Increasing the modulation index () generally increases the effective bandwidth of the FM spectrum.
Another parameter useful for analyzing the spectrum of simple FM is called the harmonicity ratio, .
For , the carrier and modulator frequencies are equal and the resulting spectrum is harmonic, with the fundamental frequency equal to the carrier frequency (
).
For , the carrier frequency is double the modulator frequency and the resulting spectrum is harmonic, though with fundamental frequency equal to the modulator frequency.
For , the spectrum is harmonic with odd integer multiples of the fundamental .
In general, if has the form , where and are positive integers, the resulting spectrum will be harmonic.
If is not of this form, the spectrum will be inharmonic.