I can’t answer that - I too see differences in the output, I am not sure which one is correct

The difference in the graphs might have to do with the bin size. The amplitude displayed is basically a count of how many times that frequency occurred, so if your bins are larger, you get larger amplitudes, even if the data is the same. The spiky-looking graph from pymavlink suggests it’s using smaller bins. You can try increasing the bin count in the MP FFT grapher to see if you can get a graph that matches. I don’t know if the pymavlink one lets you change them.

Hi Rick,

Thank you for your reply. I noticed that in pymavlink the function is scaling the amplitude. Moreover on the wiki page in ardupilot speaking about IMU batch sampler it is written that FFT should be used only to locate frequency bands. Amplitude values are not scaled to a useful value. So I suppose that the pymavlink amplitude values are more appropriate?

The actual value of the amplitude is essentially meaningless, so just ignore it. The important part is the general shape of the graphs, which is why I mentioned the data binning, as that can affect not just the amplitude but also the shape (small bins makes it spiky).

But they’re both valid and both show the same thing - that you’ve got a vertical resonance and ~110 Hz and a harmonic at 220.