Nortel Networks Circuit Card - Manual (Page 739)

NTAK20 Clock Controller daughterboard Page 739 of 906
Circuit Card Description and Installation
Functional description
The main functional blocks of the NTAK20 architecture include:
• phase difference detector circuit
• digital Phase Locked Loop (PLL)
• clock detection circuit
• digital-to-analog converter
• CPU MUX bus interface
• signal conditioning drivers and buffers
• sanity timer
• microprocessor
• CPU interface
• external timing interface
Phase difference detector circuit
This circuit, under firmware control, enables a phase difference measurement
to be taken between the reference entering the PLL and the system clock.
The phase difference is used for making frequency measurements and
evaluating input jitter and PLL performance.
Digital phase lock loops
The main digital PLL enables the clock controller to provide a system clock
to the CPU. This clock is both phase and frequency locked to a known
incoming reference.
The hardware has a locking range of + 4.6 ppm for Stratum 3 and + 50 ppm
for Stratum 4 (CCITT).
A second PLL on the clock controller provides the means for monitoring
another reference. Note that the error signal of this PLL is routed to the phase
difference detector circuit so the microprocessor can process it.