Synchronization system for reducing slipping

• US 6,178,215 B1
• Filed: 12/24/1997
• Issued: 01/23/2001
• Est. Priority Date: 10/09/1997
• Status: Expired due to Fees

• Current Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Zhang, Genzao, Ferland, Roger, Smith, Roland, Oprea, Dan
• Primary Examiner(s): Pham, Chi H.
• Assistant Examiner(s): Bayard, Emmanuel
• Application Number:
  US08/997,981
• Time in Patent Office: 1,126 Days
• Field of Search: 375/352, 375/354, 375/357, 375/359, 375/362, 375/371, 375/373, 375/375, 375/376, 375/294, 329/319, 329/325, 331/17, 327/147, 327/148, 455/182.2, 455/192.2, 455/208, 455/255, 455/256, 455/257, 455/258, 455/260, 455/264, 455/265
• US Class Current: 375/371
• CPC Class Codes: H03L 7/08 Details of the phase-locked loop
• CPC Class Codes: H04J 3/0688 Change of the master or reference, e.g. take-over or failure of the master
• CPC Class Codes: H04L 7/0083 taking measures against momentary loss of synchronisation, e.g. inhibiting the synchronisation, using idle words or using redundant clocks

Abstract

An improved local synchronization module which uses Frequency-phase adaptive double locked loop (FPADLL) to control a stable controllable oscillator is disclosed. A single physical feedback loop is implemented which can operate in either a phase locked loop mode or a frequency locked loop mode. The sync module includes a controller which determines in which mode the feedback loop operates. The controller also uses slipping information from a network reference recovery interface to reduce slipping. Also effects of ageing of the stable controllable oscillator are predicted and compensated for.

Claims

1. A synchronization system comprising:

a reference signal recovery unit for recovering a reference signal, said reference signal having phase and frequency signal characteristics, said recovery unit including a slip buffer;

a controllable signal source for producing an output signal having phase and frequency signal characteristics;

a signal detector connected to an output of said recovery unit and an output of said controllable signal source, said signal detector detects signal differences in one of said signal characteristics between said output signals, and produces a difference signal indicative of said differences; and

a controller connected to receive said difference signal as an input and to receive an indication from the recovery unit when a slip occurs and for producing a control signal for controlling said controllable signal source in order to reduce slipping.

2. A synchronization system as claimed in claim 1, wherein said controllable signal source comprises a controllable oscillator and wherein said signal detector comprises a phase detector for producing a phase difference signal indicative of the phase difference between said oscillator output and said reference signal and wherein said controller further comprises means for processing said difference signal in order to estimate a long term phase error in order to determine whether the system is approaching a slip.

3. A synchronization system as claimed in claim 2 further comprising means for resetting said estimate of the long term phase error whenever an indication of a slip is received and sending a reset signal to said phase detector.

4. A synchronization system as claimed in claim 3, wherein said controller maintains multiple threshold levels for controlling said oscillator and determines which threshold level to use based on said estimate of the long term phase error.

5. A synchronization system as claimed in claim 3, wherein said means for processing said difference signal in order to estimate the long term phase error in order to determine whether the system is approaching a slip includes means for determining the value of said the long term phase error and the direction of change of said long term phase error.

6. A synchronization system as claimed in claim 5, wherein said controller maintains multiple threshold levels for controlling said oscillator and determines which threshold level to use based on the determined value and direction of change of the long term phase error.

7. A synchronization system as claimed in claim 3, wherein said controller includes means for forcing a slip to occur during initialization.

8. A synchronization apparatus for synchronizing a locally generated signal in a network element with other network elements comprising:

reference signal means for receiving a reference signal from at least one other network element, said reference signal having phase and frequency signal characteristics, said reference signal including a slip buffer;

controllable signal means for producing an output signal having phase and frequency signal characteristics;

signal detector means, connected to an output of said reference signal means and an output of said controllable signal means, for detecting signal differences in one of said signal characteristics between said output signals, and for producing a difference signal indicative of said differences; and

control means, connected to receive said difference signal as an input and to receive an indication from the reference signal means when a slip occurs, for producing a control signal for controlling said controllable signal means in order to reduce slipping.

9. A synchronization apparatus as claimed in claim 8, wherein said controllable signal means comprises a controllable oscillator and wherein said signal detector means comprises phase means for producing a phase difference signal indicative of the phase difference between said oscillator output and said reference signal and wherein said control means further comprises means for processing said difference signal in order to estimate a long term phase error in order to determine whether the system is approaching a slip.

10. A synchronization apparatus as claimed in claim 9 further comprising means for resetting said estimate of the long term phase error whenever an indication of a slip is received and sending a reset signal to said phase detector.

11. A synchronization apparatus as claimed in claim 10, wherein said control means includes means for maintaining multiple threshold levels for controlling said oscillator and determines which threshold level to use based on said estimate of the long term phase error.

12. A synchronization apparatus as claimed in claim 11, wherein said means for processing said difference signal in order to estimate the long term phase error in order to determine whether the system is approaching a slip includes means for determining the value of said the long term phase error and the direction of change of said long term phase error.

13. A synchronization 14 as claimed in claim 12, wherein said control means includes means for maintaining multiple threshold levels for controlling said oscillator and determines which threshold level to use based on the determined value and direction of change of the long term phase error.

14. A synchronization apparatus as claimed in claim 10, wherein said control means includes means for forcing a slip to occur during initialization.

15. A synchronization apparatus as claimed in claim 13, wherein said control means includes means for forcing a slip to occur during initialization.