A Wideband Microstrip Common-Mode Suppression Filter Using Signal Interference Techniques

A fully-planar common-mode (CM) suppression filter is presented. Because of destructive interference between two different CM signal paths, multiple transmission zeros are created to reject CM signals. The positions of transmission zeros can be controlled, and the desired rejection level and wide stopband can be obtained by adjusting the characteristics impedance of the transmission lines of the topology. The design concept is validated through the development of a microstrip prototype. The fabricated filter exhibits a measured CM rejection better than 20 dB in a 44% bandwidth and a measured DM insertion loss lower than 0.5 dB within the CM stopband.