Analysis And Design Of High-Speed Differential Transmission Line Models

In the low-speed circuits, the impedance discontinuity of microstrip and striplinedon’t have an impact on the signal transmission. However, as the data transfer raterising to the GHz, the single-ended signal lines will influenced by various noise sources.In contrast, differential lines have a variety of advantages. Today’s high-speed digitalcommunications use low-voltage differential signal pairs (LVDS) as high-speed datatransmission lines，which is used to improve signal integrity and reduce EMC.Due to the intensity of component placement and routing complexity increased, inthe actual design of PCB, unavoidably there will be a corner in the line, and those makethe length of the two transmission lines inconsistent which will cause a transmissiondelay. If the length deviation exceeds the space extension by20%, it will makes part ofthe differential signal edge distortion and makes a differential signal into a commonmode signal，which will cause common mode noise and lead to signal integrity issueand radiation effects. For the generation of common mode noise, this paper comparesfour models，put forwards its own model, and use S-parameter to make an analysis of itscommon-mode noise. Then this paper describes a method of compensation capacitorand makes an analysis of it. Finally, two new compensation methods are proposed andthe experiments show that the first compensation method is not effective to reducecommon noise due to the distance between transmission lines have been changed.However, the second compensation method is effective to reduce the asymmetrybetween the differential lines, which can suppress differential to common-mode signalconversion. At the same time, the physical structure of differential vias and how itaffects the physical structure of the signal transmission quality are detailed analysis inthis paper, including impact of the pore size, pads and anti-pad of differential vias onsignal integrity, so as to determine that we should choose the differential vias with asmall pore size, a small pad radius and an anti-pad radius, which provide a basis for thedifferential vias design on differential lines.The proposed compensation method for the corner on differential lines and thestudy of differential vias design in this paper have provided a theoretical basis for thedifferential transmission lines.