Signal Integrity Analysis On High-Speed Digital Circuit With Board-Level Multiple Load Termination

With the increasing complexity of the functions of the electronic system and the continuous improvement of the signal data rate,the memory consumption of terminal devices are forced to increase and the design of DDR circuit is also stricter.There are many different function lines in high-speed parallel bus of DDR,and the number of loads mounted on them is also different.There is much discontinuous impedance on high speed circuits with board-level multiple loads,which make signal integrity more and more serious.Because of the limitation of the routing space on the PCB and the complex package chip,the design of circuit with board-level multiple loads is more and more difficult.The balance between cost and performance is the biggest contradiction on high-speed circuit,but it is a key factor to promote the development of signal integrity subject.In this thesis,an optimal design method for reflection and crosstalk is proposed on high speed circuits with board-level multiple loads,which is caused by discontinuous interconnection and many traces network.And it improves the performance of signal transmission under the requirement of low cost design.(1)In view of the reflection problem on the multiple loads circuit,the formation mechanism of the reflection is analyzed by electromagnetic wave.The main contradiction of optimization is between matched impedance and the cost control about the reflection problem.This thesis focuses on the Fly-by topology,and proposes a new reflection optimization method.By controlling the time delay,the positive and negative reflection of each reflection node can be offset to each other,and the number of matched impedance is reduced.First,according to the transmission path of signal,the expression of reflection coefficient and transmission coefficient of each reflection node is derived theoretically in fly-by topology.Then we analyze the influence of stub delay and main transmission line delay on signal transmission performance.The method is given that the positive and negative reflection signals are cancelled each other.Finally,the feasibility and effectiveness of the scheme are verified by simulation.(2)Aiming at crosstalk in multiple loads circuits,it is often used to suppress crosstalk to add guard trace with grounded via in engineering.But it also increases the cost of the circuit.This thesis analyzes the important role of guard trace in crosstalk noise suppression,and focuses on the research of grounded via in micro-strip line and coplanar line.The optimal method of grounded via number in guard trace is proposed.We eliminate high cost caused by overdesign of guard trace.First,the optimal number of grounded via is determined by analyzing the mechanism of crosstalk formation for the micro-strip line.Then,according to the coplanar line structure without ground plane reference,the optimal number of via in the coplanar guard trace is determined by extracting the circuit parameters of the coplanar line and analyzing the transmission path of the signal.Finally,the effectiveness of the optimization method about the number of grounded via in the micro-strip and coplanar guard trace is verified by the three-dimensional simulation software HFSS.According to the research,It reduces the reflection noise and crosstalk noise on high speed circuits with board-level multiple loads.And the number of grounded via on guard trace is optimized.It not only improves the performance of the circuit,but also effectively reduces the design cost of the circuit.The theories of the research are verified by simulation,and the achievements can be directly applied to the engineering design on high speed circuits with board-level multiple loads.