| Summary: | 博士 === 國立成功大學 === 電機工程學系碩博士班 === 90 === This research is about the design of compact microwave planar filters with low passband insertion loss, high stopband rejection, better shape factor, transmission zeros at certain frequencies, and tunable higher-order responses. All components (resonators and feed lines) and their layouts (tapped-line feed positions, coupling structures between resonators, etc.) have been investigated.
To begin with, stepped-impedance resonators have been thoroughly studied. Two equations for odd-mode and even-mode resonance are derived from a network model. The size and resonant frequencies of the resonator could then be determined based on these two equations. A cross-coupled miniaturized hairpin filter with an extra tunable transmission zero has been designed to verify these equations. A new resonator-embedded cross-coupled filter, constructed by stepped-impedance hairpin resonators and miniaturized hairpin resonators, has also been proposed. These two filters are very compact and the positions of their first spurious responses are successfully tuned to be higher than the double of their center frequencies. Furthermore, the level of the first spurious response of the new resonator-embedded filter is also lower than those of conventional filters.
Secondly, the feed structure of filters has been investigated. A skew-symmetric (zero-degree) feed structure, which can create two extra transmission zeros, has been proposed. This new feed structure is analyzed by using transmission matrices. It is found that the two transmission zeros are near and on the opposite sides of the passband, and hence the out-of-band rejection of the designed filter is significantly improved. Two second-order filters and the previous fourth-order filters have been designed or modified with the new feed structures to demonstrate the applications. The theoretical analyses are successfully verified by these experiment results. Furthermore, a method for tuning the frequencies of these two transmission zeros using impedance transformers has been proposed. Relations between the circuit parameters and the position of the transmission zero are discussed. Design equations of the impedance transformers for creating a desired transmission zero are also presented. Two second-order hairpin filters with different impedance transformers have been designed to demonstrate this approach.
The last part of this thesis involves the analysis of two types of coupled-line structures: coupled lines with loads at one end and folded coupled lines. The equation for the transmission zero of coupled lines with loads at one end has been derived from its network model. Based on this equation, coupled lines with different loads are analyzed and the rules for controlling the transmission-zero frequency are given. This type of coupled-line structures is experimentally verified with several second-order filters. The use of the skew-symmetric feed structure in these filters is also discussed and an example is given. Next, a folded coupled-line structure, which can create a transmission zero, has also been studied. The estimate of its transmission-zero frequency is given. A compact second-order filter with these folded coupled-line structures and a skew-symmetric feed structure has been designed. The improved shape factor and out-of-band response of this new filter are compared with those of a conventional second-order filter. Finally, this new filter topology is applied to the design of diplexers to illustrate the other applications.
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