideband FM systems occupy far more spectrum. In practice, wideband FM is used for entertainment broadcasting (e.g., commercial radio and television audio), whereas narrowband FM is preferred for mobile and professional communication services such as military, police, ambulance, and fire networks, where maximum deviations of 5–10 kHz fit within channel spacings (15–30 kHz) only slightly wider than those of AM systems.
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