Digital To Analog Converter - Meridian Nortel 1 Option 11C Technical Reference Manual

NTAK20 clock controller Page 509 of 544
Autorecovery and chatter
If the command "track to primary" is given, the clock controller tracks to the
primary reference and continuously monitors the quality of both primary and
secondary references. If the primary goes out of specification, the clock
controller automatically tracks to secondary if that is within specifications.
On failure (both out of specification), the clock controller enters the
HOLDOVER mode and continuously monitors both references. An
automatic switchover is initiated to the reference that recovers first. If the
secondary recovers first, then the clock controller tracks to the secondary, but
will switch over to the primary when the primary recovers. If the primary
recovers first, the clock controller tracks to the primary and continues to do
so even if the secondary recovers.
If the command "track to secondary" is given, the clock controller tracks to
the secondary reference and continuously monitors the quality of both
primary and secondary references. If the secondary goes out of specification,
the clock controller automatically tracks to primary provided that is within
specifications. On failure (both out of specification), the clock controller
enters the HOLDOVER mode and continuously monitors both references. An
automatic switchover is initiated to the reference that recovers first. If the
primary recovers first, the clock controller tracks to the primary, but switches
over to the secondary when the secondary recovers. If the secondary recovers
first, the clock controller tracks to the secondary and continues to do so even
if the primary recovers.
To prevent chatter due to repeated automatic switching between primary and
secondary reference sources, a time-out mechanism of at least 10 seconds is
implemented.

Digital to analog converter

The DAC (digital to analog converter) allows the microprocessor to track,
hold, and modify the error signal generated in the digital PLL.
The firmware uses the available memory on board the clock controller to
provide error-burst detection and correction. Temporary holdover occurs in
the momentary absence of the reference clock.
Option 11C and 11C Mini Technical Reference Guide