Method and apparatus for adjusting reference oscillator frequency in a mobile wireless device
A method and apparatus for using a conventional oscillator in a cellular telephone transceiver as a source of a reference signal for a GPS receiver. In one embodiment, the method comprises using a voltage-controlled oscillator (“VCXO”) within a cellular telephone transceiver to generate a reference frequency signal for the GPS receiver. Circuitry within the telephone transceiver generates a frequency error signal. Both of these signals are coupled to GPS circuitry and used to control a carrier numerically controlled oscillator (“NCO”) and a code NCO. The NCOs produce a tuning signal and a timing signal, respectively. The GPS circuitry uses the NCO generated signals to process GPS signals.
1. Within a mobile device comprising cell phone circuitry and GPS circuitry, a method for producing a reference signal for a GPS receiver comprising:
producing a cellular oscillator signal and a frequency error signal within the cell phone circuitry, where the frequency error signal is for controlling an oscillator to operate the cell phone circuitry within cell phone network specifications;
coupling the oscillator signal to at least one numerically controlled oscillator within GPS circuitry;
processing the frequency error signal to produce a frequency control value that is coupled to the at least one numerically controlled oscillator;
generating, within the at least one numerically controlled oscillator, a signal in response to the cellular oscillator signal and the frequency control value that is used for processing GPS satellite signals;
searching for a pilot tone or a carrier signal of a cellular telephone by tuning a frequency of an oscillatory signal generator;
fixing the frequency of the oscillatory signal generator when the pilot tone or the carrier signal are not found;
searching for the GPS satellite signals; and
returning to searching for a pilot tone or a carrier signal when the GPS satellite signals are not found.
2. The method of claim 1 wherein, the frequency error signal is maintained by the cell phone circuitry within the cell phone transmission specifications.
3. The method of claim 1 wherein, the frequency error signal represents the frequency difference between the pilot tone or the carrier signal and a signal derived from the cellular oscillator signal.
4. The method of claim 1 wherein, the frequency control value is a digital word.
5. The method of claim 1 wherein, the at least one numerically controlled oscillator comprises:
a carrier numerically controlled oscillator for generating a carrier tuning signal; and
a code numerically controlled oscillator for generating a code timing signal.
6. The method of claim 5 wherein, the carrier tuning signal substantially reduces frequency error caused by Doppler shift.
7. The method of claim 1 , wherein searching for a pilot tone or carrier signal occurs for a first predetermined time;
fixing the frequency occurs when the first predetermined time lapses;
searching for the GPS satellite signals occurs for a second predetermined time; and
returning to searching for a pilot tone or carrier signal occurs after the second predetermined time lapses.
8. The method of claim 7 , wherein the first predetermined time is in a range of about 20 to 30 seconds.
9. The method of claim 7 , wherein the second predetermined time is in a range of about 20 to 30 seconds.
10. The method of claim 1 , further comprising:
fixing the oscillatory signal generator to the frequency corresponding to the pilot tone or the carrier signal; and
using a voltage-controlled oscillator to adjust the frequency of the oscillatory signal generator to track the pilot tone or the carrier signal.
11. The method of claim 10 , further comprising: establishing a frequency threshold for the frequency of the oscillatory signal generator.
12. The method of claim 11 , further comprising: adjusting the frequency of the oscillatory signal generator towards a center frequency of the cellular oscillator signal in response to the frequency error signal exceeding the frequency threshold.
13. The method of claim 10 , further comprising: adjusting the frequency of the oscillatory signal generator towards a center frequency of the cellular oscillator signal.
14. The method of claim 10 , further comprising: adjusting the frequency of the oscillatory signal generator towards another frequency other than a center frequency of the cellular oscillator signal.
15. The method of claim 10 , further comprising: adjusting the frequency of the oscillatory signal generator to operate the cell phone circuitry within cell phone network specifications.
16. The method of claim 1 , further comprising: providing the signal from the at least one numerically controlled oscillator to at least one tuner to generate a carrier tuning signal.
17. The method of claim 16 , further comprising: using at least one decimation circuit to sample at a sampling frequency the carrier tuning signal generated by the at least one tuner.
18. The method of claim 17 , further comprising: using at least one code numerically controlled oscillator to control the sampling frequency of the at least one decimation circuit.
19. The method of claim 17 , further comprising: coupling the sampled carrier tuning signal to at least one correlator.