Not too familiar with communication theory myself, so i thought i would post this here.

when we talk about frequency modulation, we usually have an equation in the form of

y = sin(2*pi*(f+f_mod))

This is fine, but i am wondering if FM applies the same way to the harmonics of sinusoidal signals. For example, a square wave is composed of an infinite number of harmonics plus the fundamentals frequency. If i were to apply FM to a square wave, would the higher order harmonics be affected by the same way as the fundamental frequency, or would something else happen? i.e. would the frequency just change by f_mod?

tia