code: def phase_psd_from_qd(self, tau0=1.0):
          
          return self.qd*2.0*pow(2.0*np.pi, self.b)*pow(tau0, self.b+1.0)
docs: 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)