High Selectivity Broadband Bandpass Filter Using Stub-Loaded Quadruple-Mode Resonator

Abstract

—In this paper, a compact and high selectivity broadband microstrip bandpass filter (BPF) using stub-loaded resonator is proposed. The basic block of the proposed filter is a quadruple-mode resonator consisting of a stepped-impedance resonator and three shunt open stubs. The proposed resonator can provide two even-mode, two odd-mode frequencies and two transmission zeros. The even-mode resonant frequencies can be flexibly controlled by the centre stub, while the odd-mode frequencies are fixed. The two other symmetrical stubs can both affect even- and odd- mode frequencies. The proposed filter exhibits high selectivity profiting from the two transmission zeros (TZs) near the passband created by two types of stubs. The measured –3 dB bandwidth is 1.0 GHz at the centre frequency of 2.5 GHz, and the S ₂₁ exhibits a wide spurious free response up to 6 GHz. Measured results show good agreements with EM simulation results.