Taylor, Richard Roy (2007). COMPACT BAND PASS FILTERS USING DIELECTRIC RESONATORS AND HIGH TEMPERATURE SUPERCONDUCTING MATERIALS PhD Thesis, School of Information Technology and Electrical Engineering, University of Queensland.

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Author Taylor, Richard Roy
School, Centre or Institute School of Information Technology and Electrical Engineering
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Dr John Ness
Abstract/Summary Mobile wireless service providers are facing a problem. Subscriber numbers are growing exponentially and the new services offered to mobile users require increasingly complex networks. Existing microwave filters cannot meet the emerging needs of higher filter selectivity combined with lower insertion loss, and new technologies must be found quickly. We need to ask what new technologies will provide microwave filters for mobile wireless services with superior performance? This thesis contends that new materials and innovative design can produce microwave filters with superior capabilities to meet the demands of the new generation cellular networks. It demonstrates that a novel integration of high temperature superconductor materials and new ceramics used as dielectric resonators will allow the design and manufacture of compact, very sharp, narrow bandwidth filters in the 800 MHz to 3 GHz range. Such filters are suitable for application in mobile wireless base stations. The thesis is a multidisciplinary study which builds on the theoretical models for high temperature superconductors and their application to microwave design, and on models of dielectric resonators in cylindrical cavities. High temperature superconductors, because of their low losses, enable the design of filters with high Qo, and consequently very low insertion loss. A relevant question is, "What properties of ceramic materials make them appropriate for microwave applications?" As a starting point, to determine the relative permittivity of ceramic materials at microwave frequencies, a test device was designed and built, and the microwave properties of a number of ceramic materials were studied. Since high temperature superconductor materials exhibit their superconducting properties at cryogenic temperatures, a test environment with cryocooler, vacuum chamber, and associated measurement and control electronics was designed and built for the study of microwave high temperature superconductor devices and cooled ceramic dielectric resonators. If ceramics are to be used with high temperature superconducting materials, it is necessary to understand their behaviour when cooled. The test environment can also be used to investigate a range of cryogenic cooling systems, metal filter holders, spacers and clamps, and thermal insulation materials and their performance, all at low temperatures under vacuum. Thin film microstrip filters suitable for receive filters and for moderate power throughput transmit filters, plus resonant structures with a ceramic dielectric resonator in a cylindrical cavity were built, measured using the c~yogenic test environment and analysed. For the investigation of the microwave properties of a dielectric resonator in a cavity, high-capability desk-top algebraic analysis software was first used to assist in determining the relative permittivity of the ceramic, and subsequently to identify the modes to be measured. The results demonstrate that substantial miniatusisation can be achieved with the use of high temperature superconductors and dielectric resonators, either separately or together. Such miniaturization enables the designer to combine the advantage of the high Q of high temperature superconductor material and certain ceramics and the consequent advantage of low filter insertion loss with the high selectivity that results from using a large number of resonators. Thin film high temperature superconductor filters, and dielectric resonator cavity filters are suitable for the design of transmit filters which need to handle higher power throughput. In the course of this work, further areas for study have emerged of significance to the ongoing application of these materials to microwave filter design. Will such filters meet the emerging needs of mobile wireless service providers? Extensive acceptance of high temperature superconducting products in commercial applications requires that cryocoolers be economically priced, reliable and appropriately priced for the application. Reported data indicate that cryocooling units in the field are reliable enough to be acceptable to wireless network providers. Consequently, the necessity for incorporating a cryogenic system will not be a barrier to wide application of the filters. This thesis shows that by exploiting the characteristics of high temperature superconductors and ceramics at cryogenic temperatures, filters with low insertion loss and high selectivity can be developed to meet the emerging needs.

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Created: Fri, 21 Nov 2008, 16:00:17 EST