code: def flat_unity(length, delta_f, low_freq_cutoff):
      
      fseries = FrequencySeries(numpy.ones(length), delta_f=delta_f)
      kmin = int(low_freq_cutoff / fseries.delta_f)
      fseries.data[:kmin] = 0
      return fseries
docs: Returns a FrequencySeries of ones above the low_frequency_cutoff.
  
      Parameters
      ----------
      length : int
          Length of output Frequencyseries.
      delta_f : float
          Frequency step for output FrequencySeries.
      low_freq_cutoff : int
          Low-frequency cutoff for output FrequencySeries.
  
      Returns
      -------
      FrequencySeries
          Returns a FrequencySeries containing the unity PSD model.