New dual-mode dual-band bandpass filter with quasi-elliptic function passbands and controllable bandwidths

New dual-mode dual-band bandpass filters with quasi-elliptic function responses are devised based on a single stepped-impedance resonator. The center of the resonator is grounded through a short high-impedance section which is equivalent to an inductor for providing coupling between the resonant modes. In addition, in order to increase the degree of freedom for the dual-band design, an interdigital capacitor is introduced so that both center frequencies and bandwidths of the two passbands can be controlled. Moreover, with the aid of source-load coupling, two extra transmission zeros can be created close to the passband edges and hence greatly improve the filter selectivity between the two passbands. Two filters are fabricated and measured to validate the proposed idea. The measured results show good agreement with the simulation.