Compensating for temperature-dependent hysteresis in a temperature compensated crystal oscillator

Patent metadata

Field	Value
Publication number	US-10164643-B2
Application number	US-201615261239-A
Country	US
Kind code	B2
Filing date	Sep 9, 2016
Priority date	Sep 9, 2016
Publication date	Dec 25, 2018
Grant date	Dec 25, 2018

Abstract

Hysteresis causes the temperature dependent frequency characteristic of the crystal of a crystal oscillator to be different when the temperature is rising from a previous colder state and when the temperature is falling from a hotter state. A rising temperature-to-frequency mapping polynomial and a falling temperature-to-frequency mapping polynomial are generated and their evaluations are weighted based on a current temperature and past temperature(s). The weighted evaluations are combined and used in temperature-based frequency compensation of the crystal oscillator.

First claim

What is claimed is:

1. A method comprising:
weighting a first evaluation of a first temperature-to-frequency mapping for rising temperatures and a second evaluation of a second temperature-to-frequency mapping for falling temperatures based, in part, on temperature measurements associated with a temperature compensated crystal oscillator and generating first and second weighted evaluations;
combining the first and second weighted evaluations and generating a combined output; and
compensating for temperature dependent frequency change of a crystal in the temperature compensated crystal oscillator using the combined output.

2. The method as recited in claim 1 wherein the first temperature-to-frequency mapping is a rising polynomial and represents a first temperature to frequency characteristic of the crystal when moving from a first temperature to a second temperature higher than the first temperature and the second temperature-to-frequency mapping is a falling polynomial and represents a second temperature to frequency characteristic of the crystal when moving from a third temperature to a fourth temperature lower than the third temperature.

3. The method as recited in claim 1 , further comprising:
determining a first difference between a current temperature measurement and a previous temperature measurement;
comparing the first difference to a rising threshold;
if the first difference is above the rising threshold, increasing a weight of the first evaluation corresponding to the rising temperatures by a first amount.

4. The method as recited in claim 3 , further comprising:
decreasing a weight of the second evaluation corresponding to the falling temperatures by an amount corresponding to the first amount.

5. The method as recited in claim 3 , further comprising:
determining a second difference between the previous temperature measurement and the current temperature measurement;
comparing the second difference to a falling threshold;
if the second difference is above the falling threshold resulting in another weighting event, increasing a weight of the second evaluation corresponding to the falling temperatures by a second amount.

6. The method as recited in claim 5 , further comprising:
decreasing a weight of the first evaluation corresponding to the rising temperatures by an amount corresponding to the second amount.

7. The method as recited in claim 5 , wherein the previous temperature measurement was a temperature at a most recent weighting event.

8. The method as recited in claim 2 , further comprising:
adjusting the weighting of the first and second evaluations corresponding to the rising and falling temperatures responsive to a predetermined passage of time to move weighting of the first and second evaluations, corresponding respectively to the rising temperatures and the falling temperatures, towards an even weighting.

9. The method as recited in claim 8 , further comprising:
determining the predetermined passage of time using a counter that is independent of weighting events.

10. The method as recited in claim 8 , further comprising:
determining the predetermined passage of time using a counter; and
resetting the counter at each weighting event to begin counting the predetermined passage of time responsive to each weighting event.

11. An apparatus comprising:
weighting logic to apply a first weight to a first evaluation of a first temperature-to-frequency mapping corresponding to rising temperatures and generate a first weighted evaluation and to apply a second weight to a second evaluation of a second temperature-to-frequency mapping corresponding to falling temperatures and generate a second weighted evaluation;
weighting control logic to determine the first and second weights based on temperature history measurements associated with a temperature compensated crystal oscillator; and
a combining circuit to combine the first and second weighted evaluations and generate a combined output for use in compensating for temperature dependent frequency change of a crystal in the temperature compensated crystal oscillator.

12. The apparatus as recited in claim 11 further comprising:
a phase-locked loop including a feedback divider having a divide ratio determined, based in part, on the combined output.

13. The apparatus as recited in claim 11 wherein the first temperature-to-frequency mapping is a rising polynomial and represents a first temperature to frequency characteristic of the crystal when moving from a first temperature to a second temperature higher than the first temperature and the second temperature-to-frequency mapping is a falling polynomial and represents a second temperature to frequency characteristic of the crystal when moving from a third temperature to a fourth temperature lower than the third temperature.

14. The apparatus as recited in claim 11 wherein the first temperature-to-frequency mapping is first frequency error data stored in a first lookup table and represents a first temperature to frequency characteristic of the crystal when moving from a first temperature to a second temperature higher than the first temperature and the second temperature-to-frequency mapping is second frequency error data stored in a second lookup table and represents a second temperature to frequency characterisitic of the crystal when moving from a third temperature to a fourth temperature lower than the third temperature.

15. The apparatus as recited in claim 11 , wherein the weighting control logic is further configured to:
determine a difference between a current temperature measurement and an earlier temperature measurement;
compare the difference to a rising threshold; and
if the difference is above the rising threshold, increase the first weight of the first evaluation.

16. The apparatus as recited in claim 15 , wherein the weighting control logic is further configured to decrease the second weight of the second evaluation by an amount corresponding to the increase of the first weight of the first evaluation.

17. The apparatus as recited in claim 11 , wherein the weighting control logic is further configured to:
determine a difference between an earlier temperature measurement and a current temperature measurement;
compare the difference to a falling threshold; and
if the difference is above the falling threshold, increase the second weight of the second evaluation.

18. The apparatus as recited in claim 17 , wherein the weighting control logic is further configured to decrease the first weight of the first evaluation by an amount corresponding to the increase of the second weight of the second evaluation.

19. The apparatus as recited in claim 15 , wherein the earlier temperature measurement was a temperature measurement at a most recent weighting event.

20. The apparatus as recited in claim 11 , wherein the weighting logic is further responsive to passage of a predetermined time period to adjust the first and second weights of the respective first and second evaluations towards an even weighting.

21. The apparatus as recited in claim 20 , wherein the weighting logic further comprises:
a counter to count the predetermined time period independent of weighting events.

22. The apparatus as recited in claim 20 , wherein the weighting logic further comprises:
a counter to count the predetermined time period; and
wherein the counter is configured to begin counting the predetermined time period responsive to each weighting event.

Assignees

• Silicon Lab Inc

Inventors

• Seethamraju Srisai R
• Cali Joseph D
• Thirugnanam Rajesh
• Juhn Richard J

Classifications

• H03L7/093

  using special filtering or amplification characteristics in the loop (H03L7/087 - H03L7/091 take precedence) · CPC title

• H03L1/028

  of generators comprising piezoelectric resonators (H03L1/021, H03L1/022 take precedence; oscillation generators with a piezoelectric resonator H03B5/32) · CPC title

• H03L1/022

  by indirect stabilisation, i.e. by generating an electrical correction signal which is a function of the temperature (H03L1/021 takes precedence) · CPC title

• H03L7/099

  concerning mainly the controlled oscillator of the loop · CPC title

• H03L7/197

  a time difference being used for locking the loop, the counter counting between numbers which are variable in time or the frequency divider dividing by a factor variable in time, e.g. for obtaining fractional frequency division {(H03L7/1806 takes precedence)} · CPC title