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Benefits of Signal Integrity Simulation

Benefits of Signal Integrity Simulation

By Timothy Coyle

Why You Need to Simulate Today’s Digital Systems

Increasing system design complexity has made design margins smaller and smaller. With faster edge rates and increased bus frequencies designers are faced with advanced material loss and every interconnect in the system looking like a transmission line. With more loss comes less design margin for setup and hold timing as well as shrinking noise level margin.

Most designers today are faced not with just one interface but multiple interface technologies across multiple platforms. You might have a DDR2 memory interface and a multi-gigahertz differential interface on the same system. With DDR memory interfaces you typically have tight timing requirements so you need to carefully understand how the skew between your different signal groups is going to affect your final setup and hold time. With multi-gigahertz differential interfaces you need to worry about meeting a minimum eye diagram requirement and need to understand how the material loss in your PCB laminate, connector, and other interconnect are going to close your eye.

In the chart above you can see how device complexity has naturally increased with frequency. We went from PCI to DDR2 with on-die termination to LVDS with differential signaling and then on to SerDes like PCI Express with equalization. Each new device complexity requires another layer of management from the designer in order to optimize the performance of the overall system.

So what’s a designer to do? This is where simulation comes into play. Simulation is all about virtual prototyping, allowing the designer to do what-if analysis on the flow and to not only predict the behavior of a system but also to influence it. Should you use loosely coupled or tightly coupled differential pairs? What on-die termination for your DDR interface will give you the most margins? These question and more can be answered by using simulation.

We’re not talking about pushing buttons with a blind eye towards sound engineering but about using simulation intelligently. Simulation gives you a window to view what’s really going on in a system. Used at the right times it can save you from costly board spins and a failing product.

References and Links

I’m going to recommend one of my favorite books on Signal Integrity if you want to learn more about the theory and practical analysis behind Signal Integrity analysis.

“High-Speed Digital System Design: A Handbook of Interconnect Theory and Design Practices”, Hall, Hall, and McCall. ISBN # 978-0471360902.




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