Patent ID: 7058372

Claim:
A continuous automatic tuning circuit for a transmitter circuit having a resonant circuit that includes an antenna and a power amplifier for driving the resonant circuit, the automatic tuning circuit comprising: a capacitance bank coupled between the power amplifier and the resonant circuit, the capacitance bank having an input that is coupled to an output of the power amplifier, an output that is coupled to the resonant circuit, and a control input, where a capacitance of the capacitance bank is determined by a control word received at the control input; a first phase shifter having an input coupled to the input of the capacitance bank, where the first phase shifter is configured to phase shift a resonance signal received at the input of the second phase shifter by a predetermined phase shift amount (Φ) in order to generate a first phase shifted signal at an output of the first phase shifter; a second phase shifter having an input coupled to the input of the power amplifier, where the second phase shifter is configured to phase shift an input signal received at the input of the second phase shifter by the predetermined phase shift amount plus or minus ninety degrees (Φ±90) in order to generate a second phase shifted signal at an output of the second phase shifter that is substantially in quadrature with the first phase shifted signal; a mixer having a first input coupled to the output of the first phase shifter, a second input coupled to the output of the second phase shifter, and an output, where the mixer is configured to mix the first phase shifted signal and the second phase shifted signal in order to generate an error signal at an output of the mixer; a control word generator having an input coupled and an output coupled to the control input of the capacitance bank, where the control word generator is configured to generate the control word at the output of the control word generator responsive to a control signal received at the input of the control word generator; and a compare and control circuit having an input coupled to the output of the mixer, where the compare and control circuit is configured to generate a control signal at an output of the compare and control circuit responsive to the error signal, where the control signal is configured to drive the control word generator in order to adjust the control word.