Dithering Control of Oscillator Frequency to Reduce Cumulative Timing Error in a Clock

1 . A method for correcting time error in an oscillator operated clock, comprising:

at selected times, determining at least one of a time error in the clock and a frequency difference between the oscillator and a reference oscillator by detecting a time reference signal;

determining a change in the at least one of the time error and the frequency difference between a first one and a second one of the detecting the time reference signals; and

adjusting a frequency of the oscillator so as to substantially cancel a cumulative time error between the second one of the detecting the time reference signal and a selected detecting the time reference signal.

2 . The method of claim 1 further comprising measuring a temperature of a crystal associated with the oscillator and adjusting a frequency of the oscillator in response to the detected temperature.

3 . The method of claim 1 wherein the time reference signal comprises global positioning system satellite signals.

4 . The method of claim 1 wherein the adjusting the frequency is performed over a selected time interval.

5 . The method of claim 4 wherein a rate of change of the adjusting, a maximum amplitude of the adjusting and a length of the selected time interval are selected based on a magnitude of the time error.

6 . The method of claim 4 wherein the adjusting the frequency comprises sinusoidally varying the oscillator frequency and wherein an average value of the sinusoidal variation is selected to substantially cancel the cumulative time error.