I have a signal that looks like this.
I analyse it using fast Fourier transforms to identify the frequency with the largest peak, which is always close to zero. (There are no other clear peaks.) If I use windows of different sizes (by "window" I mean size of the snippet I use – I don't know if this is the correct use of the terminology, however), the result shows that the dominant peak gets closer and closer to zero as the size of the windows increase. That is to say, if I plot 1/frequency against window size, it is linear.
My initial thought is that if I had an infinitely long window, then the peak would be located at zero implying a DC offset, but the signal is centred on zero in an effort to avoid such an offset. Can someone explain why I see this phenomenon and how I can avoid it?
PS I even tried this using sliding windows so that I can get frequencies based on different parts of the data, but the issue persists.