Compact and Wide Upper-Stopband Triple-Mode Broadband Microstrip BPF

Abstract

A broadband microstrip bandpass filter (BPF) with compact size, sharp skirt and wide upper-stopband performance is proposed using the triple-mode stub-loaded resonator. The resonator can generate one odd mode and two even modes in the desired passband. Due to the main path signal counteraction, two transmission zeros T z1 , T z2 can be created near the passband edge to achieve sharp skirt. Two transmission zeros T z4 , T z5 are created between two harmonic frequencies h m2 , h m3 by the folded open stub and the interdigital coupling feeding lines, respectively. Owing to the source-load coupling, one zero T z3 is introduced to deepen the stopband. Meanwhile, the zero T z1 is shifted to the lower cut-off frequency and the other three ones T z2 , T z4 , T z5 can be slightly turned to suppress three harmonic frequencies h m1 , h m2 , h m3 . O ne filter prototype with the fractional bandwidth 34% is designed , fabricated and measured .