I need to compute FFT components and autocorrelation of a discrete signal. When I do not use any window function before taking FFT of the signal I get a reasonable AC function from FFT that includes some information about period of the signal:
However when I apply a window function (Hanning) before applying FFT I got an AC function that seems to be distorted:
What is the effect of window function on an AC function that is computed from FFT?