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Reference Monitoring: Phase

The quality of input references can be assessed by comparing their phase offsets against the current system reference, and against each other. This is called Reference Monitoring.

Reference Monitoring helps to understand and predict system behavior and as such is useful for interference mitigation. It can also be used to manually re-organize reference priorities e.g., by assigning a lower reference priority to a noisy reference or a reference with a significant phase offset, or to automatically failover to a different reference if certain quality thresholds are no longer met (see Smart Reference Monitoring).

NetClock allows Reference Monitoring by comparing the phase data of references against the System Ontime Point. The phase values shown are the filtered phase differences between each input reference 1PPS, and the internal disciplined 1PPS.

The data is plotted in a graph in real-time. The plot also allows you to display historic data, zoom in on any data range or on a specific reference. A data set can be exported, or deleted.

To monitor the quality of references:

  • Navigate to TOOLS > SYSTEM: Reference Monitor. The Reference Monitor screen will display:
  • On the left side of the screen, Status information is displayed for the System and the References. Note that the Reference Status panel also displays the latest PHASE OFFSET reading (1) for active references against the System Ontime Point. The reading is updated every 30 seconds.

    This Reference Phase Offset Data is plotted over time (abscissa) in the Reference Monitor panel in the center of the screen. Use the check boxes in the References panel (2) to select the reference(s) for which you want to plot the phase offset data. Use the handles (3) to zoom in on a time window.

    The scale of the axis of ordinate (4) is determined by the largest amplitude of any of the references displayed in the current time window. Use the checkboxes in the References panel on the right to remove references from the graph, or add them to it.

Smart Reference Monitoring

Spectracom's Smart Reference Monitoring uses phase error validation in combination with automatic failover:

The phase error validation calculates long-term averages and standard deviations of the phase offset between the monitored external reference and the internal system reference. The standard deviation is used to calculate two validity thresholds, a higher and a lower one (the latter acts as a hysteresis buffer, preventing the status flip/flopping if the actual phase error validation value varies closely around the outer threshold). The thresholds are not user-configurable.

If the higher threshold value is exceeded, the automatic failover will cause NetClock to fall back to its next lower reference (if available).

If no other reference is found, the unit will transition into a 1200-second coasting period. During this coasting period, the TIME and 1PPS references will continue to be considered valid, but NetClock's oscillator will flywheel. Note that the PPS reference status light will turn yellow. After expiration of the 1200 seconds the unit will transition into Holdover.

Should, however, the above-mentioned higher threshold value no longer be exceeded, the unit will remain in the 1200-second flywheel mode until either (a) the lower threshold value is no longer exceeded, or (b) the 1200-second flywheel period expires. In both cases the PPS status light will turn green again.

Smart reference monitoring is OFF by default. To turn it ON:

  1. Navigate to MANAGEMENT > OTHER: Disciplining.
  2. In the Status panel on the left, click the GEAR icon. The Oscillator Settings window will open.
  3. Check the box next to 1PPS Phase Validation and click Submit.