MIT/PCM-based RF Switches for Tunable Filter Application

Abstract

Tunable filters are promising for reducing the size of communication systems by replacing switch filter bank, also they are gaining more attention for satellite applications which will need to change the frequency bands. One of the main requirements for tunable filters is to maintain low passband insertion loss and high stopband rejection over the tuning range. The key to achieving this goal is using high quality factor (high-Q) resonators such as three-dimensional (3D) structure resonators (dielectric resonators, coaxial resonators, waveguide resonators, etc.). For high-Q tunable filters, not only do the resonators need to have high unloaded Q, the tuning elements also need to have low loss. Over the past years, various tuning techniques have been employed to realize tunable filters, including solid-state tuning, mechanical tuning, piezoelectric tuning, magnetic tuning, and MEMS tuning. All of these techniques have both advantages and drawbacks in terms of insertion loss, DC power consumption, tuning speed, non-linearity, and reliability. The objective of the research presented in this thesis is to investigate the feasibility of using metal-insulating transition (MIT) material and phase-change material (PCM) to realize high-Q tunable/ reconfigurable filters for base station in communication and satellite applications. Fabrication methods of MIT material VO2 (Vanadium Dioxide) and PCM GeTe (Germanium Telluride) are first investigated to obtain optimal MIT/PCM materials with high resistivity ratio between different states. RF switches based on both VO2 and GeTe integrated with micro-heaters are then developed. VO2-based variable attenuators are realized at both X-band and Ka-band. A switch capacitor bank is also designed and fabricated, integrating GeTe RF switches and MIM (metal-insulator-metal) capacitors in a six-layer microfabrication process. A novel structure for wideband dielectric substrate-based filters is developed. The cavity of the filter is separated into two thin dielectric substrates (DSs), a metal frame for holding the dielectric substrates, and two metal covers. The filter is compact in size and ideal for integration with antennas because of its relatively thin profile. Since it uses separated DSs attached to the two sides of the metal frame, the input/output probe can be placed at the middle of the cavity, leading to a significant increase in the input/output coupling compared to conventional dielectric resonator filters. Additionally, the filter structure lends itself to realizing relatively large values of inter-resonator coupling. A five-pole Chebyshev filter with a fractional bandwidth of 9.6%, a four-pole filter having two transmission zeros with a fractional bandwidth of 9%, and a three-pole filter with a fractional bandwidth of 13% are designed, fabricated and tested. The filter structure promises to be useful in sub-6 GHz 5G base station applications that require the use of low-cost, miniature, high-Q wideband filters with a thin profile. Furthermore, a novel tuning structure with multiple strip lines is developed in this work. The tuning structure is designed to employ RF switches to tune 3D filters, eliminating the need to use variable capacitor loading, which is known to degrade the filter’s loaded Q over the tuning range. Two- and three-pole filters with combline configuration are designed, fabricated and measured with wire bonding, MEMS switches, and VO2 switches. With wire bonding, all of the tuning states demonstrate measured Q higher than 2000, whereas with the fabricated VO2 switches, the measured Q was lower. The test data confirm however the tunability and feasibility of using the proposed tuning scheme to realize a reasonable high-Q tunable filters that maintain Q values over the tuning range. A DS-loaded three-pole tunable filter is designed, fabricated, and measured. The results show the potential of realizing a tunable dielectric filter with a low loss VO2-based switch.