Researches On Novel Planar Multiband Filters

Recent advances in the development of communication industry, especially the newly developed wireless local area networks (WLANs), have greatly stimulated the multiband communication systems. The multiband bandpass filter plays an important role in a multiband transceiver, and many researches about it have been carried out. This dissertation gives a detail study on the design technique of planar multiband filters, some resonators with easily controlled resonance frequencies and coupling structure are presented, and different multiband filters of good characteristics are designed using these resonators. The main contributions of the dissertation can be summarized as follows.(I) Based on the mechanism of the transmission zero, the pseudo-interdigital coupling structure is analyzed in detail. This structure can generate transmission zeros in the insertion loss response to improve the selectivity of the filter, and exhibits a size reduction of 50% compared with the traditional direct coupling structure at the same frequency. High performance dual-,tri- and quad-band filters using pseudo-interdigital coupling structure are presented in this dissertation.(II) Based on the lumped equivalent circuit of the multiband filters, assembled resonator is proposed to design multiband filters, and the mechanism of the transmission zeros is explained concretely. Without a significant increase in circuit size, the coupling structure can introduce transmission zeros between passbands, and the bandwidth of each passband can be tuned in a wider range because of the extra coupled sections.(III) Based on the lossless transmission line model analysis, the resonant characteristics of different kinds of stub-loaded resonators are presented. Due to the large size of the open stub-loaded resonator, the short stub-loaded resonator is analyzed and used to design the dual-band filters. Due to the difficultly controlled center frequencies of the tri-band filter, crossed resonator which contains an open stub and a short stub is proposed to design tri-band filters, the passband frequencies of which can be tuned conveniently. It is found that the first three resonance frequencies of the crossed resonator can be controlled. Benefiting from this feature, the resonator can be utilized to design not only the tri-band bandpass filter, but also the UWB filter and cascaded triplet filter. (IV) The stub-loaded resonator and its application to high-order dual-band bandpass filters with independently controlled characteristics are presented. Usually, the two passband bandwidths of the dual-band filter are determined by the same coupling structure, therefore, the bandwidths of two passbands are related, thus can't be controlled independently. A Novel multi-stub loaded resonator is presented to design the dual-band filter, and a key merit of the filter configuration is that the characteristics (passband frequency and bandwidth) of the second passband can be conveniently controlled whereas that of the first passband are fixed. As the multi-stub loaded resonator contains six symmetric stubs which can provide sufficient coupled sections between adjacent resonators, it is realizable to build the high-order dual-band filters using the proposed resonators.