I am sampling a sinusoidal signal with an oscilloscope and viewing its spectrum using the FFT mode. The signal in time domain is this:
and I am getting this spectrum:
The peak above the red line is clearly the 30 kHz sine signal but, what are all those peaks below the red line? They are clearly not harmonics of my signal... There are peaks even at frequencies below the fundamental... Might this be caused by aliasing? Or what is this?
If the input is a square wave like this one:
I get this spectrum:
Following the guidelines in the answer of Ali Chen I have verified that all those "strange peaks" are due to aliasing, just sweeping the frequency and watching how the spectrum moved. I had prepared a gif but cannot find how to embed it here. So I left a link to the gif.