# FFT looking messy after filtering process

I have a vibration signal (25.6 kHz) which has been through the following filters:

1: highpass (2000 Hz) -> (I'm not interesten in the harmonics of the system. I then perform spectral kurtosis to find the optimal bandpass bandwidth.) 2: Bandpass (8750 Hz,9250 Hz) -> 3: Rectification (abs(xi)^2) -> 4.Lowpass (2000 Hz)

The fft of the final signal looks like this:

The Bandpass filter bandwidth and center frequency was selected based on the kurtogram of signal after initial lowpass.

I am no expert in signal processing- does the FFT look "correct"? It seems odd that the amplitudes rarely go below 0.05...

The original signal (25.6 kHz):

Zoomed in in original signal (25.6 kHz):

Final signal in time domain:

FFT of original signal (before step 1)

Objective of project:

• To look for any fault development in the early stages of a gearbox fault on a wind turbine. I have signal data 412 days, in the form of 412 10-second intervals.
• Want to extract features from time and freq. domain and cluster the 412 intervals, to see if there is a pattern.
• Any chance you can attach the signal plot as well? – DSP Novice Mar 25 at 10:55
• Hm, why would you first in 1: cut off anything above 2000 Hz, then in 2: cut off anything below 8750 Hz? That very much sounds like a design mistake. If your low-pass filters are good, you don't have any signal after that. – Marcus Müller Mar 25 at 10:58
• The 25.6Hz in your question is a typo, correct? It should be 25.6kHz based on your previous questions.. – jithin Mar 25 at 11:44
• @DSPNovice added some more plots. Let me know if you would like to see more. – user10971344 Mar 25 at 14:38
• @MarcusMüller I updated the process. The optimal bandwidth is around 9000 Hz from the spectral kurtosis. – user10971344 Mar 25 at 14:40