docs: def feedback_results_to_measurements_frame(feedback_result):
      '''
      Extract measured data from `FeedbackResults` instance into
      `pandas.DataFrame`.
      '''
      index = pd.Index(feedback_result.time * 1e-3, name='seconds')
      df_feedback = pd.DataFrame(np.column_stack([feedback_result.V_fb,
                                                  feedback_result.V_hv,
                                                  feedback_result.fb_resistor,
                                                  feedback_result.hv_resistor]),
                                 columns=['V_fb', 'V_hv', 'fb_resistor',
                                          'hv_resistor'],
                                 index=index)
      df_feedback.insert(0, 'frequency', feedback_result.frequency)
      return df_feedback