A compact common-mode filter for GHz differential signals using defected ground structure and shorted microstrip stubs

A miniaturized common-mode suppression filter for GHz differential signals is realized using two C-shaped and one Hshaped defected ground structure (DGS), and one of the C-shaped DGS units is combined with two symmetrical shorted microstrip stubs to further reduce the area occupied. The equivalent transmission line model of the DGS unit with shorted microstrip stubs is established with good agreement to the full-wave simulation. It is found that the length of the stub and resonant frequency is inversely proportional. A sample board is fabricated on FR4 substrate. It is proved that the common-mode noise can be reduced by over 15dB from 3.68GHz to 8.43GHz, and the differential signal is not affected. The filter size is about 0.29λgX 0.32λg, and the fractional bandwidth of the stopband is 78%. It is the first common-mode filter using shorted microstrip stubs with the most compact design to our best knowledge.