Differential Wideband Bandpass Filters With Enhanced Common-Mode Suppression Using Internal Coupling Technique

A differential wideband bandpass filter (WBPF) with enhanced common-mode (CM) suppression is presented. The study about this modified branch-line structure with internal coupling sections shows that the proposed filter demonstrates a good wideband bandpass filtering response under the differential mode (DM) operation, meanwhile it also functions as a CM bandstop filter with wide and high rejection owing to the multiple transmission zeros brought by the internal coupling mechanism. To verify the design concept, one filter example has been designed, fabricated and measured. The fabricated differential WBPF has a DM 3 dB fractional bandwidth (FBW) of 59.5%. Moreover, it demonstrates an ultra-wide CM stopband from 0.82 to 2.8 GHz over 40 dB, representing a 40 dB FBW of 109.4%.