NTAK20 Clock Controller daughterboard Page 737 of 906Circuit Card Description and Installation• jitter filtering• use of an algorithm to detect crystal aging and qualify clockinginformationClocking modesThe clock controller can operate in one of two modes: tracking ornon-tracking (also known as free-run).Tracking modeIn tracking mode, one or more DTI/PRI cards supply a clock reference to theNTAK20 clock controller daughterboard. When operating in tracking mode,one DTI/PRI card is defined as the Primary Reference Source (PREF) forclock synchronization. The other DTI/PRI card is defined as the SecondaryReference Source (SREF). PREF and SREF are defined in LD 73.There are two stages to clock controller tracking:• tracking a reference• locking on to a referenceWhen tracking a reference, the clock controller uses an algorithm to match itsfrequency to the frequency of the incoming clock. When the frequencies arealmost matched, the clock controller locks on to the reference. The clockcontroller makes small adjustments to its own frequency until both theincoming and system frequencies correspond.If the incoming clock reference is stable, the internal clock controller tracksit, 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 trackingstage. The green LED flashes until the clock controller is locked on to thereference again.If the incoming reference is unstable, the internal clock controllercontinuously tracks, and the LED continuously flashes green. This conditiondoes not present a problem. It shows that the clock controller is continuallyattempting to lock onto the signal. If slips occur, there is a problem with theclock controller or the incoming line.
