Miniature Microstrip Bandpass Filters Based on Capacitive Loaded Coupled-Lines and Lumped-Element K-Inverters

Yo-Shen LIN  Chien-Chun CHENG  

IEICE TRANSACTIONS on Electronics   Vol.E90-C   No.12   pp.2218-2225
Publication Date: 2007/12/01
Online ISSN: 1745-1353
DOI: 10.1093/ietele/e90-c.12.2218
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Recent Technologies for Microwave and Millimeter-wave Passive Devices)
bandpass filter,  capacitive loaded,  cross-coupled,  K-inverter,  lumped-element,  parallel-coupled,  

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This study presents a class of miniature parallel-coupled bandpass filters with good selectivity and stopband rejection. Capacitive terminations are introduced to the conventional anti-parallel coupled-lines, and lumped-element K-inverters are employed, to achieve both size reduction and spurious suppression. Additionally, the capacitive cross-coupling effect can be introduced to obtain three transmission zeros to enhance the selectivity. Suitable equivalent-circuit models, along with design formulae, are also established. Specifically, via design examples, this work demonstrates the feasibility of proposed filter structures in microstrip configuration. Compared to the conventional parallel-coupled filters, the proposed filters exhibit over 60% size reduction, improved selectivity, and wider stopbands up to four times the center frequency.