I have an almost-sinusoidal signal (see attached file), which I believe is composed of a mixture of maybe 2 (or 3) Fourier modes:
I wanted to verify this by plotting its Fourier transform. I expected a flat signal with a bunch of peaks at the signal harmonics. However, using fft() on the signal gives me a 2d plot that makes absolutely no sense to me. Can someone please help me to understand what is going on?