Why We Simulate

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When I was cutting my teeth in high-speed PCB design some 25 years ago, speeds were slow, layer counts were low, dielectric constants and loss tangents were high, design margins were wide, copper roughness didn’t matter, and glass-weave styles didn’t matter. We called dielectrics “FR-4” and their properties didn’t matter much. A fast PCI bus operated at just 66 MHz.

As speeds increased in the 1990s and beyond, PCB fabricators acquired software tools for designing stackups and dialing in target impedances. In the process, they would acquire PCB laminate libraries, providing proposed stackups to their OEM customers late in the design process, including material thicknesses, copper thickness, dielectric constant, and trace widths—all weeks or months after initial signal-integrity simulation and analysis should have taken place.

Speeds continued to increase in the 2000s; design margins continued to tighten, and OEM engineers began tracking signals in millivolts (mV) and picoseconds (ps). Figure 1 illustrates these trends starting in 2000, emphasizing the trajectory of PCI Express trajectory, from PCIe 3.0 in 2010 to PCIe 6.0, which is just on the doorstep.

In webinars and training events I often pose this question: “Why do we simulate?” I ask because the answers tell me a lot about the audience, and some wise older person long ago told me and my fellow students to “always know your audience.”

When I ask that, I get answers like faster signaling speeds, calculating impedance or loss, opening eyes and avoiding inter-symbol interference, controlling crosstalk, etc. These are all good answers, but a bit on the periphery in my view.

One astute signal integrity practitioner offered that we simulate for only two reasons:

  1. To make design decisions (i.e., evaluate tradeoffs during design).
  2. To verify a design before manufacturing (verification).

The only question that designers really care about is, “Will it work and by how much?” This implies that the simulation should be able to produce tangible metrics that can be related to design success or failure. Fair enough.

This is a good description for “why we simulate,” but so far, I’ve never heard anyone mention the most fundamental reason, in my opinion, for signal integrity (SI) or power integrity (PI) simulation: To predict the negative impact that the physical world has on the electrical world, and to mitigate or prevent the negative effects proactively.

To read this entire article, which appeared in the April 2021 issue of Design007 Magazine, click here.



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