Compact wide-stopband balanced bandpass filter with intrinsic common-mode suppression

Abstract

A compact balanced bandpass filter (BPF) with wide-stopband and intrinsic common-mode (CM) suppression is proposed. The balanced bandpass filter is composed of a pair of shunt-connected coupled-line resonators. By short circuiting at the middle point of one side of the shunt-connected coupled-line resonator, therefore CM signal suppression is realized under differential- mode (DM) excitation. In addition, the coupled feeding position of the filter is chosen at the point which divides the branch of the single coupled-line resonator with the ratio of 1:2 and consequently wide-stopband is achieved. In addition, three transmission zeros (TZs) are generated to improve the skirt-effect. To validate the idea, the equivalent transmission line model of the proposed filter is given and the design equations are derived for theoretical analysis. Finally, the proposed balanced filter is fabricated for experiment, and the measurement and simulation are in good agreement based on which the design is verified.