code: def edf_greenhall_simple(alpha, d, m, S, F, N):
      
      L = m/F+m*d # length of filter applied to phase samples
      M = 1 + np.floor(S*(N-L) / m)
      J = min(M, (d+1)*S)
      inv_edf = (1.0/(pow(greenhall_sz(0, F, alpha, d), 2)*M))* \
                 greenhall_BasicSum(J, M, S, F, alpha, d)
      return 1.0/inv_edf
docs: Eqn (13) from Greenhall2004