TCXO replacement for GPS
View Patent ↗To determine the level of frequency drift of a crystal oscillator as a result of a change in the its temperature, the temperature of the crystal oscillator is sensed and used together with previously stored data that includes a multitude of drift values of the frequency of the crystal oscillator each associated with a temperature of the crystal oscillator. Optionally, upon initialization of a GPS receiver in which the crystal oscillator is disposed, an initial temperature of the crystal oscillator is measured and a PLL is set to an initial frequency in association with the initial temperature. When acquisition fails in a region, the ppm region is changed. The temperature of the crystal oscillator is periodically measured and compared with the initial temperature, and the acquisition process is reset if there is a significant change in temperature. The GPS processor enters the tracking phase when acquisition is successful.
1. A device comprising:
a phase-locked-loop circuit;
a crystal oscillator coupled to the phase-locked-loop circuit;
a temperature sensor configured to determine a temperature associated with the crystal oscillator during at least first and second times; and
a controller configured to:
calculate a difference between the temperatures determined during the at least first and second times;
compare the temperature difference with a threshold value; and
vary an acquisition range of the phase-locked-loop circuit in accordance with the comparison result.
2. The device of claim 1 wherein the phase-locked-loop circuit comprises a sigma-delta modulator.
3. The device of claim 1 wherein the controller comprises a plurality of configuration and control registers for configuring and controlling the phase-locked-loop circuit.
4. The device of claim 1 wherein the controller is configured to operate the phase-locked-loop circuit in an acquisition phase and in a tracking phase.
5. The device of claim 4 wherein the controller is configured to:
record a frequency offset value of the crystal oscillator in association with a determined current temperature in the tracking phase;
store the frequency offset value in a memory; and
construct a set of the frequency offset values as a function of the determined current temperature.
6. The device of claim 1 wherein the controller is operable to store the frequency difference associated with the measured temperature.
7. The device of claim 1 wherein the controller is operable to apply a frequency correction to the phase locked loop based on a measured temperature at an initialization of an acquisition phase.
8. A method of adjusting an acquisition range of a phase-locked loop, the method comprising:
determining a temperature associated with a crystal oscillator coupled to the phase-locked loop during at least first and second times;
calculating a difference between the temperatures determined during the at least first and second times;
comparing the temperature difference with a threshold value; and
varying the acquisition range of the phase-locked-loop circuit in accordance with the comparison result.
9. The method of claim 8 further comprising:
recording a frequency offset value of the crystal oscillator in association with a determined current temperature in a tracking phase;
storing the frequency offset value in a memory; and
constructing a set of the frequency offset values as a function of the determined current temperature.
10. The method of claim 9 wherein constructing the set of the frequency offset values comprises:
building a plurality of affine functions, wherein each of the plurality of affine functions fits a subset of the set of the frequency offset values.
11. The method of claim 9 wherein constructing the set of the frequency offset values comprises:
building a polynomial function to best fit the set of the frequency offset values.