Nortel Networks 553-3001-211 manual Digital to analog converter, Autorecovery and chatter

NTAK20 Clock Controller daughterboard Page 729 of 894

without software intervention. If both references are out of specification, the clock controller provides holdover.

Autorecovery and chatter

If the command “track to primary” is given, the clock controller tracks to the primary reference and continuously monitors the quality of both primary and secondary references. If the primary goes out of specification, the clock controller will automatically “track to secondary” if the secondary is within specifications. On failure (both out of specification), the clock controller enters the HOLDOVER mode and continuously monitors both references. An automatic switchover is initiated to the reference that recovers first. If the secondary recovers first, then the clock controller tracks to the secondary, then switches over to the primary when the primary recovers. If the primary recovers first, the clock controller tracks to the primary and continues to do so even if the secondary recovers.

If the command “track to secondary” is given, the clock controller tracks to the secondary reference and continuously monitors the quality of both primary and secondary references. If the secondary goes out of specification, the clock controller automatically tracks to primary provided that is within specifications. On failure (both out of specification), the clock controller enters the HOLDOVER mode and continuously monitors both references. An automatic switchover is initiated to the reference that recovers first. If the primary recovers first, the clock controller tracks to the primary, but switches over to the secondary when the secondary recovers. If the secondary recovers first, the clock controller tracks to the secondary even if the primary recovers.

To prevent chatter due to repeated automatic switching between primary and secondary reference sources, a time-out mechanism of at least 10 seconds is implemented.

Digital to analog converter

The Digital to Analog Converter (DAC) enables the microprocessor to track, hold, and modify the error signal generated in the digital PLL.

The firmware uses the available memory on the clock controller to provide error-burst detection and correction. Temporary holdover occurs in the momentary absence of the reference clock.

Circuit Card Description and Installation