return frequency power spectral density coefficient h_a
            for the noise type defined by (qd, b, tau0)

            Colored noise generated with (qd, b, tau0) parameters will
            show a frequency power spectral density of

            S_y(f) = Frequency_PSD(f) = h_a * f^a
            where the slope a comes from the phase PSD slope b:
            a = b + 2

            Kasdin & Walter eqn (39)

def frequency_psd_from_qd(self, tau0=1.0):
        """ return frequency power spectral density coefficient h_a
            for the noise type defined by (qd, b, tau0)

            Colored noise generated with (qd, b, tau0) parameters will
            show a frequency power spectral density of

            S_y(f) = Frequency_PSD(f) = h_a * f^a
            where the slope a comes from the phase PSD slope b:
            a = b + 2

            Kasdin & Walter eqn (39)
        """
        a = self.b + 2.0
        return self.qd*2.0*pow(2.0*np.pi, a)*pow(tau0, a-1.0)