Tunable balanced bandpass filter with high common-mode suppression

This Letter presents a novel tunable balanced bandpass filter with high common-mode (CM) suppression by using discriminating coupling. Four half-wavelength resonators terminated with varactors are utilised to obtain the tunable responses. The high CM suppression is realised by discriminating coupling between feeding structures and resonators, which blocks the CM signals. An experimental tunable balanced bandpass filter is implemented which achieve the continuous tuning range from 0.84 to 1.15 GHz and more than 50 dB CM rejection levels.