Mechanically Tunable RF/Microwave Filters: from a MEMS Perspective

• Main Author: Yan, Dong
• Format: Others
• Language: en
• Published: 2007
• Subjects: Tunable filter; RF MEMS; latching mechanisms; thermal actuators; Electrical and Computer Engineering
• Online Access: http://hdl.handle.net/10012/3113

RF/microwave tunable filters are widely employed in radar systems, measurement instruments, and communication systems. By using tunable filters, the frequency bandwidth is utilized effectively and the system cost and complexity is reduced. In the literature, various tuning techniques have been developed to construct tunable filters. Mechanical tuning, magnetic tuning, and electrical tuning are the most common. In terms of quality factor, power handling capability, and linearity, mechanical tuning is superior to the other two tuning techniques. Unfortunately, due to their bulky size, heavy weight, and low tuning speed, mechanically tunable filters have limited applications. MicroElectroMechanical Systems (MEMS) technology has the potential to produce highly miniaturized tunable filters; however, most of the MEMS tunable filters reported so far have a relatively low quality factor. The objective of the research described in this thesis is to investigate the feasibility of using MEMS technology to develop tunable filters with a high quality factor. The integration of MEMS tuning elements with a wide range of filter configurations is explored, from micromachined filters to traditional dielectric resonator filters, from planar filters to cavity filters. Both hybrid integration and monolithic integration approaches are carried out. To achieve tunability, MEMS tuning elements are embedded within RF and microwave filters. Tuning is accomplished by disturbing the electromagnetic fields of resonators with nearby MEMS tuning elements, which in turn change the resonant frequency of the resonators. First, the proposed tuning concept is experimentally demonstrated by integrating a surface micromachined planar filter with MEMS thermal actuators as the tuning elements. Then, a novel micromachined ridge waveguide filter embedded with similar MEMS tuning elements is proposed and constructed by using the EFAB^{TM} micromachining technique. A power handling analysis is performed for the newly devised 3D micromachined filter, and potential failure mechanisms such as air breakdown are identified. For the first time, a tunable dielectric resonator bandpass filter, incorporating vertical long-throw MEMS thermal actuators as tuning elements, is developed to achieve a wide tuning range, high quality factor, and large power handling capability. Several prototype tunable filter units are fabricated and tested. The experimental results reveal that the tunable filters maintain a relatively high quality factor value over a wide tuning range. In addition to the hybrid integration approach, a monolithic integration approach is investigated. A novel surface micromachining process is developed to allow monolithic integration of MEMS tuning elements into micromachined filters. Due to a stress mismatch, MEMS actuators fabricated by this process obtain a vertical deflection of several hundred microns, resulting in a wide tuning range. Various latching mechanisms are created, based on the micromachining processes that are used to fabricate the MEMS tuning elements. These out-of-plane latching mechanisms with multi-stable states have the potential to be adopted not only for tunable filter applications but also for switches and phase shifters.