Miniaturized Wideband Bandpass Filter Utilizing Square Ring Resonator and Loaded Open-Stub

In this paper, a miniaturized wideband bandpass fllter utilizing a square ring resonator and loaded open-stubs is proposed. One pair of bent open-stubs characterized as perturbations is installed outside the diagonal corners of the ring, and another pair of loaded open-stubs is added inside the ring resonator. By stretching the perturbation stubs more than half-wavelength of the ring, three pairs of degenerated modes in a ring are split for wideband operation. The flrst two split modes form the dominant passband. Meanwhile, the loading efiect introduced by the loaded open-stubs could move the third split mode into the dominant passband. At the same time, an additional transmission zero is generated by the loaded open- stub, which improves the skirt selectivity. Due to the applying of the perturbation stubs as long as more than half-wavelength, this kind of wideband microstrip ring resonator fllter occupies a smaller size than those conventional ones that based on ring resonator. To verify the mechanism above, a wideband bandpass fllter centered at 3.5GHz is designed, implemented, and fabricated. Measured results of experimental circuit show good agreement with simulated responses.