September 25, 2006

Chapter 1: Overview

EXTERNAL SHELF CLOCK BACKUP CIRCUIT

The EMU-830 List 6A includes a backup circuit for the 2.048 MHz external shelf clock. This circuit will continue to provide a 2.048 MHz clock to the DSL cards in the event the 2.048 MHz external shelf clock is lost. The software reports the status of this circuit and allows the user to set the severity of the alarm generated when the clock is lost. A block diagram of the clock backup circuit is shown in Figure 1-8 on page 1-11.

Under normal operation the external shelf clock and the backup circuit synchronize their respective clocks through the EMUs phase-locked loop (PLL) device. In the event the external shelf clock is lost, the oscillator in the backup circuit will continue to supply the 2.048 MHz clock. A Loss of External Clock alarm is reported to the management station.

External

Clock

Backplane input

EMS-83x Shelf

Slot 1

LTU/STU-C

Slot 16

LTU/STU-C

Slot 17

Reference

PLL

2.048 MHz

Source

Oscillator

EMU-830 List 6

Figure 1-8. External Shelf Clock and EMU-830 List 6A Backup Circuit

LTPE-UM-3159-02

1-11

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ADC user manual External Shelf Clock Backup Circuit, External Shelf Clock and EMU-830 List 6A Backup Circuit