return phase power spectral density coefficient g_b
            for noise-type defined by (qd, b, tau0)
            where tau0 is the interval between data points

            Colored noise generated with (qd, b, tau0) parameters will
            show a phase power spectral density of
            S_x(f) = Phase_PSD(f) = g_b * f^b

            Kasdin & Walter eqn (39)

def phase_psd_from_qd(self, tau0=1.0):
        """ return phase power spectral density coefficient g_b
            for noise-type defined by (qd, b, tau0)
            where tau0 is the interval between data points

            Colored noise generated with (qd, b, tau0) parameters will
            show a phase power spectral density of
            S_x(f) = Phase_PSD(f) = g_b * f^b

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