The Study Of Common-mode Noise Filters For High-speed Differential Signals

With the ever upgrading of semiconductor manufacturing process,large capacity,ultra-wideband network information system platform,together with the continuous expanding of innovative design chain,the modern high-speed digital communication systems have been developed rapidly in recent years.When the I/O signaling enters Gbps magnitude,the stringent requirements of noise immunity and dislocation clock contribute to the implementation of differential signaling technology with clock and data recovery functions.Compared with the single-ended signal transmission,differential signaling has excellent noise immunity performance.It is insensitive to the return path and also has electromagnetic interference cancellation feature.However,the different line lengths,etching changes,proximity effects,trace bends and other factors in actual circuits,will inevitably lead to the asymmetry of differential lines and result in the common mode noise(CMN).The harmful CMN will degrade the signal/power integrity of high-speed circuits and may also cause serious electromagnetic compatibility problems.Therefore,wideband CMN suppression with good differential transmission is desired in high-speed circuit design.In this paper,three kinds of filters are designed for CMN suppression in high-speed differential circuits.Firstly,based on the traditional dumbbell-shaped defected ground structure(DGS),a novel wideband CMN suppression filter is designed.A periodic stub-loaded structure is introduced to compensate the discontinuity of the odd-mode characteristic impedance.It also contributes to the suppression of CMN.In order to enhance the common-mode stopband and maintain the circuit size,the slow-wave structures are utilized along the differential traces.Secondly,a low cost wideband CMN filter is designed by etching coplanar waveguide(CPW)quarter-wavelength resonators on the ground plane.The equivalent circuit model considering the coupling between differential traces and quarter-wavelength resonators,as well as the CPW discontinuities,is introduced to evaluate the effectiveness of the proposed structure.Also,wideband CMN filter is achieved by cascading two units.Thirdly,based on the Complementary Split Ring Resonator(CSRR),a wideband CMN filter is proposed based on ? pattern DGS,an equivalent circuit model is proposed and the working principle is given.The simulated and measured results demonstrate the good performance of the CMN filter.