Research And Design Of A New GHz Multimode Ultra-wideband Common Mode Filter

Differential signaling scheme has been widely utilized to transmit high-speed digital signals due to its high immunity to external noise and crosstalk.However,in practical circuits,the common-mode(CM)noise is inevitable due to lots of asymmetric factors including the asymmetrical signal paths,unbalance of digital signals in amplitude and phase,which will seriously degrade the signal integrity of differential signals.To eliminate the CM noise,based on the asymmetric short-stub loaded resonator,dual coupling paths and the traditional microwave filter theory,two kinds of common-mode filters(CMFs)and new topologies for common-mode filter(CMF)are presented in this paper.The detailed research contents are summarized as follows:(1)In the asymmetric short-stub loaded resonator(ASSLR),a shorted stub is loaded at the position off the center point of a uniform impedance resonator to introduce multiple resonant modes,and the influence of the electrical length ratios on resonant frequencies is investigated.By applying the ASSLR to the CMF design,the initial CMF with four transmission zeros is proposed.To give a physical insight of CM noise suppression,the equivalent circuit models for the initial CMF are established and analyzed,and characteristic improvements for the initial CMF are also proposed.The modified CMF shows excellent performances including high differential-mode cutoff frequency,ultra-wideband CM noise suppression,and high CM rejection-level.(2)A novel CMF based on hybrid transmission line resonators with dual coupling paths is proposed to suppress wideband CM noise.To give a physical insight of the CMF operating mechanism,the whole structure is divided into two sub-CMFs with single coupling path,and the even-mode equivalent circuit models for them are established and analyzed respectively.Subsequently,according to the network analysis and transmission line theory,the formulas for CM transmission zeros are derived to give a comprehensive design procedure.The measured results show that the proposed CMF has multiple CM transmission zeros,ultra-wide CM stopband and high differential-mode cutoff frequency.(3)Based on the traditional microwave filter theory,new CMF topologies with one,two and four transmission zeros are proposed in sequence.By utilizing the odd and even-mode analysis,the CMF topology with four transmission zeros is analyzed in detail,and the formulas for differential-mode transmission coefficient and CM transmission zeros are derived to predict the topology performances,which has great significance on guiding the following CMF design.According to the simulations,the proposed CMF topology exhibits excellent performances including ultra-wide CM stopband,high CM rejection-level and high differential-mode cutoff frequency.In conclusion,the proposed CMFs in this dissertation can be used to keep the signal integrity of differential signals while suppressing the ultra-wideband CM noise for high-speed digital systems.