'not really a method
just for comparison, not efficient for large n or long acf
this is also similarly use in tsa.stattools.periodogram with window'

def acf2spdfreq(self, acovf, nfreq=100, w=None):

    if (w is None):
        w = np.linspace(0, np.pi, nfreq)[:, None]
    nac = len(acovf)
    hw = ((0.5 / np.pi) * (acovf[0] + (2 * (acovf[1:] * np.cos((w * np.arange(1, nac)))).sum(1))))
    return hw
