A common mode filter using DGS-based coupled multiple resonance points for all-pass differential circuit

An ultra wideband common-mode filter based on coupling multiple resonance points is proposed in the letter. The coupling multiple resonant points are realized by four compact defected ground structure (DGS) cells, of which both the ends are C-type DGS cells and the middle are H-type DGS cells. The structure and size of these DGS units are different, which leads to their different resonance points. Because of the close distance between DGS cells, coupling inductors are generated between them, which can be used for smoothing filtering. A coupled LC resonator equivalent circuit model is built to explain the filter's working principle. The DGS-based filter with an area of 20 mm x 12 mm are implemented under the differential microstrip line. The simulated results shows the filter can achieves a wide 15-dB common-mode suppression of frequency range from 2.9 to 12.6 GHz. Good agreement between full-wave simulation and measured results is observed.