Design of Pseudo-Elliptical Wideband Bandpass Filter Using Stub Loaded Short-Circuited Parallel-Coupled Three-Line Units

Chun-Ping CHEN  Junya TAKAHASHI  Zhewang MA  Tetsuo ANADA  Jui-Pang HSU  

IEICE TRANSACTIONS on Electronics   Vol.E93-C   No.7   pp.1022-1031
Publication Date: 2010/07/01
Online ISSN: 1745-1353
DOI: 10.1587/transele.E93.C.1022
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Recent Progress in Microwave and Millimeter-Wave Technologies)
filter synthesis,  pseudo-elliptical filter,  ultra-wideband bandpass filter,  stub-loaded,  parallel-coupled three-line,  

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The purpose of this paper is to propose an efficient and effective design scheme to implement compact, high-performance wideband bandpass filters based on a novel compound three-line unit consisting of a stub-loaded short-circuited parallel-coupled three-line structure and two lead lines. Firstly, a simulative investigation is conducted on the transmission characteristics of the proposed coupling unit. The results show that the bandwidth of such structure can be predicted by a symmetric parallel-coupled short-circuited three-line unit, whereas the transmission zeros by a three-section stub composed of the loaded stub, one of the parallel-coupled three lines and the lead line. Accordingly, given specifications, a pseudo-elliptical filter can be designed in an novel three-coupled-line based two-step design scheme: 1. after the derivation of the new closed-form synthesis formulae, a Chebyshev ultra-wideband (UWB) filter is synthesized on a desired passband using symmetric three-line coupling units. 2. By designing the stubs and choosing the proper lengths of the lead lines, multiple transmission zeros are then introduced to improve the skirt and stopband characteristics, whereas the equiripple characteristics are kept in passband. As an example, a UWB bandpass filter covering the Japan's lower UWB band (BW: 3.1-4.8 GHz, FBW: 43%) is designed to describe the proposed design procedure. The measured filtering characteristics agree very well with the theoretical predictions, which validate the effectiveness of the proposed new coupling structure and corresponding filter design technique. In addition, the designed filters exhibit good characteristics, such as steep skirt selectivity, very wide stopbands, a compact size compared with the filter based on short-circuited three-line structure, etc.