Currently trying to plot the magnitude spectrum of a single tone FM modulated signal in Matlab. I made a small error where the modulating frequency (fm) was of the same order of ten as the carrier frequency (fc). For example, both were in the MHz range instead of having fm in the kHz. The magnitude spectrum I plotted when the mistake was present was great with multiple sidebands etc and what I expected. I picked up my error, corrected it and the plot changed to only showing one spike at fc. Why is this? It only seems to plot well when fc and fm are of the same order of ten.
This might be more of a Matlab question but I would love to know any theory behind it. :) Thanks