Spectracom PRISMA VelaSync User Manual page 152

3.3 Managing References
For more information on the TFOM value and Phase Error Limit, see "Configuring the
Oscillator" on page 143.
What is the recommended setting for the Holdover Timeout period?
The factory default Holdover period is 2 hours (7200 seconds) . The value can be increased to
up to 5 years. During this time period, VelaSync will be useable by its NTP clients (or other con-
sumers) after GNSS reception has been lost.
The length of time is really based on the type of oscillator installed in a unit, and what the typ-
ical accuracy requirements are for the NTP clients. The longer it can run in Holdover mode
before it expires, the longer it can continue being a central time source for all of its clients. But
the longer VelaSync runs in Holdover, the larger the offset to true UTC time will become,
because the undisciplined oscillator will drift over time:
The better the type of oscillator installed, the more stable it is while in Holdover and therefore,
the less its time will drift away from true UTC time. This results in more accurate timing, over
extended durations upon the loss of GPS input. For instance, a Rubidium oscillator will main-
tain significantly better time over a longer Holdover duration than a TCXO oscillator (TCXOs
are considerably less stable than a Rb oscillator).
Oscillator Phase Drift
The chart below provides typical stability performance for the oscillator types that can be found
in VelaSync units. These numbers are based on the oscillator being locked to a reference for
two weeks, but then loses GPS reception for an extended period of time, while the ambient tem-
perature remains stable.
This data can help you determine how long of a Holdover period can be tolerated, based on
how much time drift may occur after GPS input is lost. The larger the time error that can be tol-
erated by VelaSync clients, based on the oscillator installed, the larger the Holdover timeout
period can be set to.
Table 3-3:
1PPS Phase Drift in Holdover
- 4 hours
- 24 hours
- 7 days
140
Note: Changes made to the Holdover Timeout always take effect immediately. If
VelaSync is in Holdover and the Holdover timeout is changed to a value that is
less than the current time period that VelaSync has been in Holdover Mode, the
unit will immediately declare loss of synchronization.
Estimated Phase Drifts
(no reference available)
OCXO
OCXO
(high performance)
3 µs 2.8 µs
40 µs 30 µs
1.2 ms 0.6 ms
CHAPTER •
3
CSAC
1µs
7 µs
100 µs
VelaSync User's Manual Rev. 1