Automatic antenna tuning method and circuit

A method of matching a resonant frequency of an antenna resonant circuit to an output frequency of a transmitter output stage, the method comprising the steps of:

• a) providing a powering signal from said transmitter output stage to a powering winding of a transformer, said powering signal being substantially a square wave having rising and falling edges, the frequency of said powering signal not precisely known but known to fall between a minimum and a maximum frequency;
• b) providing by said transformer at a second winding thereof, an antenna signal in response to said powering signal;
• c) receiving at said antenna resonant circuit said antenna signal, said antenna resonant circuit comprising a coil, a capacitor, and a tuning circuit, said coil and said capacitor being serially connected between the first and second terminals of said second winding and defining a resonant circuit output at the connection between said coil and said capacitor, and said tuning circuit being in parallel with said capacitor and being operable to variably modify the resonance frequency of said resonant circuit from the initial resonance frequency of the combination of said coil and said capacitor;
• d) providing at said output of said antenna resonant circuit a phase correlation signal, the form of which is substantially a sine wave having an apex which:
• i) lags the falling edge of said powering signal when the resonance frequency of said antenna circuit is less than the frequency of said powering signal;
• ii) is approximately at the falling edge of said powering signal when the resonance frequency of said antenna circuit is approximately at the frequency of said powering signal;
• and
• iii) leads the falling edge of said powering signal when the resonance frequency of said antenna circuit is greater than the frequency of said powering signal,
• the magnitude of the distance between the apex of said sine wave and the falling edge of said powering signal being the phase difference, said phase difference being related to the magnitude of the frequency difference between said resonance frequency and said powering signal;
• e) receiving by a zero crossing detector said correlation signal and conditioning this signal to be substantially a square wave for compatibility with digital logic circuitry;
• f) receiving by a phase shifter said powering signal, said phase shifter inducing a phase shift in said powering signal to correspond to the phase shift which is inherently induced in said zero crossing detector;
• g) receiving in a phase comparator the outputs of said zero crossing detector and said phase shifter, said phase comparator providing an output of a first logic level when the phase difference is less than a reference value and of a second logic level when the phase difference is greater than a reference value;
• and
• h) adjusting the resonant frequency of said resonant circuit by a control unit in response to the received output from said phase comparator.

A circuit arrangement for matching the resonant frequency of an antenna resonant circuit to the output frequency of a transmitter output stage comprising:

• a) a transmitter output stage providing a powering signal;
• b) an antenna resonant circuit for receiving said powering signal, said antenna resonant circuit having a predetermined resonant frequency and comprising a tuning circuit wherein said resonant frequency is dependent upon the value of said tuning circuit, said antenna resonant circuit having at its output a phase correlation signal, the phase of said phase correlation signal relative to said powering signal having a known relationship to the frequency difference between said resonant frequency and said powering signal;
• and
• c) a phase comparator for receiving said powering signal and said phase correlation signal and measuring the phase difference therebetween, said phase comparator further for controlling said tuning circuit to adjust the resonant frequency of said resonant circuit depending upon said phase difference.

A circuit arrangement for matching a resonant frequency of an antenna resonant circuit to an output frequency of a transmitter output stage comprising:

• a) a transmitter output stage providing a powering signal;
• b) an antenna resonant circuit for receiving said powering signal, said antenna resonant circuit having a predetermined resonant frequency and comprising a tuning circuit wherein said resonant frequency is dependent upon the value of said tuning circuit, said antenna resonant circuit having at its output a phase correlation signal, the phase of said phase correlation signal relative to said powering signal having a known relationship to a frequency difference between said resonant frequency and said powering signal;
• and
• c) a phase comparator for receiving said powering signal and said phase correlation signal and measuring the phase difference therebetween, said phase comparator providing a binary signal having a first polarity when said powering signal is of a greater amplitude than said phase correlation signal and a second polarity when said powering signal is of a lesser amplitude than said phase correlation signal;
• d) a control unit for receiving said binary signal from said phase comparator and for controlling said tuning circuit to adjust the resonant frequency of said resonant circuit depending upon said phase difference between said phase correlation signal and said powering signal.

A method of matching a resonant frequency of an antenna resonant circuit to an output frequency of a transmitter output stage, the method comprising the steps of:

• a) providing a powering signal from said transmitter output stage to said antenna resonant circuit, said antenna resonant circuit comprising a coil and a capacitor wherein the resonant frequency of said coil and capacitor is chosen to be outside known extreme of the frequency of said powering signal, said antenna resonant circuit further comprising a tuning circuit having a network of capacitors in parallel with said coil and said capacitor, said antenna resonant circuit providing a phase correlation signal at an output thereof;
• b) receiving at said antenna resonant circuit said powering signal, the phase of said phase correlation signal relative to said powering signal having a known relationship to the frequency difference between said resonant frequency and said powering signal;
• c) receiving in a phase comparator said powering signal and said phase correlation signal;
• and
• d) adjusting the resonant frequency of said resonant circuit in response to a received output of said phase comparator by changing the capacity of the tuning circuit in steps until the resonant frequency of said resonant frequency is approximately equal to the frequency of said powering signal.

A circuit arrangement for matching a resonant frequency of an antenna resonant circuit to an output frequency of a transmitter output stage comprising:

• a) a transmitter output stage providing a powering signal;
• b) an antenna resonant circuit for receiving said powering signal, said antenna resonant circuit comprising an electrically coupled coil and capacitor, wherein the resonant frequency of said electrically coupled coil and capacitor is chosen to be outside known extreme of the frequency of said powering signal, said antenna resonant circuit further comprising a tuning circuit comprising a network of capacitors electrically coupled to said resonant circuit wherein said tuning circuit is operable to modify said resonant frequency, said antenna resonant circuit further providing at its output a phase correlation signal, the phase of said phase correlation signal having a known relationship to the frequency difference between said resonant frequency and said powering signal;
• and
• c) a phase comparator for receiving said powering signal and said phase correlation signal and measuring the phase difference therebetween, said phase comparator further for controlling said tuning circuit to adjust the resonant frequency of said resonant circuit in response to the comparison between the powering signal and the phase correlation signal until the resonant frequency of said resonant frequency is approximately equal to the frequency of said powering signal.

A circuit arrangement for matching a resonant frequency of an antenna resonant circuit to an output frequency of a transmitter output stage comprising:

• a) a transmitter output stage providing a powering signal;
• b) a modulator for modulating said powering signal with information to be transmitted to a transponder, said modulation creating a modulated powering signal wherein said modulated powering signal comprises discontinuities;
• c) an antenna resonant circuit for receiving said modulated powering signal, said antenna resonant circuit having a predetermined resonant frequency and comprising a tuning circuit wherein said resonant frequency is dependent upon the value of said tuning circuit, said antenna resonant circuit having at its output a phase correlation signal, the phase of said phase correlation signal relative to said powering signal having a known relationship to the frequency difference between said resonant frequency and said modulated powering signal;
• and
• d) a phase comparator for receiving said modulated powering signal and said phase correlation signal and measuring the phase difference therebetween, said phase comparator further for controlling said tuning circuit to adjust the resonant frequency of said resonant circuit depending upon said phase difference.