Quadruple-mode broadband BPF with sharp skirt and wide upper-stopband performance

Abstract

In this article, a broadband bandpass filter (BPF) with sharp skirt and wide upper-stopband performance is proposed using the quadruple-mode stub-loaded resonator.The resonator formed by attaching three stepped-impedance open stubs to high impedance microstrip line can generate two even-mode resonant frequencies fm1 and fm3 and two odd-mode ones fm2 and fm4 in the desired band. The fm2 can be approximately determined by the high impedance microstrip line. The open stub at the center plane can only control the even-modes and two identical open stubs are mainly applied to adjust the resonant frequencies fm3 and fm4. Two transmission zeros are created near the cutoff frequencies by the open stub at the center plane and two interdigital coupling feeding lines, leading to sharp shirt. Two additional transmission zeros are generated in the upper-stopband by two identical open stubs and two interdigital coupling feeding lines to suppress two main high harmonic resonant frequencies. A broadband filter prototype is simulated, fabricated, and measured. The measured results agree well with the electromagnetics (EM) simulations. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26051