High selective compact dual-band filter using ring resonator with quarter-wavelength stubs

A ring resonator loaded with five open stubs is proposed for the design of dual-band bandpass filters. To shrink the filter layout, a pair of loaded quarter-wavelength stubs is utilised to produce the first passband. On the other hand, the introduced coupling between the quarter-wavelength stubs and the ring resonator produced transmission zeros for enhancing the selectivity. Both bandwidth and central frequency ratio of the two passbands can be adjusted freely by the circuit parameters. Two coupling lines are added beside the resonator as feed-lines to construct a filter with high selectivity and compact size. A filter at 1.57/2.45 GHz is designed, fabricated and measured for verification.