I am analyzing a signal with FFT and I found an effect, that I do not understand. I am expecting to have an amplitude modulated signal. As far as I know, if you compute the power of the FFT of an amplitude-modulated signal, you observe sidebands to the left and right of the carrier frequency.
E.g., if you have a sinusoidal carrier at frequency $w_c$, and you modulate the carrier's amplitude with frequency $w_m$, you see sidebands appearing at $w_c \pm w_m$.
However, in my signal's power spectrum, I see the following. Note, that the x-axis shows not the absolute frequency, but the delta from the carrier.
At the frequency, where I would expect peaks in the spectrum, I see drops. How can that be? What does that tell me about the signal? If I look at the signal in the time domain. I can see, that amplitude modulation is actually happening!