Systems and methods for maintaining a drive signal to a resonant circuit at a resonant frequency

1. A system for maintaining a drive signal to a resonant circuit at a resonant frequency comprising:

an oscillator configured to provide an output to a phase comparator and a drive circuit, the drive circuit configured to provide a drive signal to a resonant circuit;

a phase detector configured to receive a filtered version of the drive signal by sensing a voltage at a junction of an inductor and a capacitor of the resonant circuit and provide a phase-indicating signal to the phase comparator; and

the phase comparator, wherein the phase comparator is configured to provide a signal based on a phase difference between the oscillator output and the phase-indicating signal, wherein the signal from the phase comparator is used to control the frequency of the oscillator such that the phase difference converges to a fixed value.

2. The system of claim 1 , wherein the resonant circuit comprises at least one of a second order low-pass filter and a second order high-pass filter, and wherein the fixed value is 90 degrees, thereby maintaining the drive signal to the resonant circuit that converges to the resonant frequency.

3. The system of claim 2 , further comprising an integrator configured to vary a control signal supplied to the oscillator based on the signal provided by the phase comparator, thereby adjusting the oscillator such that there is a phase shift between the oscillator output and the resonant circuit output that converges to ninety degrees, thereby maintaining the drive signal to the resonant circuit that converges to the resonant frequency.

4. The system of claim 3 , wherein the integrator has an input-to-output transfer function of a first order integrator.

5. The system of claim 3 , wherein the integrator has an input-to-output transfer function of a first order low-pass filter.

6. The system of claim 3 , wherein the resonant circuit comprises at least one capacitor effectively in series with an inductor.

7. The system of claim 1 , wherein the resonant frequency is in the range of between 15 kHz and 100 kHz.

8. The system of claim 1 , wherein the resonant frequency is in the range between 20 kHz and 26 kHz.

9. The system of claim 1 , wherein the resonant frequency is greater than 100 kHz.

10. A system for maintaining a drive signal to a resonant circuit at a resonant frequency comprising:

an oscillator configured to provide an output to a phase comparator, a drive circuit having variable gain, and a low-pass filter, the drive circuit configured to provide a drive signal to a resonant circuit, the low-pass filter having a characteristic frequency that is substantially the same as the resonant frequency of the resonant circuit;

a phase detector configured to receive a filtered signal from each of the low-pass filter and the resonant circuit, and responsively provide a phase-indicating signal to the phase comparator representing the phase of whichever of the phase detector inputs is stronger; and

the phase comparator, wherein the phase comparator is configured to provide a signal based on a phase difference between the oscillator output and the phase-indicating signal, wherein the signal from the phase comparator is used to control the frequency of the oscillator.

11. The system of claim 10 , further comprising an integrator configured to vary a control signal supplied to the oscillator based on the signal provided by the phase comparator, thereby adjusting the oscillator such that there is phase shift between the oscillator output and the phase-indicating signal that converges to ninety degrees and maintaining the drive signal to the resonant circuit that converges to the resonant frequency.

12. The system of claim 11 , wherein the integrator has an input-to-output transfer function of a first order integrator.

13. The system of claim 11 , wherein the integrator has an input-to-output transfer function of a first order low-pass filter.

14. The system of claim 10 , wherein the resonant circuit comprises at least one capacitor in series with an inductor.

15. The system of claim 10 , wherein the resonant frequency is in the range of between 15 kHz and 100 kHz.

16. The system of claim 10 , wherein the resonant frequency is in the range between 20 kHz and 26 kHz.

17. The system of claim 10 , wherein the resonant frequency is greater than 100 kHz.

18. A method for maintaining a drive signal to a resonant circuit at a resonant frequency comprising:

using an oscillator to provide an output to a phase comparator and a drive circuit, the drive circuit configured to provide a drive signal to a resonant circuit;

using a phase detector to receive a filtered version of the drive signal by sensing a voltage at a junction of an inductor and a capacitor of the resonant circuit and provide a phase-indicating signal to the phase comparator;

using the phase comparator to provide a signal based on a phase difference between the oscillator output and the phase-indicating signal; and

using the signal from the phase comparator to control the frequency of the oscillator such that the phase difference converges to a fixed value.

19. The method of claim 18 , wherein the resonant circuit comprises at least one of a second order low-pass filter and a second order high-pass filter, and wherein the fixed value is 90 degrees, thereby maintaining the drive signal to the resonant circuit that converges to the resonant frequency.

20. The method of claim 19 , further comprising using an integrator to vary a control signal supplied to the oscillator based on the signal provided by the phase comparator, thereby adjusting the oscillator such that there is a phase shift between the oscillator output and the resonant circuit output that converges to ninety degrees, thereby maintaining the drive signal to the resonant circuit that converges to the resonant frequency.

21. The method of claim 20 , wherein the integrator has an input-to-output transfer function of a first order integrator.

22. The method of claim 20 , wherein the integrator has an input-to-output transfer function of a first order low-pass filter.

23. The method of claim 18 , wherein the resonant circuit comprises at least one capacitor effectively in series with an inductor.

24. The method of claim 18 , wherein the resonant frequency is in the range of between 15 kHz and 100 kHz.

25. The method of claim 18 , wherein the resonant frequency is in the range between 20 kHz and 26 kHz.

26. The method of claim 18 , wherein the resonant frequency is greater than 100 kHz.

27. A method for maintaining a drive signal to a resonant circuit at a resonant frequency comprising:

using an oscillator to provide an output to a phase comparator, a drive circuit having variable gain, and a low-pass filter, the drive circuit configured to provide a drive signal to a resonant circuit, the low-pass filter having a characteristic frequency that is substantially the same as the resonant frequency of the resonant circuit;

using a phase detector to receive a filtered signal from each of the low-pass filter and the resonant circuit, and responsively provide a phase-indicating signal to the phase comparator representing the phase of whichever of the phase detector inputs is stronger;

using the phase comparator to provide a signal based on a phase difference between the oscillator output and the phase-indicating signal; and

using the signal from the phase comparator to control the frequency of the oscillator.

28. The method of claim 27 , further comprising using an integrator to vary a control signal supplied to the controlled oscillator based on the signal provided by the phase comparator, thereby adjusting the oscillator output such that there is a phase shift between the oscillator output and the phase-indicating signal that converges to ninety degrees and maintaining the drive signal to the resonant circuit that converges to the resonant frequency.

29. The method of claim 28 , wherein the integrator has the input-to-output transfer function of a first order integrator.

30. The method of claim 28 , wherein the integrator has the input-to-output transfer function of a first order low-pass filter.

31. The method of claim 27 , wherein the resonant circuit comprises at least one capacitor in series with an inductor.

32. The method of claim 27 , wherein the resonant frequency is in the range of between 15 kHz and 100 kHz.

33. The method of claim 27 , wherein the resonant frequency is in the range between 20 kHz and 26 kHz.

34. The method of claim 27 , wherein the resonant frequency is greater than 100 kHz.

35. The method of claim 27 , wherein the phase difference converges to a ninety degree phase shift between the oscillator output and the phase-indicating signal, thereby maintaining the drive signal to the resonant circuit that converges to the resonant frequency.

36. The system of claim 10 , wherein the phase difference converges to a ninety degree phase shift between the oscillator output and the phase-indicating signal, thereby maintaining the drive signal to the resonant circuit that converges to the resonant frequency.