Managing The Oscillator - Spectracom VersaSync User Manual

3.4 Managing the Oscillator
manually configured Leap Second is asserted by a user (leap seconds do not affect the GPS
and TAI time scales).
If no external references (such as GNSS or IRIG) are available when a Leap Second is sched-
uled to occur, manual Leap Seconds can also be applied to the UTC or Local time base; see
"Leap Seconds" on page 123.
If the Holdover Timeout has expired, do I need to reset the clock once GPS
becomes available again?
No, the Holdover timer is automatically reset as soon as at least one reference has been
restored/returned for at least one second. If GPS is restored and then lost again moments later,
the Holdover timer starts again with its full value. If its set to one week in this case, it then gets
another week of Holdover operation before NTP goes to Stratum 16 (if GPS remained unavail-
able for the entire week).
Holdover mode and the User/User reference
If the only available input reference is a manually set User time, and VersaSync is subsequently
rebooted or power cycled, time sync will be lost when VersaSync powers back-up. The time will
need to be set manually again in order for VersaSync to return to its fully synchronized state.
See "The "User/User" Reference" on page 134 and "Manually Setting the Time" on page 118
for more information.
3.4

Managing the Oscillator

The purpose of the built-in oscillator is to provide VersaSync with an accurate and very stable
internal frequency source. This allows VersaSync to go into a holdover mode in the event that
external time or frequency references are lost or become invalid. However, the oscillator can
also be used as a legitimate 1PPS reference during normal operation, in conjunction with an
external time reference (for more information, see "Configuring Input Reference Priorities" on
page 131.)
VersaSync's internal oscillator is normally disciplined to an input reference (such as GNSS,
IRIG input, 1PPS input, etc.) in order to provide the highest degree of oscillator accuracy and
to account for oscillator drift. While disciplining (with a 1PPS input reference input present and
valid), the oscillator's output frequency is monitored and based on the measured frequency, the
oscillator is steered to maintain a very accurate 10 MHz output. If no valid 1PPS input ref-
erences are present (or input references are present but not considered valid), the oscillator will
be in Freerun mode instead.
If no external input reference such as GNSS, IRIG, etc. is available (or is temporarily lost), Ver-
saSync may become an NTP Stratum 2 or higher reference. If so configured, VersaSync can
use a reference such as an NTP daemon, referred to as a Host Reference . If the Host Reference
becomes active, it will automatically take over the disciplining of the oscillator. This built-in func-
tionality is referred to as Host Disciplining .
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