Adaptive tuning for handheld transmitters

This paper presents a closed loop impedance tuner for a WCDMA handset power amplifier, with the goal of increasing overall efficiency by reducing the reflected power when the antenna is mismatched. A self-assessment circuit provides the feedback signal. A directional coupler between the amplifier and the antenna routes the reflected power to a detector where it is compared to the incident power. The output signal from the detector is digitized and fed to a FPGA containing the control algorithm for impedance tuning. The tuner is a simple varactor double-stub design and provides VSWR ≪ 3 at 1.95 GHz for loads with a mismatch up to VSWR = 81. The tuner exhibits loss as low as 0.3 dB and uses two separate control voltages between 0 and 6V.