'raw spectral density, returns Fourier transform
n is number of points in positive spectrum, the actual number of points
is twice as large. different from other spd methods with fft'

def spd(self, npos):

    n = npos
    w = ((fft.fftfreq((2 * n)) * 2) * np.pi)
    hw = self.fftarma((2 * n))
    return ((((hw * hw.conj()).real * 0.5) / np.pi), w)
