Compensating for thermal lag in temperature compensated crystal oscillators

1. A method for compensating for temperature-dependent frequency change of a crystal in a temperature compensated crystal oscillator comprising:

sensing a temperature in a temperature sensor and supplying sensed temperature data;

delaying application of temperature-dependent frequency compensation using a delay circuit having a selectable delay, the temperature-dependent frequency compensation based on the sensed temperature data, the delay circuit being a low-pass filter;

selecting a pole location of the low-pass filter to select the selectable delay; and

selecting the pole location of the low-pass filter according to the sensed temperature data to thereby dynamically delay the application of the temperature-dependent frequency compensation.

2. The method as recited in claim 1 , wherein the selectable delay is based on a time lag between when the temperature is sensed and when the temperature affects a crystal used in the crystal oscillator.

3. The method as recited in claim 1 , wherein a time lag associated with the delaying application of the temperature-dependent frequency compensation is determined from measured data.

4. The method as recited in claim 1 , further comprising measuring the temperature in a temperature sensor on a semiconductor die forming a part of the temperature compensated crystal oscillator.

5. The method as recited in claim 1 , further comprising:

averaging the sensed temperature data and supplying an average; and

adjusting the selectable delay based on the average.

6. The method as recited in claim 1 , further comprising:

applying the temperature-dependent frequency compensation to a phase-locked loop coupled to the crystal oscillator.

7. A temperature compensated crystal oscillator (TCXO) comprising:

a crystal oscillator;

a temperature sensor to provide sensed temperature data; and

a delay circuit providing a delay to delay temperature-dependent frequency compensation, the temperature-dependent frequency compensation based on the sensed temperature data, the delay to compensate for a difference between when the temperature sensor reflects a change in temperature and when a frequency of a signal supplied by the crystal oscillator is affected by the change in temperature;

wherein the delay circuit is a low-pass filter and a pole location of the low-pass filter determines the delay; and

wherein the pole location varies according to the sensed temperature data to thereby adjust the delay according to the sensed temperature data.

8. The TCXO as recited in claim 7 , wherein the delay is determined from measured data.

9. The TCXO as recited in claim 7 further comprising:

a phase-locked loop coupled to receive an oscillator signal supplied by the crystal oscillator and to correct a phase-locked loop signal generated by the phase-locked loop according to the temperature; and

wherein the temperature sensor is on a semiconductor die including the phase-locked loop.

10. The TCXO as recited in claim 7 , wherein the temperature-dependent frequency compensation is applied by changing a divider value in a feedback divider of a phase-locked loop.

11. A temperature compensated crystal oscillator (TCXO) comprising:

a crystal oscillator;

a temperature sensor to provide sensed temperature data; and

a low-pass filter for delaying temperature-dependent frequency compensation, the temperature-dependent frequency compensation based on the sensed temperature, a delay of the low-pass filter to account for a difference between when the temperature sensor reflects a change in temperature and when a frequency of a signal supplied by the crystal oscillator is affected by the change in temperature;

wherein the delay of the low-pass filter is adjusted based on the sensed temperature data by adjusting a pole location of the low-pass filter.

12. The TCXO as recited in claim 11 , wherein the delay varies according to temperature.