Triple- and Quadruple-Mode Wideband Bandpass Filter Using Simple Perturbation in Single Metal Cavity

Abstract

This paper proposes a novel class of wideband metal cavity multiple-mode bandpass filters (BPFs), consisting of a triple-mode filter and a quadruple-mode filter. Two ${\rm TE}_{11}$ degeneration modes in a single metal cavity are excited by using an off-centered approach instead of a traditional corner-cut approach. They work together with the ${\rm TM}_{10}$ mode to constitute a three-pole wide passband with a fractional bandwidth of 30% at the central frequency of 3.2 GHz. Moreover, an off-centered metal cylinder can properly excite an extra ${\rm TM}_{11}$ mode in the passband. As such, this ${\rm TM}_{11}$ mode, ${\rm TM}_{10}$ mode, and a pair of degeneration ${\rm TE}_{11}$ modes can be simultaneously utilized to form a quadruple-mode BPF. Compared with the triple-mode BPF, the quadruple-mode BPF has better performance in term of the steepness of rejection skirt. The fractional bandwidth of this BPF can achieve up to 31%. In final, the two proposed BPFs are fabricated by using the silver plated aluminum technology. Measured $S$ -parameter frequency response and group delay have satisfactorily matched with the simulated results.