Modulation index is also called as Modulation depth. The modulation index of a modulation scheme describes by how much the modulated variable of the carrier signal varies around its unmodulated level.

how modulation index varies for FM

how modulation index varies for FM

Modulation Index is the ratio of the maximum deviation frequency to the frequency of modulation. In other words it is the ratio of the spread in frequency spectrum to the frequency that was used to modulate the carrier.

For FM,

modulation index is given by the formula

mf= df/f

where,

mf=modulation index for FM

df=difference in carrier frequency

f=frequency of the signal

The modulation index in AM can range from zero to any number. But

if distortion is to be avoided, then the index must not exceed 100%.