Communicating with an implanted wireless sensor

1. A method for determining a resonant frequency of a wireless sensor, comprising:

generating a first set of energizing signals, wherein the first set of energizing signals comprises a plurality of energizing signals, each with a different frequency;

receiving a first set of sensor signals, wherein each sensor signal corresponds to one of the energizing signals;

determining a phase for each of the sensor signals in the first set of sensor signals;

determining a first group phase delay and a second group phase delay using the phases of the sensor signals;

comparing the first group phase delay to the second group phase delay; and

determining the resonant frequency of the wireless sensor based on the group phase delay comparison.

2. The method of claim 1 , wherein the first set of energizing signals comprises:

a first energizing signal having a first frequency below an estimation of the resonant frequency of the wireless sensor;

a second energizing signal having a second frequency at the estimation of the resonant frequency; and

a third energizing signal having a third frequency above the second frequency.

3. The method of claim 2 , further comprising:

generating a second set of energizing signals, the second set of energizing signals comprising:

a first energizing signal having a first frequency below a second frequency;

a second energizing signal having the second frequency; and

a third energizing signal having a third frequency above the second frequency; and

selecting the second frequency of the second set based on the group phase delay comparison.

4. The method of claim 3 , further comprising:

selecting the first frequency of the second set and the third frequency of the second set based on the group phase delay comparison.

5. The method of claim 3 , wherein selecting the second frequency of the second set comprises:

selecting the second frequency of the second set as a higher frequency than the second frequency of the first set if the group phase delay comparison is positive; and

selecting the second frequency of the second set as a lower frequency than the second frequency of the first set if the group phase delay comparison is negative.

6. The method of claim 1 , wherein the first frequency and third frequency are offset from the second frequency by a predetermined amount.

7. The method of claim 1 , wherein comparing the first group phase delay and the second group phase delay comprises subtracting the first group phase delay from the second group phase delay.

8. The method of claim 1 , further comprising:

receiving a calibration signal during a calibration cycle;

processing the calibration signal during a first period within the calibration cycle to create an IF calibration signal;

determining a first phase difference between the IF calibration signal and a reference signal; and

based on the first phase difference adjusting the phases of the energizing signals to reduce the first phase difference.

9. The method of claim 1 , wherein determining the resonant frequency of the wireless sensor based on the group phase delay comparison comprises:

selecting a second energizing signal of a subsequent set of energizing signals until the group phase delay comparison results in zero.

10. A method for determining a resonant frequency of a wireless sensor, comprising:

receiving a first sensor signal having a first phase;

receiving a second sensor signal having a second phase;

receiving a third sensor signal having a third phase;

generating a first group phase delay by subtracting the first phase from the second phase;

generating a second group phase delay by subtracting the second phase from the third phase;

comparing the first group phase delay to the second group phase delay by subtracting the first group phase delay from the second group phase delay; and

based on the group phase delay comparison, determining the resonant frequency of the wireless sensor.

11. The method of claim 10 , further comprising:

selecting an energizing signal frequency based on the group phase delay comparison.

12. The method of claim 11 , further comprising:

receiving a calibration signal;

processing the calibration signal to create an IF calibration signal;

determining a first phase difference between the IF calibration signal and a reference signal; and

based on the first phase difference selecting the phase of the energizing signal to reduce the first phase difference.

13. The method of claim 12 , wherein selecting a phase of an energizing signal is repeated until the first phase difference between the IF calibration signal and the reference signal corresponds to a first predetermined value.

14. The method of claim 11 , further comprising:

using the resonant frequency of the sensor to determine a physical parameter associated with the sensor.

15. A system for determining a resonant frequency of a wireless sensor, comprising:

a base unit for transmitting a first set of energizing signals and receiving a first set of sensor signals, wherein each of the energizing signals of the first set has a different frequency and each of the sensor signals of the first set has a phase;

wherein the base unit comprises a signal processor for measuring the phase for each of the sensor signals, determining a first group phase delay and a second group phase delay based on the sensor signal phases, comparing the first group phase delay and second group phase delay, and determining a resonant frequency of the wireless sensor based on the comparison of the first group phase delay and second group phase delay.

16. The system of claim 15 , wherein the base unit further comprises a demodulator for demodulating each of the sensor signals.

17. The system of claim 15 , wherein the base unit further comprises a controller for at least one of:

controlling a switch on an antenna between transmit and receive mode;

sending the energizing signals to an RF power amplifier; and

controlling switches for receive mode.

18. The system of claim 15 , wherein the signal processor uses the resonant frequency of the sensor to determine a physical parameter associated with the sensor.

19. The system of claim 18 , wherein the base unit is in communication with a user interface, the signal processor being adapted to cause a numerical or graphical representation of the physical parameter to appear on the user interface.

20. The system of claim 15 , wherein the signal processor determines the resonant frequency of the sensor by:

generating a first group phase delay by subtracting a first phase from a second phase;

generating a second group phase delay by subtracting the second phase from a third phase; and

comparing the first group phase delay to the second group phase delay by subtracting the first group phase delay from the second group phase delay.

21. The system of claim 15 , wherein the first set of energizing signals comprise:

a first energizing signal having a frequency below an estimation of the resonant frequency;

a second energizing signal having a frequency at the estimation of the resonant frequency;

a third energizing signal having a frequency above the estimation of the resonant frequency; and

wherein the first set of sensor signals comprises:

a first sensor signal corresponding to the first energizing signal, the first sensor signal having a first phase;

a second sensor signal corresponding to the second energizing signal, the second sensor signal having a second phase; and

a third sensor signal corresponding to the third energizing signal, the third sensor signal having a third phase.

22. The system of claim 21 , wherein the signal processor,

identifies the resonant frequency as the second energizing frequency if the group phase delay comparison result is zero;

instructs a controller to transmit a second set of energizing signals having frequencies higher than the first set if the group phase delay comparison result is positive; and

instructs the controller to transmit the second set of energizing signals having frequencies lower than the first set if the group phase delay comparison result is negative.

23. The system of claim 15 , wherein the signal processor is a digital signal processor.