Short range, license-free, wireless applications operating in the Industrial Scientific Medical (ISM) bands (e.g. door openers, tire pressure monitoring systems (TPMS), wireless mice and wireless local area networks (WLANs)) have become popular in recent years. Such applications typically comprise one or more transmitting devices, which transmit a radio signal to one or more receiving devices. On receipt of a signal from the transmitting device(s), the receiving device(s) take an appropriate action.
Transmitting device(s) are typically mobile, handheld devices, which comprise a transmitter (for generating signals), an antenna (for transmitting the signals) and a battery (as an internal energy source for such transmissions). However, the batteries are usually quite small. Thus, power consumption is a critical factor in the design of such transmitting device(s).
The antenna in a transmitting device is a load on the device's transmitter. Maximally efficient power transfer between a transmitter and an antenna is only achieved when the transmitter impedance is the complex conjugate of the antenna impedance. However, the sensitivity of antenna impedance to surrounding conditions is making the task of matching the impedances of antennas and transmitters quite difficult. Attempts to solve this problem have been ongoing for some time.
Integration Associates (Automatic Antenna Tuning RF Transmitter IC Applying High Q Antenna—White Paper Ver. 1.0 (IA ISM-WP1), Integration Associates Inc., 2004) and Micrel (MICRF103 QwikRadio (trademark) ASK Transmitter FINAL, June 2002) have proposed a solution in which a single capacitor adjusts the reactive part of antenna impedance. However, any reduction in impedance mismatch achieved by this method would be limited, as it does not take into account the resistive components of the antenna impedance and transmitter impedance. Moreover, the Integration Associates and Micrel solutions are only applicable to source current transmitters, as the measurements used to match the impedances are phase differences between the base and collector of a transistor in a switched current source configuration.
U.S. Patent Application US2005219132 describes a system in which two capacitors of a PI matching network are tuned. Impedance matching is achieved by measuring the voltage and the phase difference between the input and output of the matching network. However, the system described in US2005219132 requires A/D convertors and a phase comparator and would be difficult to implement at the frequency of 868 MHz typically employed in automotive applications.
U.S. Patent Application US2005003771 describes a circuit for automatically tuning a resonant circuit in a transceiver. The circuit adjusts a single capacitor connected between the two outputs of a differential amplifier, wherein the adjustments are made on the basis of the phase difference between the input and the output of the amplifier. However, in a similar manner to the Integration Associates and Micrel solutions, the circuit in US2005003771 only provides reactive impedance matching. Furthermore, the circuit does not work with Class C amplifiers.
More generally, by using only a single adjustable capacitor, prior art solutions available at frequencies of 434-868 MHz have limited ability to match the impedances of a transmitter and antenna. In particular, whilst the prior art solutions provide reactive impedance cancellation, they do not provide conjugate matching.