6.4.2 Modulation Index And Deviation Ratio
The modulation index (mf) of an FM wave is the ratio of the frequency deviation of the carrier to the modulating frequency:
The modulation index determines the amplitudes of the various frequency components (sidebands) in the FM spectrum, as discussed earlier. It is also commonly denoted by β and is dimensionless, though it represents an angular deviation in radians. Because the modulation index depends on the frequency of the modulating signal, its value changes as the modulation frequency varies. It is therefore not a fixed design parameter for an FM system.
We therefore define the deviation ratio as the value of the modulation index when both the frequency deviation and the modulating frequency are at their maximum values:
The deviation ratio is specified during system design and remains constant for a given FM system. Typical values depend on application—narrowband FM systems use D<1, while wideband systems (such as broadcast or satellite FM) may have D≫1.
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