Tunable CPW Ring Resonator-Based Bandpass Filters Utilizing RF MEMS Varactors

Abstract

Radio Frequency (RF) Micro-Electro-Mechanical Systems (MEMS) switches are mostly used in many applications such as: tunable filters, phase shifters and oscillators because they offer wide bandwidth, low insertion loss, excellent isolation and outstanding linearity in comparison with their electronic counterparts. The objective of this research is to design and implement tunable RF MEMS based bandpass filters (BPF) using coplanar waveguide (CPW) ring resonator with both frequency control, and to achieve tunable BPF that demonstrate low insertion loss, high return loss, high quality factor, and compact size. Tunable bandpass filters that are RF MEMS based have the potential to advance wireless communications and radar systems due to their superior RF performance in comparison with conventional tunable filters. Four new types of bandpass filters using CPW technology with open ring resonators are proposed in this work which being tuned using RF MEMS capacitive switches to achieve wide tuning range with multiple DC biasing circuits. The advantage of designing the filter with CPW structure is having the ground plane and the signal conductor on the same level. This feature allows achieving compact structures in comparison with microstrip technology and will allow the integration with other technology such as RF MEMS. Two tunable ring resonators and two second order BPF are introduced in this thesis. The ring resonator that is the basic block of the proposed tunable filters is of a circular shape and open type where the RF MEMS capacitive switches are placed in the open area of the ring resonator between the edges to tune the electrical length and achieve tunability. For the second order BPF, two ring resonators are coupled with two couplers and a CPW waveguide. All proposed filters are built on an Aluminum Oxide (Al2-O3) ceramic substrate with the thickness of 675 μm. The first and second designs of tunable ring resonator are designed at 2 GHz. These two designs are used in wireless communications, WiMAX (Worldwide interoperability Microwave Access), mobile GSM and ISM bands. The third and fourth designs of tunable bandpass filter are designed at 20 GHz, and they can be used in mobile satellite applications.