Page508 of 544 NTAK20clock controller
553-3011-100 Standard14.00 January 2002
System clock specification and characteristics
Since the accuracy req uirements for CCITT and EIA Stratum 3ND are so
different, it is necessary to have two TCVCXO which feature different values
of frequency tuning sensitivity.
EIA/CCITT compliance
The clock controller complies with 1.5 Mb EIA Stratum 3ND, 2.0 Mb CCITT
or 2.56 basic rate. The differences between these requirements mainly affect
PLL pull in range. Stratum 4 conforms to international markets (2.0Mb) while
stratum 3 conforms to North American market. (1.5 Mb).
Monitoring references
The primary and secondary synchronization references are continuously
monitored in order to provide autorecovery.
Reference switchover
Switchover may occur in the case of reference degradation or loss of signal.
When performance of the reference degrades to a point where the system
clock is no longer allowed to follow the timing signal, then the reference is
out of specification. If the reference being used is out of specification and the
other reference is still within specification, an automatic switchover is
initiated without software intervention. If both references are out of
specification, the clock controller provides holdover.
Table 1 59
System clock specification and characteristics
Specifications CCITT EIA
Base Frequency 20.48 MHz 20.48 MHz
Accuracy + 3 ppm + 1 ppm
Operating Temperature 0 to 70 C + 1 ppm 0 to 70 C + 1 ppm
Drift Rate (Aging) + 1 ppm per year + 4 ppm in 20 years
Tuning Range (minimum) + 60 ppm min.
+ 90 ppm max.
+ 10 ppm min.
+ 15 ppm max.
Input Voltage Range 0 to 10 volts, 5V center 0 to 10 volts, 5V center