NTAK20 clock controller Page505 of 544
Option11C and 11C Mini TechnicalReference Guide
Tracking mode
There are two stages to clock controller tracking:
•tracking a reference
•locking on to a reference.
When tracking a reference, the clock controller uses an algorithm to match its
frequency to the frequency of the incoming clock. When the frequencies are
very near to being matched, the clock controller is locked on to the reference.
The clock controller will make small adjustments to its own frequency until
both the incoming and system frequencies correspond.
If the incoming clock reference is stable, the internal clock controller will
track it, lock on to it, and match frequencies exactly. Occasionally, however,
environmental circumstances will cause the external or internal clocks to
drift. When this happens, the internal clock controller will briefly enter the
tracking stage. The green LED will flash momentarily until the clock
controller is locked on to the reference once again.
If the incoming reference is unstable, the internal clock controller will
continuously be in the tracking stage, with the LED flashing green all the
time. This condition does not present a problem, rather, it shows that the clock
controller is continually attempting to lock onto the signal. If slips are
occurring, however, it means that there is a problem with the clock controller
or the incoming line.
Free-run (non-tracking)
In free-run mode, the clock controller does not synchronize on any source, it
provides its own internal clock to the system. This mode can be used when
the Option 11C is used as a master clock source for other systems in the
network. Free-run mode is undesirable if the Option 11C is intended to be a
slave. It can occur, however, when both the primary and secondary clock
sources are lost due to hardware faults or when invoked by using software
commands.