Investigations On The Miniaturized High-Performance Microstrip Filters

With the rapid development of wireless and mobile communication technology, the tendency of modern microwave and millimeter-wave communication systems is miniaturization, low-weight, reliable, multifunctional and low-cost. As an important part of system, the filters with high-quality and miniaturized size have caught greater attention from the world, and have been investigated intensive and extensive in this dissertation from three aspects. Aiming at designing miniaturized and high-quality filter, this dissertation provides several solutions from three aspects.Firstly, a miniaturized and high-quality filter can benefit a lot from the optimization of the resonator. This dissertation proposed several miniaturized resonators including a quasi-lumped resonator, several dual-mode resonators, a multi-mode resonator and a composite resonator. The proposed quasi-lumped resonator is not only compact in size, but is convenient to generate transmission zeros to compose quasi-elliptic response. The presented dual-mode resonator is developed from an open-circuited or short-circuited half wavelength resonator. When it is under certain perturbation, a quasi-elliptic response with transmission zeros to improve the transition can be obtained using the transmission zero technique given in the dissertation. Further, a multi-mode resonator composed of a half wavelength resonator loaded with two shunt series LC circuit is also investigated intensive. Its resonant characteristics and application in wideband filter design are also studied extensive. In addition, a composite resonator and its application in dual-band filter design are also explored in the dissertation. Measurement results prove that such filters based on these above-mentioned resonators are all small in size, easy to fabricate, low in loss, and of high selectivity.Secondly, with the help of extra transmission zeros to compose quasi-elliptic function, a compact low-order filter can meet the performance requirements that are usually achieved by high-order filters. To realize quasi-elliptic function in the filter design, the method of adjacent cross coupling to provide two out-phase coupling path between two adjacent resonant node is discussed in this dissertation to create finite transmission zeros. Applying this method on bandpass filter and lowpass filter design, this dissertation provided several compact filter topologies with excellent performance. The measured results show good agreement with the simulated results and validate the structure of the proposed filters.Finally, a periodical Electromagnctic Bandgap structure named as CMRC (compact microstrip resonant cell) is employed to compose lowpass filter. Using this periodical CMRCs, and owing to a simple tapering consisting of a periodicity break attained by varying the size of the near- and far-end CMRC sections, the proposed structure has successfully addressed simultaneously the problems of compactness, matching, selectivity, insertion loss, and the suppression of spurious frequency bands.With all the good features, these above proposed filters are effective to realize miniaturized high quality filter, and promising for future microwave communication systems.