| Summary: | 博士 === 國立成功大學 === 電機工程學系碩博士班 === 94 === This research is about the improved coupled-line filter designs including the correction for the distortions caused by different even- and odd-mode phase velocities and the development of synthesis procedure for dual-band filters. This thesis starts from the fundamental studies of the effects of resonator Q and the concept of component Q distribution is introduced. The effects of component losses for the resonator in each stage of filters are evaluated, and they are found quite different on the transmission response in the entire passband, especially at the band edges. The critical components should undoubtedly be paid more attention to and be carefully manufactured. On the other hand, the others could be replaced with low-Q alternatives for the cost and circuit size reduction. When resonators with different Q are used in a filter, it was found that a bell-shaped Q distribution should be selected to achieve optimal passband responses.
In the second part of this thesis, the coupled-line filter synthesis is first reviewed. The coupled-line sections at the outer stages of the filter are treated as impedance transformers, and some designs for the improvement on the tight couplings are suggested. When coupled microstrip-line structures are employed in filter designs, the difference in the mode velocities is inevitable. This may cause the degradations in the passband such as increased reflection and altered bandwidth. A new concept of effective even- and odd-mode characteristic impedances is then introduced for improving the passband responses. This approach has taken the effects of velocity difference into account and allows the microstrip filters designed with the classical filter synthesis procedures.
In the last part of this thesis, rigorous synthesis methods of distributed dual-band filters are developed. Different topologies of transmission-line circuits such as the combinations of open and short stubs in series and parallel are fully investigated for the implementation of dual-band resonators. Suitable structures of dual-band inverters are also given and can be easily merged with the adjacent resonators. In the dual-band filter designs, the coupled-line circuits are the most important elements since they have the properties of the resonators and inverters, and thus can make the dual-band filters more compact. In order to increase the feasibility of the coupled-line circuits, several stepped-impedance coupled-line structures are proposed and their equivalent circuits and design equations are then derived, which are helpful to the dual-band filter designs. The characteristics of the proposed dual-band filters are analyzed and compared, and all of them have been experimentally verified.
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