A Comprehensive Investigation of a Common-Mode Filter for Gigahertz Differential Signals Using Quarter-Wavelength Resonators

This paper presents a comprehensive investigation on an inexpensive and wideband common-mode noise suppression filter that uses a quarter-wavelength resonator. An equivalent transmission line model is also used to evaluate the effectiveness of the proposed structure in a common-mode filter (CMF). Moreover, more important parameters, such as coupling coefficients are presented to investigate the characteristics of CMF unit. The performance of CMF is then characterized using two coupling coefficients between two differential traces and between differential traces and a quarter-wavelength resonator. A simple design approach is also presented for a wideband CMF. Numerical results demonstrate that CMF with three different lengths of quarter-wavelength resonators have rejection bands of 3.94-8.74 GHz and 3.53-10.1 GHz at cutoff frequencies of -20 dB and -10 dB, respectively Time-domain common-mode noise is decreased by approximately 60%. Moreover, analyses of differential insertion loss and group delay indicate that the differential signals maintain sufficient signal integrity when the wideband CMF is used in single- and two-pair differential interconnects. Finally, CMF is validated by an eye diagram and by measurements of time/frequency-domain common-mode noise and I/O cable common-mode current.