A New Broadband Common-Mode Noise Absorption Circuit for High-Speed Differential Digital Systems

A novel concept of the absorptive common-mode filter (A-CMF) is proposed for solving electromagnetic interference and RF interference problems in high-speed differential digital systems. The A-CMF is realized by a balanced four-port circuit with both horizontal and vertical symmetry. The common-mode (CM) noise can be terminated (or absorbed) by the resistors in the A-CMF and is thus transferred to heat. In addition, the signal integrity of differential signals can be well preserved by the A-CMF. Theory and design procedures are developed to determine corresponding element values of the A-CMF. A design sample is realized by an integrated passive device process on glass substrate. The circuit size is only 1 mm². Results of measurement and full-wave simulation have good consistency. It will be found that the CM power loss (or absorption) ratio is over 80% from 4 to 14 GHz in measurement, and the differential-mode cutoff frequency can maintain up to 9 GHz with insertion loss, which is good enough for high-speed digital system applications. Finally, the eye diagram at the A-CMF output does not degrade significantly even when the differential data rate is up to 5 Gb/s.