Tunable dual-mode dual-band filter with constant bandwidth and high selectivity

A tunable dual-mode dual-band filter composed of varactor-loaded folded square loop resonator is presented. A pair of the resonators is mirror coupled to form the higher band, while two varactors with the same bias are symmetrically loaded at the corner of each loop, splitting dual modes and form the tunable lower band. As the bias voltage increases, the lower band frequency increases from 1.1 GHz to 1.5 GHz with a constant bandwidth, while the higher band one remains at 2.1 GHz. The dual-mode effect and source-load coupling introduce multiple transmission zeros that effectively improves the out-of-band selectivity. (C) 2016 Wiley Periodicals, Inc.