A dual-mode dual-band ring resonator bandpass filter with controllable in-between isolation

In this paper, a dual-mode dual-band bandpass filter with controllable in-between isolation is presented using a single microstrip ring resonator. By installing two dispersion-controlled coupled-line sections on the ring resonator at the two ports with 90°-separation, two transmission poles can be easily emerged and distributed in each passband. Meanwhile, two transmission zeros are generated between the two passbands, which can be used to control the rejection level of the in-between isolation. Instead of perturbing the gap distance between one of the open ends of the coupled-line section and the ring, a pair of open-circuited stubs is further installed along the symmetrical plane of the ring resonator, which can effectively enlarge the tuning range of the two transmission zeros. A prototype dual-band filter is finally designed, fabricated, and verified through the experiment.