Modulation Index

The modulation index is the ratio of the frequency deviation of the modulated signal to the message signal bandwidth. For FSK modulation, the modulation index h is defined as the ratio of the spacing between consecutive frequencies in the FSK symbol map, to the symbol rate. More precisely,

h=2×f_dT/(M-1)

where

M is the modulation format
T is the symbol period
f_d is the peak frequency deviation

For example, in 4-FSK, the spacing between consecutive frequencies equals 2f_d / 3, hence the modulation index equals (2f_d×T) / 3.

For more information about amplitude modulation, refer to modulation depth.