DOI link for Frequency Modulation
Frequency Modulation book
The modulation index is a function of the frequency deviation of the system and the applied modulating signal:
Where: MI = the modulation index Fd = frequency deviation Mf = modulating frequency
The higher the MI, the more sidebands produced. It follows that the higher the modulating frequency for a given deviation, the fewer number of sidebands produced, but the greater their spacing.