Study On Common Mode Suppression Methods For High Speed Interconnect Networks

The upgrading of semiconductor technology has promoted the rapid development of digital communication systems.The process has developed toward high density,high speed,low power consumption and high power output.At the same time,the system clock frequency has been continuously increased,and the rise and fall times of drivers have been continuously reduced and transmitted.The everincreasing rate has put forward a requirement for noise immunity to the system.How to ensure the normal operation of high-speed digital systems and normal communication between systems has become an important issue in system design.When the input and output data rates reach the order of Gbps,the differential technology is widely used in high-speed digital circuits due to its characteristics such as clock and data recovery.At the same time,differential signal lines are widely used in high-speed interconnect standards due to their strong anti-noise capability and strong anti-EMI interference capability.However,in the actual circuit design,the wiring design such as the trace bend will cause the difference of the differential line length,causing the transmission delay difference,which causes some differential mode signals to be converted into common mode signals,resulting in common mode noise.Therefore,it is necessary to design a reasonable model for the differential line corner to suppress the common mode noise of the differential line.In this paper,the causes of common mode noise in high-speed interconnected networks are studied.Two differential line corner models based on the principle of step impedance are studied and designed.Firstly,based on the study of stepped impedance structure,a right angle corner model based on asymmetric stepped impedance structure is proposed.The working principle is analyzed and the equivalent circuit is given.The model increases the transmission delay of the differential line by periodic impedance changes,reduces the transmission delay difference between the internal and external traces,and suppresses the differential signal from being converted into a common mode signal.The consistency of the simulation and test results verifies that the proposed structure has the performance of suppressing common mode noise.Secondly,based on the study of actual circuit traces,a 135-degree angular corner model based on asymmetric stepped impedance structure is proposed.The simulation results show that the structure can not only suppress the conversion of partial differential signals to common-mode signals,but also ensure the difference.Excellent transmission characteristics of the signal.At the same time,the asymmetric stepped impedance structure has the characteristics of small size,low cost and small damage to the original structure,and is suitable for the differential interconnection corner design of the high-speed circuit.