A narrow bandpass balanced filter with back-to-back structure

A narrow bandpass balanced filter with good common-mode suppression using back to back structure is designed in this paper. Under the differential- or common-mode excitation, the proposed configuration is separated as a bandpass filter or a bandstop characteristic, respectively. Resonators are coupled with each other through four apertures which are cut away from the common ground. One open stub is added in the middle to construct different electric lengths so as to suppress the common-mode response. Finally, a narrow bandpass balanced filter is designed at the center frequency of 1 GHz and 3-dB FBW of 10% with a good common-mode suppression more than 30 dB.