Q.Draw the frequency spectrum of an FM wave.

Unlike AM, a single-tone FM wave contains the carrier plus theoretically an infinite number of sideband pairs spaced at multiples of the modulating frequency, so its spectrum is a comb of lines whose heights follow Bessel functions. …

[!TLDR]

An FM spectrum is the carrier plus many sideband pairs at $f_c \pm n f_m$, with amplitudes given by Bessel functions $J_n(m_f)$.

When a carrier is frequency-modulated by a single tone of frequency $f_m$, the modulated wave can be expressed as a sum of many frequency components:

• the carrier at $f_c$, and
• an infinite series of sideband pairs at $f_c \pm f_m, f_c \pm 2 f_m, f_c \pm 3 f_m, \dots$

The amplitude of the carrier and of each sideband pair depends on the modulation index $m_f$ through the Bessel functions of the first kind: the carrier amplitude is proportional to $J_0(m_f)$, the first sideband pair to $J_1(m_f)$, the second to $J_2(m_f)$, and so on. Because these values fall off, only a finite number of sidebands carry significant power; the rest are negligible. …