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July 2016 • The PCB Design Magazine 37 Those layer transitions often create the largest impedance discontinuities seen in the signal path, and play a key role in the accuracy of the overall channel model. Full-wave 3D solutions are computationally intensive, and it can easily take several hours to extract a set of differential pairs coupled to- gether. The 3D meshing done along the long uniform PCB traces is a significant culprit in the overall solution time, and is unnecessary for these largely 2D structures. A "cut-and-stitch" methodology can be used to focus the more ex- pensive 3D field solution where it is warranted, while traditional 2D techniques can be used to quickly solve uniform transmission lines. This "full wave where you need it" approach, com- bining multiple field solvers, can be automated by cutting out the serial link path from the over- all design, breaking it up into "cutting zones," and designating those zones for the solver of choice. The project can be automatically bro- ken down, solved in pieces, and re-combined into a composite S-parameter. With accurate S-parameter extraction com- pleted, pre-layout test benches can quickly be updated, incorporating the detailed intercon- nect models into the analysis. Similar attention needs to be paid to device modeling. Multi-gigabit interfaces generally uti- lize equalization to achieve the required signal quality. Interfaces running over 10Gbps usually use adaptive equalization techniques, where the equalization behavior changes over a period of time. Traditional time step-based circuit simu- lation cannot run enough data traffic in order for the equalization to adapt, produce enough samples in the eye distribution, and enable bit error ratio (BER) to be extrapolated. These types of interfaces require channel simulation, which uses impulse response techniques to simulate high-capacity traffic. Channel simulation also enables IBIS-AMI device models to be used. These types of device models incorporate exe- Figure 7: 3D via array structure. Figure 8: Cut and stitch approach. SIGNAL INTEGRITY TOOLS AND DESIGN METHODOLOGY IN THE MODERN AGE