All frequencies in between are equally spaced and don't overlap. SPSS has a fast Fourier transfrom built in for these analyses. Spectrum analysis significance tests use upper and lower bounds of a confidence interval that are derived using a c 2 distribution. The degrees of freedom will depend on what kind of smoothing was used. This confidence interval can be superimposed on the Power Spectrum so that significant values may be seen.
References There are many references out there for time series analysis.
Most refer to applications involving econometrics or social sciences, but most techniques can be applied to the biological sciences. Most of the web pages involve vaery advanced theories and techniques. Books held by the SFSU Library Box, G.E.P., G.M. Jenkins, and G.C. Reinsel. USA A great introductory section, although the rest of the book is very involved and mathematically in-depth. Chatfield, C. 1996.