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

摘要

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 hot cycling 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 Vdc 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.