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