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Practical Differential Via Modeling Made Easy
Thursday, July 14, 2011 | Bert Simonovich, Lamsim Enterprises

You are a backplane designer and have been assigned to engineer a new high-speed, multi-gigabit serial link architecture from several line cards to multiple fabric switch cards across a backplane. These links must operate at 6GB/s on Day One and be 10GB/s ready for product evolution. The schedule is tight, and you need to come up with a backplane architecture to allow the rest of the program to progress on schedule.

You come up with a concept you think will work, but the backplane will be thick with over 30 layers. There are some long traces over 30 inches and some short traces of less than 2 inches between card slots. There is strong pressure to reuse the same connector you used in your last design, but your gut tells you its design may not be good enough for this higher speed application.

You are worried about the size and design of the differential via footprint used for these connectors because you want to maximize the routing channel through the connector field. This requires you to shrink the anti-pad dimensions so the tracks will be covered by the reference planes.  But you can’t easily quantify the consequences to the via of doing so.

You have done all you can think of, based on experience, to make the vias as transparent as possible without simulating. Removal of non-functional pads on the inner layers and planning to back-drill the connector via stubs will help, but is it enough? You know in the back of your mind the best way to answer these questions, and to help you sleep at night, is to put in the numbers.

So you decide to model and simulate the channel. But to do so, you need accurate models of the vias to plug into your favorite circuit simulator. But how do you get these? You have heard it all before: For high-speed, the best way to model a via is with a 3D electromagnetic field solver.  Although this might be true, what if you don’t have access to such a tool, because they cost more than your company wants to spend, or because you don’t have the expertise or the time to learn how to build a model you can trust to make a timely decision?

Furthermore, 3D field solvers typically produce S-parameter behavioral models. Since they represent only one sample of a given construction, it is impossible for you to perform what-if, worst-case, min/max analysis with a single behavioral model. Because of this, many iterations of the model are required, causing further delay in getting your answer.

A circuit model, on the other hand, is a schematic representation of the actual device. For any physical structure, there can be more than one circuit model to describe it. All can give the same performance up to some bandwidth. When run in a circuit simulator, it predicts a measurable performance of the structure. These models can be parameterized so that worst-case analysis can be explored quickly and easily.

In the past, it was next to impossible to develop a circuit model of a differential via structure without a behavioral model to calibrate it. These behavioral models were developed through empirical formulas, measured data, or through the use of 3D EM field solvers.

Now there is another way. As I promised you in my last column, “The Three Amigos: Twin-Rod, Rod-Over-Plane and Coax,” I will reveal how you can call on these newfound friends to develop equations, to help you calculate the parameters of a practical differential via circuit model.

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