Dynamic antenna tuning apparatus and method

1. A near-field transmitter, comprising:

a power amplifier having an input for receiving a communication signal, and an output for providing a differential output signal;

a resonant network coupled to said output of said power amplifier and having a tunable reactive element whose reactance value is tuned by a tuning signal;

an envelope detector coupled to said output of said power amplifier for providing an envelope signal in response to said differential output signal; and

an antenna tuning circuit for adjusting said tuning signal in response to said envelope signal during operation of the near-field transmitter.

2. The near-field transmitter of claim 1 , wherein said resonant network comprises:

said tunable reactive element comprises a tunable capacitor having a first terminal coupled to a first output of said power amplifier, a second terminal coupled to a second output of said power amplifier, and a tuning input for receiving a first component of said tuning signal; and

an inductor having a first terminal coupled to said first output of said power amplifier, a second terminal coupled to said second output of said power amplifier.

3. The near-field transmitter of claim 2 , wherein said resonant network further comprises:

a tunable resistor having a first terminal coupled to said first output of said power amplifier, a second terminal coupled to said second output of said power amplifier, and a tuning input for receiving a second component of said tuning signal.

4. The near-field transmitter of claim 2 , wherein said antenna tuning circuit comprises:

a digital state machine having an input coupled to said output of said envelope detector, and an output coupled to said tuning input of said tunable reactive element, wherein said digital state machine adjusts said output continuously to maximize said envelope signal.

5. The near-field transmitter of claim 4 , wherein said digital state machine is coupled to said output of said envelope detector through:

a voltage divider; and

an analog-to-digital converter coupled to said voltage divider to provide a digital signal representative of a voltage of said envelope signal to said input of said digital state machine.

6. The near-field transmitter of claim 1 , wherein said envelope detector comprises:

a rectifier having inputs coupled to said output of said power amplifier, and an output for providing said envelope signal; and

a clamp coupled to said output of said rectifier for clamping a voltage on said output of said rectifier.

7. The near-field transmitter of claim 6 , wherein said rectifier is formed by a plurality of electrostatic discharge protection diodes.

8. The near-field transmitter of claim 1 , wherein said communication signal comprises audio information.

9. The near-field transmitter of claim 1 , wherein said communication signal is modulated using pulse-code modulation (PCM).

10. The near-field transmitter of claim 9 , wherein said communication signal is further modulated using offset quadrature phase-shift keying (OQPSK) modulation.

11. A near-field transmitter for transmitting a signal using near-field communication, comprising:

a modulator having an input for receiving a communication signal, and an output for providing a differential output signal to first and second integrated circuit terminals;

a power amplifier having an input coupled to said output of said modulator, and first and second outputs adapted to be coupled to an inductor, for providing a differential output signal;

a tunable reactive element coupled between said first and second outputs of said power amplifier whose reactance is tuned by a tuning signal;

an envelope detector coupled to said output of said power amplifier for providing an envelope signal in response to said differential output signal; and

an antenna tuning circuit for adjusting said tuning signal in response to said envelope signal.

12. The near-field transmitter of claim 11 , wherein said communication signal comprises audio information.

13. The near-field transmitter of claim 11 , wherein said communication signal is modulated using pulse-code modulation (PCM).

14. The near-field transmitter of claim 13 , wherein said communication signal is further modulated using offset quadrature phase-shift keying (OQPSK) modulation.

15. The near-field transmitter of claim 11 , further comprising:

a tunable resistor having a first terminal coupled to said first output of said power amplifier, a second terminal coupled to said second output of said power amplifier, and a tuning input for receiving a second component of said tuning signal.

16. The near-field transmitter of claim 11 , wherein:

said modulator, said power amplifier, said tunable reactive element, said envelope detector, and said antenna tuning circuit are combined on a monolithic integrated circuit, and said inductor is external to said monolithic integrated circuit.

17. A method of tuning a radio transmitter, comprising:

forming a communication signal at a carrier frequency;

amplifying a power of said communication signal and forming a differential output signal in response to said amplifying;

providing said differential output signal to a resonant network having a tunable reactive element;

rectifying said differential output signal to provide a rectified signal;

determining a level of said rectified signal; and

adjusting a value of a reactance of said tunable reactive element to an optimum amplitude of said rectified signal during operation of the radio transmitter.

18. The method of claim 17 , wherein adjusting said value of said tunable reactive element comprises:

setting said value to an initial amount;

measuring an initial voltage of said rectified signal;

increasing said value to a second amount; and

measuring a subsequent voltage of said rectified signal;

comparing said subsequent voltage to said initial voltage; and

determining an optimum value in response to said comparing.

19. The method of claim 18 , wherein if said subsequent voltage is less than said initial voltage, adjusting said value of said tunable reactive element further comprises:

repetitively decreasing said value of said tunable reactive element until a next voltage of said rectified signal is less than a current voltage of said rectified signal; and

setting said value to an amount corresponding to said current voltage of said rectified signal.

20. The method of claim 18 , wherein if said subsequent voltage is greater than said initial voltage, adjusting said value of said tunable reactive element further comprises:

repetitively increasing said value of said tunable reactive element until a next voltage of said rectified signal is less than a current voltage of said rectified signal; and

setting said value to an amount corresponding to said current voltage of said rectified signal.