This dissertation presents the research on several different projects. The first project is a via-less CPW RF probe pad to microstrip transition; The second, the third, and the fourth one are reconfigurable microwave circuits using RF MEMS switches: an X-band reconfigurable bandstop filter for wireless RF frontends, an X-band reconfigurable impedance tuner for a class-E high efficiency power amplifier using RF MEMS switches, and a reconfigurable self-similar antenna using RF MEMS switches. The first project was developed in order to facilitate the on-wafer measurement for the second and the third project, since both of them are microstrip transmission line based microwave circuits. A thorough study of the via-less CPW RF probe pad to microstrip transition on silicon substrates was performed and general design rules are derived to provide design guidelines. This research work is then expanded to W-band via-less transition up to 110 GHz. The second project is to develop a low power reconfigurable monolithic bandstop filter operating at 8, 10, 13, and 15 GHz with cantilever beam capacitive MEMS switches. The filter contains microstrip lines and radial stubs that provide different reactances at different frequencies. By electrically actuating different MEMS switches, the different reactances from different radial stubs connecting to these switches will be selected, thus, the filter will resonate at different frequencies. The third project is to develop a monolithic reconfigurable impedance tuner at 10 GHz with the cantilever DC contact MEMS switch. The impedance tuner is a two port network based on a 3bit-3bit digital design, and uses 6 radial shunt stubs that can be selected via integrated DC contact MEMS switches. By selecting different states of the switches, there will be a total of 2^6 = 64 states, which means 64 different impedances will be generated at the output port of the tuner. This will provide a sufficient tuning range for the output port of the power amplifier to maximize the power efficiency. The last project is to integrate the DC contact RF MEMS switches with self-similar planar antennas, to provide a reconfigurable antenna system that radiates with similar patterns over a wide range of frequencies.

In modern broadband microwave applications, receivers are very sensitive to interference signals which can come from the system itself or from hostile emitters. Electronically tunable bandstop filters can be used to eliminate these interference signals with adaptation to changing frequency conditions. In this thesis, electronically tunable bandstop filter design techniques are investigated for microwave frequencies. The aim is to find filter topologies which allow narrowband bandstop or &lsquo / notch&rsquo / filter designs with low-Q resonators and with tuning capability. Tunability will be provided by the use of electronically tunable capacitors, specifically varactor diodes. For this purpose, firstly direct bandstop filter techniques are investigated and their performances are analyzed. Then phase cancellation approach, which enables high quality bandstop filter design with lossy circuit elements, is introduced and analyzed. Lastly, a novel notch filter design technique called as all-pass filter approach is introduced. This approach allows a systematic design method and enables to design very good tunable notch filter characteristics with low-Q resonators. Three filter topologies using this approach are given and their performances are analyzed. Also prototype tunable notch filters operating in X-Band are designed and implemented by using these three topologies.