Common-Mode Noise Reduction Schemes for Weakly Coupled Differential Serpentine Delay Microstrip Lines

This paper proposes design schemes to reduce the common mode noise from weakly coupled differential serpentine delay microstrip lines (DSDMLs). The proposed approach is twofold: we leverage strongly coupled vertical-turn-coupled traces (VTCTs) instead of weakly coupled VTCTs (conventional pattern) and add guard traces. Time-and frequency-domain analyses of the proposed schemes for reducing the common-mode noise are performed by studying the transmission waveform and the differential-to-common mode conversion using the circuit solver HSPICE and the 3-D full-wave simulator HFSS, respectively. Compared to the conventional design of the weakly coupled DSDMLs, the proposed solutions yield a reduction of about 54% of the peak-to-peak amplitude of the common-mode noise, while the differential impedance remains matched along the complete length of the DSDML. Moreover, the range of frequencies, over which the magnitude of the differential-to-common mode conversion is now significantly reduced, is very wide, i.e., about 0.3-10 GHz. Furthermore, the differential insertion and reflection loss introduced by the newly proposed designs are almost the same as the ones achieved by using the conventional design. Finally, a favorable comparison between simulated and measured results confirms the excellent common-mode noise reduction performance of the proposed schemes.