Novel Design of Microstrip Bandpass Filters with a Controllable Dual-Passband Response: Description and Implementation

Sheng SUN  Lei ZHU  

IEICE TRANSACTIONS on Electronics   Vol.E89-C   No.2   pp.197-202
Publication Date: 2006/02/01
Online ISSN: 1745-1353
DOI: 10.1093/ietele/e89-c.2.197
Print ISSN: 0916-8516
Type of Manuscript: PAPER
Category: Microwaves, Millimeter-Waves
controllable dual-passband,  dual-band filter,  equivalent J-inverter network,  parallel-coupled microstrip line,  stepped-impedance resonator,  

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Novel microstrip dual-band bandpass filters with controllable fractional bandwidths and good in-between isolation are presented and implemented. A half-wavelength stepped-impedance resonator is firstly characterized, aiming at producing the two resonant frequencies at 2.4 and 5.2 GHz. Two types of coupled microstrip lines in the parallel and anti-parallel formats are then investigated in terms of unified equivalent J-inverter network. Extensive results are derived to quantitatively show their distinctive frequency-distributed coupling performances under different coupling lengths. The coupling degrees of these two coupled lines at the two resonances are properly adjusted to achieve the dual-passband response with varied or tunable bandwidths. In addition, the parallel coupled line is modeled to bring out a transmission zero between the two resonances so as to achieve the good in-between isolation. The three two-stage bandpass filters are initially designed to exhibit their dual-band response with changeable dual-band bandwidths. A three-stage dual-band filter is in final optimally designed and its predicted performance is confirmed in experiment.