- Increasing the modulation index (
*I*) generally increases the effective bandwidth of the FM spectrum. - Another parameter useful for analyzing the spectrum of simple FM is called the harmonicity ratio,
*H*=*f*_{m}/*f*_{c}. - For
*H*= 1, the carrier and modulator frequencies are equal and the resulting spectrum is harmonic, with the fundamental frequency equal to the carrier frequency ( ). - For
*H*= 1/2, the carrier frequency is double the modulator frequency and the resulting spectrum is harmonic, though with fundamental frequency equal to the modulator frequency. - For
*H*= 2, the spectrum is harmonic with odd integer multiples of the fundamental*f*_{c}. - In general, if
*H*has the form*N*_{1}/*N*_{2}, where*N*_{1}and*N*_{2}are positive integers, the resulting spectrum will be harmonic. - If
*H*is not of this form, the spectrum will be inharmonic.

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