Hi Oskar,
Changing the window length and FFT length in the pwelch function can impact the frequency resolution and spectral leakage of the resulting power spectral density (PSD) estimate. It is often a trade-off between these two factors, where increasing the window length improves the frequency resolution but also increases spectral leakage, and vice versa.
To minimize spectral leakage, it is important to choose an appropriate window function that can reduce the sidelobe levels of the FFT. Common window functions include the Hamming, Blackman, and Kaiser windows, among others. Choosing an appropriate window function and applying it properly can help you achieve a good balance between frequency resolution and spectral leakage.
Regarding the presence of a peak at 0 Hz in the spectrum, it is possible that this peak is due to the baseline, DC offset, or bias in the signal caused by some extraneous factors such as instrument noise, bias voltage, or other disturbances. This peak can also be related to the displacement or other non-linear properties of the system being monitored, especially if it is a mechanical or vibrational system.
To resolve this issue, you may want to consider subtracting the baseline or DC offset from the signal before performing the PSD estimation, or use high-pass filtering or other techniques to remove the low frequency components. You may also want to investigate the properties and behavior of the system being monitored, including its input-output relationship, the effects of boundary conditions, and other factors that may contribute to the observed low-frequency peak in the spectrum.
with regards,
Gokul Nath S J
