This function takes the n time domain samples (real or complex)

and finds the psd by taking (fft/n)^2. The two sided spectrum is

produced by shifting the psd The array freq provides the appropriate

frequency values for plotting purposes.

By taking 10*log10(psd/max(psd)) the psd is normalized values beow 60db

are set equal to -60db-　This function takes the n time domain samples (real or complex)

　and finds the psd by taking (fft/n)^2. The two sided spectrum is

　produced by shifting the psd　The array freq provides the appropriate

　frequency values for plotting purposes.

　By taking 10*log10(psd/max(psd)) the psd is　normalized　values beow 60db

　are set equal to -60db