Nortel Networks Circuit Card User Manual

 NTAK20 Clock Controller daughterboard

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jitter filtering

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use of an algorithm to detect crystal aging and qualify clocking 
information

The clock controller can operate in one of two modes: tracking or 
non-tracking (also known as free-run).

In tracking mode, one or more DTI/PRI cards supply a clock reference to the 
NTAK20 clock controller daughterboard. When operating in tracking mode, 
one DTI/PRI card is defined as the Primary Reference Source (PREF) for 
clock synchronization. The other DTI/PRI card is defined as the Secondary 
Reference Source (SREF). PREF and SREF are defined in LD 73.

There are two stages to clock controller tracking:

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tracking a reference

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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 
almost matched, the clock controller locks on to the reference. The clock 
controller makes 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 tracks 
it, locks on to it, and matches frequencies exactly. Occasionally, 
environmental circumstances cause the external or internal clocks to vary. 
When this happens, the internal clock controller briefly enters the tracking 
stage. The green LED flashes until the clock controller is locked on to the 
reference again.

If the incoming reference is unstable, the internal clock controller 
continuously tracks, and the LED continuously flashes green. This condition 
does not present a problem. It shows that the clock controller is continually 
attempting to lock onto the signal. If slips occur, there is a problem with the 
clock controller or the incoming line.