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Page 740 of 906 NTAK20 Clock Controller daughterboard
553-3001-211 Standard 3.00 August 2005
System clock specification and characteristics
Since the accuracy requirements for CCITT and EIA Stratum 3 are different,
it is necessary to have two TCVCXOs which feature different values of
frequency tuning sensitivity. See Table 238.
EIA/CCITT compliance
The clock controller complies with 1.5 Mb EIA Stratum 3ND, 2.0 Mb CCITT
or 2.56 Mb basic rate. The differences between these requirements mainly
affect PLL pull in range. Stratum 4 conforms to international markets
(2.0 Mb) while Stratum 3 conforms to North American markets (1.5 Mb).
Monitoring references
The primary and secondary synchronization references are continuously
monitored in order to provide autorecovery.
Reference switchover
Switchover occurs 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 is out of specification and the other
reference is still within specification, an automatic switchover is initiated
Table 238
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