Define Modulation Index,Frequency Deviation and Percent Modulation For FM Wave

Modulation Index of FM Wave

The modulation index of an FM wave is defined as under:

The modulation index (mf) is very important in FM as it decides the bandwidth of the FM wave . The modulation index also decides the number of side bands having significant amplitudes. In AM, the maximum value of the modulation index m is 1. But, for FM, the modulation index can be greater than 1.

Frequency Deviation (Δf) of FM Wave

For the single tone FM i.e., the modulating signal x(t) be a sinusoidal signal of amplitude Em and frequency fm.


The unmodulated carrier is represented by the expression:

In FM, the frequency f of the FM wave varies in accordance with the modulating voltage. Thus,

Where, Δf = kf Eand it is called as frequency deviation.

Frequency deviation (Δf) represents the maximum departure of the instantaneous frequency fi(t) of the FM wave from the carrier frequency fc.

Since Δf = kf Em , the frequency deviation is proportional to the amplitude of modulating voltage (Em) and it is independent of the modulating frequency fm.

Percent Modulation of FM Wave

The percent modulation is defined as the ratio of the actual frequency deviation produced by the modulating signal to the maximum allowable frequency deviation.