How to interpret these FFT results?
Note the symmetry in your 3rd plot. That tells me the MathNet.Numerics FFT is computing a full-frequency FFT (both positive and negative frequencies), and plotting the FFT results where zero Hz should be the center of the horizontal axis. However, Problem# 1, for some reason the labeling of the horizontal axis does NOT have zero Hz in the center. I can’t explain that. Problem# 2, there’s something very wrong with the ‘flat-top’ spectral shape in your 3rd plot. Problem# 3, it seems impossible to me that your 3rd plot should have spectral components whose magnitudes are 10 raised to the 15th power. (Looking at your 1st plot I’d expect your spectral magnitude results to be on the order of 10 raised to the 7th power.) Stated in technical terms, your 3rd plot is “truly FUBAR.”

The answer, heltonbiker, is for you to determine precisely, in great detail, EXACTLY what processing is taking place in your MathNet.Numerics FFT and plotting routines. Good Luck.