A new common-mode EMI suppression technique for GHz differential signals crossing slotted reference planes

Based on a pair of symmetric quarter-wavelength stubs parallel connected to the slot line, a new low-cost technique for suppressing the common-mode electromagnetic interference (EMI) is proposed for high-speed differential signals crossing slotted ground planes. The quarter-wavelength resonator can provide a shorting path for return current around the resonance frequency of the stubs, which avoids common-mode noise to become the excitation source of the slot. To validate this idea, a test sample with using the proposed technique and a reference board without the solution are fabricated. The experimental and numerical results show the radiated energy due to the common-mode noise can be significantly reduced in broadband frequencies by using the propose resonators. The magnitude of common-mode EMI at 3 meter can be suppressed more than 5 dB at the frequency range from 2.1 GHz to 3.5 GHz, and even 10 dB during the frequency range of 2.4 GHz to 3.25 GHz.