code: def find_variable_start_frequency(approximant, parameters, f_start, max_length,
                                    delta_f = 1):
      
      l = max_length + 1
      f = f_start - delta_f
      while l > max_length:
          f += delta_f
          l = pycbc.waveform.get_waveform_filter_length_in_time(approximant,
                                                        parameters, f_lower=f)
      return f
docs: Find a frequency value above the starting frequency that results in a
      waveform shorter than max_length.