RF MEMS devices and their applications in reconfigurable RF/microwave circuits.

Abstract

Today's wireless telecommunication industry is driven by the demand for low cost and highly efficient communication technologies. At the same time, demands for multi-functional and adaptive communication systems composed of low-power consumption, low-cost, and high performance RF devices are rising. Successful development of such systems will result in a significant reduction of the device size and cost as well as a substantial improvement of the device performance. This thesis presents a novel RF MEMS analog varactor with a high tuning range of 0.1--0.3 pF and a very low loss at frequencies up to 40 GHz. The contact-less varactor has demonstrated an excellent power handling capability (up to 4 W) and high reliability (up to 108 million cycles) in <italic> hot cycling</italic> test of 3.2 W power condition with no failure observed. This is so far the best power performance that has ever been reported on RF MEMS devices in microwave frequencies. It is also a substantial improvement in power handling and reliability as compared to MEMS switches with the power handling capacity of about 100 mW/contact. A non-linear large signal circuit model is developed to characterize the varactor's intermodulation generation and its accuracy is verified by two-tone intermodulation measurements. The IIP3 of the varactor is 70 Min at Deltaf of 500 kHz and V_dc of 20 V. The minimum IIP3 at Deltaf of 500 kHz is 53.4 dBm when the varactor is at its maximum capacitance. These results are about 10--30 dB higher than those of most solid-state devices. Three reconfigurable circuits are presented in this thesis with RF MEMS devices. RF MEMS switches are used in a dual-band (6--8 GHz) amplifier for low-medium power applications (14--16 dBm). The amplifier's matching networks consist of MEMS switches which serve as digital varactors and provide an optimal matching condition at different frequency bands by switch-actuation. The developed analog varactors are applied in reconfigurable circuits for medium-to-high power applications. An X-band class-E power amplifier with 605 mW output power is demonstrated with an output impedance tuner composed of MEMS varactors. The output impedance tuner provides post-fabrication performance optimization to ensure the PA working at class-E mode with optimal power added efficiency of 52% at 10.5 GHz. A novel MEMS impedance tuner is also developed with 4 MEMS analog varactors and provides wide impedance coverage in K-Ka band. The tuner is optimized to obtain equal maximum rms voltages at the 4 varactors and achieve high power handling capability. The thesis is concluded by summarizing the main contributions and discussing the implications and directions of future work.