HIGH SELECTIVITY BROADBAND BANDPASS FILTER USING STUB-LOADED QUADRUPLE-MODE RESONATOR

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.