A high-selectivity dual-band filter with closely spaced passbands

A high-selectivity dual-band filter with closely spaced passbands implemented by two double-slot-loaded resonators(DSLR) is presented. The zero-feed structure functions as input/output structure resulting in improved frequency selectivity. The loaded open stub moves the second passband close to the first passband. The open parallel coupling microstrip line is used to offer coupling between two resonators, meantime, it generates transmission zero in upper stop-band. The etched slot line in the grounded plane provides a degree of freedom to control the performance of two passbands. The operational principle is described and the design steps are concluded. A prototype filter is designed and fabricated. Measured results of the fabricated BPF are found in agreement with the predicted ones.