Analysis and Design of Microwave Reconfigurable Filters in ESIW Technology

University essay from Högskolan i Gävle/Avdelningen för elektronik, matematik och naturvetenskap; Högskolan i Gävle/Avdelningen för elektronik, matematik och naturvetenskap

Abstract: Microwave filters are essential components in high frequency communication systems. The features required for these devices are increasing due to the increase in frequency, caused by the saturation of the electromagnetic spectrum. Among these features, the need for low cost devices with a reduced mass and volume and the need to integrate them with the current planar technology are highlighted. In addition, it is interested to have reconfigurable devices, that is, they can adjust their frequency response, replacing the need of multiple devices. So far, metallic waveguide filters have been used, but lately Substrate Intregated Waveguide (SIW) technology has appeared, and within this family, the innovative Empty Substrate Integrated Waveguide (ESIW). ESIW technology is empty, so that, it can be filled with liquid crystal (LC). The LC is a dielectric material with variable permittivity, which makes the filter can change its center frequency and bandwidth. There are several commercial tools based on numerical methods that enable to carry out the analysis and design of these structures, but they require a very high computational time during the analysis process. This affects negatively the automated design of these structures. On the one hand, an efficient and accurate analysis tool is developed in the thesis by following a strategy that consists on dividing the device under analysis in simple building blocks: waveguides filled with dielectric material, change of medium, discontinuities between guides and dielectric discontinuities. All of them are canonical structures or sufficiently simple that can be analyzed with modal methods. The Generalized Scattering Matrix (GSM) of each block is obtained, and they are linked in cascade by using an efficient technique. The accuracy and effectiveness of this tool are checked using it to analyze multiple filters, comparing the result with a commercial software. Furthermore, an analysis about how the variation of the permittivity of the liquid crystal affects in the frequency response of the filter is made. On the other hand, a tool for designing filters for high frequency communications is developed, in order to integrate it into a Computer Aided Design (CAD) tool. To do this, the classical techniques for designing waveguide filters are followed by adapting them to the new topology under design. Different synthesis and optimization strategies are implemented. These strategies are based on the synthesis of a starting point, the segmentation of the structure under design and the hybridization of different optimization algorithms. Also, a tool for calculating the values ​​of the permittivity of the LC that allows the reconfiguration of the filter is developed. These new permittivities enable to obtain a new desired frequency response. This tool has been used to design various microwave filters on H plane, using ESIW technology filled with liquid crystal. The accuracy and the effectiveness of this tool are checked. In addition, the reconfiguration tool designed is evaluated. Finally, a study of the manufacturing aspects is done, such as how to feed the filter and the solutions to polarize the liquid crystal. Taking into account all these considerations a prototype is manufactured and measured.

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