Analysis and Design Method of a Novel Absorptive Common-Mode Filter

In this paper, a bidirectional absorptive common-mode filter (A-CMF) is proposed to suppress the common-mode (CM) noise which would induce unwanted electromagnetic interference (EMI) and radio-frequency interference (RFI) problems. Different from the conventional CM filters (CMFs) which reflect the CM noise, the proposed A-CMF will absorb the CM wave, and thus suppress the radiation caused by reflected wave. An analysis and design method by using Smith chart with more physical insight is also proposed in this paper. To validate the proposed concept, the A-CMFs are implemented in the two-layer printed circuit boards. The absorbing frequency is selected at 2.4 GHz band where CM insertion loss is larger than 30 dB and the CM reflection loss is larger than 20 dB. With the proposed A-CMFs, more than 98% of the CM power can be eliminated. The proposed A-CMFs also provide a broadband CM suppression with fractional bandwidth lager than 126%, and a high bandwidth of differential-mode (DM) signal from dc to 7.4 GHz, which is about three times of the CM absorbing frequency. Eye diagrams are also measured to check the signal integrity, and the results show that the proposed A-CMF can still keep a good eye even under a high throughput of 7.5 Gb/s.