Common-Mode Filters With Interdigital Fingers for Harmonics Suppression and Lossy Materials for Broadband Suppression

Common-mode (CM) noise on differential microstrip lines can be suppressed by PCB-embedded filters. These filters use resonating or broadband structures such that only the field from the CM signal couples to them. Based on a quarter-wavelength resonator, a new PCB filter is designed having an electrical size of only 0.15 lambda x 0.065 lambda, where lambda is the effective wavelength in the CM filter. Its interdigital structure is also capable of suppressing higher order harmonics of the CM signal and can be used for USB 3.0 to mitigate electromagnetic interference. Further, a novel broadband suppression structure is described that uses magnetically lossy material to suppress the CM signal from 4.6 to 20 GHz without strongly affecting the intended differential signal.