COMPACT WIDEBAND BANDPASS FILTER WITH QUADRUPLE-MODE STUB-LOADED RESONATOR

In this letter, a novel compact wideband bandpass filter (BPF) is proposed using quadruple-mode resonator formed by attaching a short-circuited stub at the center plane and two identical impedance-stepped open stubs to high impedance microstrip line. The resonator can generate two even-modes frni, f(m3) and two odd-modes f(m2), f(m4) in the desired band. The even-mode resonant frequencies can be flexibly controlled by the short-circuited stub, whereas the odd-mode ones are fixed. When the open stubs are attached to the center plane nearby, they can be mainly applied to adjust f(m3), f(m4) into desired passband as the high odd-mode and even-mode resonant frequencies are vulnerable to their electronic lengths. Two transmission zeros near the lower and upper cut-off frequencies are separately created by the short-circuited stub and interdigital feeding lines, leading to a high rejection skirt. A wideband BPF with the fractional bandwidth 64% is simulated, fabricated and measured. The measured results agree well with the EM simulations.