Design of dual-mode dual-band bandpass filter with independently tunable bandwidths and reconfigurable filtering characteristics

In this paper, design of a tunable dual-mode dualband microstrip bandpass filter is presented. The designed filter is constructed by using two nested dual-mode resonators (DMRs) having two patch capacitances located at the lateral arms of the resonators as reference elements. Varactor diodes are used instead of perturbation elements in order to excite the degenerate modes. Linear phase and quasi elliptical filtering characteristics can be independently obtained in each passband and the band-widths can also be tuned at both filtering characteristics. For the experimental verification of the designed structure, a dual-mode dual-band microstrip bandpass filter was fabricated and measured. Center frequencies of the passbands were adjusted to 1.78-2.65 GHz and 1.83-2.7 GHz for linear phase and quasi elliptical filtering characteristics, respectively. Insertion losses in each passband were observed as better than 3 dB in measurements.