A Dual-Band Balanced Filter With High Common-Mode Suppression

A dual-band balanced filter with high common-mode (CM) suppression is designed in this paper. Besides, a hybrid cavity-microstrip structure is proposed to realize the function of differential feeding and mode coupling. The cavity acts as one dual-mode resonator, and the differential feeding networks are realized on the PCBs. Based on the analysis of electromagnetic electric coupling coefficient, TE101 and TE011 modes are generated under differential-mode (DM) excitation, and highly suppressed under CM excitation. For demonstration, a dual-band balanced filter is designed and measured. The measured results indicate that the center frequencies of the two bands are 8.22 GHz and 10.04 GHz, respectively. The insertion losses are 0.67 dB and 0.85 dB, and the CM suppressions in both passbands are under 50 dB.