Dear all,

I am dealing with some trouble using butterworth filter that I can not figure out. I am filtering LFPs sampled at 24KHz with a butterworth filter of 5th order. I am applying a bandpass filter with low and high cutoff of 0.5 and 500 hz respectively. The resulting filtered signal is a vector of NaNs. Reducing the order to 2 gives me a vector of values from which I am able to performs time frequency decomposition. I have tried different function in Matlab using both function butter and designefilt. Furthermore by downsampling the data by a factor of 24(i.e. Fs =1000) the signal becomes very low(very close to 0). If instead I apply a low pass filter on the original data(and remove the DC by demeaning) I am able to use a 5th filter order. According to what I read the higher the order of butterworth the stronger is the attenuation of the component around the pass band. Is it possible that a strong noise in lower frequency is influencing the signal I am interest in? By visually inspecting the periodogramm of the signal I can not see any kind of noise, I simply experience the normal decreasing of power as a function of frequency. I would really appreciate any suggestions on how to investigate this issue, unfortunately no one in my lab is familiar with this type of signal.

Best

Marta