FM Sidebands

The upper and lower sidebands produced by FM are given by $f_c \pm kf_m$.

Figure 10: Sidebands produced by FM synthesis.

The modulation index may be see as

$$I = \frac{d}{f_m},$$

where $d$ is the amount of frequency deviation from $f_c$.

When $d=0$, the index $I$ is also zero, and no modulation occurs. Increasing $d$ causes the sidebands to acquire more power at the expense of the power in the carrier frequency.