OSCILLATOR CALIBRATION

• US 20090163246A1
• Filed: 11/20/2008
• Published: 06/25/2009
• Est. Priority Date: 11/23/2007
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Ubiquisys Limited (Cisco Systems, Inc.)
• Inventors: Mullen, Sean
• Granted Patent: US 8,301,181 B2
• Time in Patent Office: Days
• US Class Current: 455/561
• CPC Class Codes: H04J 3/0641 Change of the master or reference, e.g. take-over or failure of the master
• CPC Class Codes: H04J 3/0667 Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays arrangements for monitoring round trip delays in packet switching networks H04L43/0864
• CPC Class Codes: H04J 3/14 Monitoring arrangements for SDH/SONET rings H04J3/085

Abstract

A basestation for a cellular communication system has an interface, for connection to a computer network, and also includes an oscillator, for generating wireless transmit and receive frequencies. A controller receives timestamped response messages from a time server over the computer network, each response message being subject to a network propagation delay, which is a sum of a minimum network propagation delay and a jitter component. For each received response message an apparent network propagation delay is determined as a function of a difference between a first timestamp applied by the time server and a second timestamp based on a clock derived from said oscillator. A subset of the received response messages are selected, whose network propagation delays include minimal jitter components. The frequency accuracy of the oscillator is then determined based on changes over time in the apparent network propagation delays of the selected received response messages. The oscillator can then be adjusted based on this frequency accuracy.

Claims

1. A method of determining an accuracy of an oscillator used to generate a local clock signal, the method comprising:

receiving a plurality of timestamped response messages from a time server over a computer network, each of said response messages being subject to a network propagation delay, said network propagation delay being a sum of a minimum network propagation delay and a jitter component;

for each received response message, determining an apparent network propagation delay as a function of a difference between a first timestamp applied by the time server and a second timestamp based on a clock derived from said oscillator;

selecting a subset of the received response messages whose network propagation delays include minimal jitter components; and

determining the accuracy of the oscillator based on changes over time in the apparent network propagation delays of the selected received response messages.

2. A method as claimed in claim 1, wherein the step of selecting the subset of the received response messages comprises:

receiving a plurality of bursts of timestamped response messages from a time server;

preselecting one response message from each burst of response messages; and

selecting the subset of the received response messages from the preselected response messages.

3. A method as claimed in claim 1, wherein the step of selecting the subset of the received response messages comprises:

defining a time interval, over which the accuracy of the oscillator is to be determined;

dividing said time interval into time windows; and

within each time window, selecting a response having a minimum network propagation delay, relative to its time position within said time window.

4. A method as claimed in claim 2, wherein the step of selecting the subset of the received response messages comprises:

defining a time interval, over which the accuracy of the oscillator is to be determined;

dividing said time interval into time windows; and

within each time window, selecting a response having a minimum network propagation delay, relative to its time position within said time window.

5. A method as claimed in claim 3, comprising:

defining a first time window;

selecting a response from the first time window;

defining a further time window starting at the selected response from the first time window; and

selecting a response from the further time window.

6. A basestation for a cellular communication system, comprising:

an interface, for connection to a computer network;

an oscillator, for generating a clock signal;

a controller, for adjusting the oscillator, wherein the controller is adapted to;

receive a plurality of timestamped response messages from a time server over the computer network, each of said response messages being subject to a network propagation delay, said network propagation delay being a sum of a minimum network propagation delay and a jitter component;

for each received response message, determine an apparent network propagation delay as a function of a difference between a first timestamp applied by the time server and a second timestamp based on the clock signal;

select a subset of the received response messages whose network propagation delays include minimal jitter components;

determine the accuracy of the oscillator based on changes over time in the apparent network propagation delays of the selected received response messages; and

adjust the oscillator based on said determined accuracy.

7. A basestation as claimed in claim 6, further comprising:

transceiver circuitry, for generating transmit and/or receive frequency signals from an output of said oscillator.