Dear Alex,
Good and fairly concise explanations of what FFT is and what mistakes
can be made in it is given in chapter 12 of the “Numerical Recipes in
C”.
You can find it here:
_http://www.library.cornell.edu/nr/bookcpdf.html_
(http://www.library.cornell.edu/nr/bookcpdf.html)
Best regards,
Axel