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When the holdover is detected and enabled it can go through the following
states:
CHAPTER
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5
WR-Z16 User Manual Rev. v5.0
1.
Locking:
During a minimum amount of time the holdover needs to perform
a rough and quick learning on a stable clock reference before using it.
2.
Learning: In order to maintains the best accuracy during enough time the
holdover is learning about its environment using adaptative algorithms.
This learning period has been set to 3 days in order to ensure to fulfill the
accuracy specifications.
If the holdover is triggered before this learning time, it will directly
enter the expired state.
Ready:
3.
Once the HO has learned enough time to ensure good performance,
the HO will be ready to be triggered at any moment (it will continue learn-
ing to slightly improve its performance).
Activated:
4.
The holdover has been triggered (by trigger_origin) and it is
actually being the active timing source of the device. The clock info will be
modified accordingly and announced to the timing network.
Expired:
5.
Reaching the holdover expired state means that the device
announce itself with a Free-Running clock_class and a clock_accuracy to
UNKNOWN. This also means that the corresponding VSC code is CRITCAL
and thus if a better timing source is detected it will switch to this one.
Worth mentioning that during the expired state, the holdover timing source
is using the OCXO oscillator that provides better performance than the
internal onboard oscillator.
Note:
Holdover and FOCA: As mentioned above FOCA only
switches between timing source when a failure is detected. This
mean that if the active timing source of a device is HO, it will stick to
it until reaching the expired states (Failure state of HO).
Note:
GNSS reference to discipline the HO: The holdover adaptative
algorithms have been optimized to learn from a GNSS reference
clock (GPS L1 signals). A better clock can be used as reference (e.g.,
Atomic Clock, ePRTC, multi bands/constellations GNSS receiver) but
using a clock reference with worth performance might not fulfil the
provided specifications.
5.7 Holdover
91
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