Design of tunable impedance matching circuit for a GSM antenna

Öztürk, Mehmet Ali
In this thesis, automatic impedance matching circuits are designed by using several tuning approaches. A typical automatic impedance matching system compensates for the impedance variations of the antenna due to the human hand and head using an electronically tunable impedance matching circuitry. Maximum output power, the linearity and efficiency of PA can be improved using automatic impedance matching circuit units. There are various tunable impedance matching circuits proposed in the literature to enhance RF front end operating conditions. These circuits are implemented by using inductors and varactors in various circuit topologies which can be utilized at the input of the low noise amplifier (LNA) in the receive chain and the output of the power amplifier (PA) in the transmit chain. Impedance coverage regions are investigated by sweeping all tunable components for various tuners. Our proposed automatic impedance networks are designed by utilizing a variation of available networks in the literature plus some novelties on the configuration and improvements on control voltage levels. A prototype of impedance tuner consisting of a 90 degree phase shifter and a variable transformer connected in cascade is implemented on an FR4 board. The automatic impedance tuning unit composed of directional couplers, attenuators, AD8302 is fabricated on an FR4 board to test the performance of the impedance tuner for various load impedances in the frequency bandwidth of 880-920 MHz. The automatic impedance tuner measurements show that significant VSWR improvement can be achieved for different load impedances. These measurements also show that the VSWR value is maintained below two for most of the mismatched loads in the frequency band of 880-920MHz. In order to study the overall efficiency of the tunable matching circuit, relative transducer gain expression is also employed to measure how much improvement or loss is introduced by the tunable matching circuit. The developed impedance tuner is compact and needs control voltage levels up to 3.3V, so that it is a good candidate for cell phone applications.