Concepts for and Synthesis of Liquid Crystal-Tunable Ka-band Filters with Independently Tunable Center Frequency and Bandwidth

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The number of radio services is steadily increasing with rapidly changing requirements in terms of frequency band allocation as well as bandwidth on demand. To meet these requirements, future communication systems such as satellites as well as 4. and 5. generation´s base stations, which have long operational times by implementation, require flexibility not only in the baseband, but also in the RF-frontend, e.g. by using reconfigurable components. A key component is the input filter. State-of-the-art are filter banks with a multitude of bandpass filters, which cover a plurality of parameters such as center frequency and bandwidth. The design is based on a fixed number of control states (i.e. filters) that can only be set discreetly and do not allow subsequent reconfiguration of filters or frequency band allocations. Furthermore, the large volume and weight of filter banks is a major problem especially for satellites. By using electronically controllable filters, parameters such as center frequency and bandwidth can be adjusted continuously within a certain tuning range. This allows an almost unlimited number of reconfigurations of the RF frontend. Furthermore, the volume and the weight of the filter is only a fraction of a filter bank. The primary objective of this research proposal is the analysis and synthesis of electrically-controllable liquid-crystal (LC) based filters reconfigurable in center frequency and bandwidth for satellite communication applications in the Ka-band. The LC technology, which had been established at TU Darmstadt provides two major benefits for the desired application in a satellite communication payload: (1) The liquid crystals have been optimized for millimeter-wave applications, showing very low losses, and thus, allowing a high filter quality. (2) Those liquid crystals are already space-approved, i.e. they are radiation hard, and therefore, suitable for the implementation into components of a satellite payload. The LC-filters to be synthesized in this project are controllable in center frequency as well as in bandwidth. Therefore, it is necessary to investigate different approaches for the realization of tunable coupling apertures and resonator structures. Suitable control electrodes are than developed with the aim to achieve the highest possible quality factor and a maximal tuning range. The LC-based higher order filter is than merged from individual investigated apertures and resonator geometries. The best suited filter topology is investigated using the coupling matrix approach. The fine tuning of the complete filter structure at the end of the synthesis process is done in a full-wave simulator. Therefore, the model has to be simplified in such a way that simulations can be accomplished in a suitable time. An investigation of the reconfiguration of the filter in operation mode as well as the implementation of a diagnosis algorithm for the realization of the thermal stability is provided at the end of this project.