A Novel Wideband Common-Mode Suppression Filter for Gigahertz Differential Signals Using Coupled Patterned Ground Structure

A wideband and compact filter design for common-mode noise suppression in high-speed differential signals is proposed based on two U-shaped and one H-shaped coupled patterned ground structure. An equivalent model of three coupled LC resonators to predict the common-mode suppression characteristics is also developed with good agreement to the full-wave simulation and measurement result. A test sample is designed and fabricated on a standard printed circuit board (PCB). It is found the common-mode noise can be reduced by the filter over 15 dB from 3.6 to 9.1 GHz and over 75% of amplitude in the time domain. More important, the differential signal integrity, in terms of insertion loss and group delay in the frequency domain and eye diagrams in the time domain, is not degraded within the wide bandwidth. The fractional bandwidth of the stopband can reach 87% and the filter size is about 0.44 lambda(g) x 0.44 lambda(g). The radiation caused by the common-mode current on the attached I/O cables is also efficiently suppressed by 10 dB on average within the designed stopband. To our best knowledge, it is the first low-cost common-mode filter designed for the gigahertz high-speed signals with the largest fractional bandwidth and most compact size on a PCB.