The amplitude coefficients 
and 
give us a measure
of how well the signal fits the kth sinusoidal
term.
A plot of versus k or versus k
is called a frequency spectrum . The
(normalized) power
spectrum amplitude is defined by
A plot of 
versus k is called the power spectrum . This graphical representation tells us how much of a
given frequency is in the original signal.
Experimental power spectra can be obtained in at least three ways:
(i) obtain a spectrum analyzer
or signal analyzer, which is a commercial
instrument dedicated to displaying real-time power spectra;
(ii) obtain a signal analyzer card for a microcomputer (this is usually
less expensive than option (i)); or
(iii) digitize your data and write an FFT for your computer
(this option is the cheapest).
Having successfully procured spectra capabilities, we now move on
to describing how to use them.
