Multimodal microwave filters

Abstract

This thesis presents the conception, design and implementation of new topologies of multimodal microwave resonators and filters, using a combination of uniplanar technologies such as coplanar waveguide (CPW), coplanar strips (CPS) and slotlines. The term "multimodal" refers to uniplanar circuits in which the two fundamental modes of the CPW propagate (the even and the odd mode). By using both modes of the CPW, it is possible to achieve added functions, such as additional transmission zeros to increase the rejection, or to attenuate harmonic frequencies to improve the out-of-band rejection. Moreover, it is demonstrated that by using multimodal circuits, it is possible to reduce the length of of a conventional filter up to 80%. In addition to bandpass filters, new topologies of compact band-stop filters are developed. The proposed band-stop filters make use of slow-wave resonators to decrease the total area of the filters and achieve compact topologies. This work also addresses the development of synthesis techniques for each multimodal filter. The design equations were obtained from generalized multimodal circuits available in the literature, which have been adapted for each particular case and modeled as basic filter components, such as immitance inverters or lumped elements. By using the proposed synthesis equations, it is possible to design filters with a desired response and relative bandwidth. The use of the proposed synthesis enables a fast analysis and design of multimodal filters using circuit simulators. As an added feature, several reconfigurable and tunable filter topologies were demonstrated, using active devices (PIN diodes and varactors) or RF-MEMS. These new topologies demonstrate the flexibility of multimodal circuits. For the RF-MEMS-based tunable filters, different capacitive and ohmic switches were designed, fabricated and measured. As an example of the additional degrees of freedom using of RF-MEMS and multimodal CPW circuits, a reconfigurable filter using RF-MEMS switchable air-bridges as a reconfiguration device has been demonstrated in this work for the first time.