FREQUENCY SPECTRUM
The spectrum is a graphical display of the coefficients of the Fourier series.
The one-sided spectrum is based on the representation
The amplitude spectrum displays Dn as the function of the frequency.
The phase spectrum displays the angle      as function of the frequency.
The frequency axis is usually drawn in units of fundamental frequency
The two-sided spectrum is based on the exponential representation
In the two-sided case, the amplitude spectrum plots |cn| while the phase spectrum
plots         versus frequency (in units of fundamental frequency)
Both spectra display equivalent information