    fmax       Last frequency value
            Nfreqs     Number of spectrum values to calculate between fmin and fmax
            Lsmin      Minimum smoothing window length (for first freq.)

 The PSD is computed by the smoothed periodogram method.
 Note that the window used by this method is quite different from the time
 window (e.g. Hanning window) used by Welch's method for PSD computation.
 The window should normally be 'boxcar', which means it does not need to
 be specified.
 A logarithmic = 'log' p