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Analyzing and Designing Vias for Multi-Gig Interconnects
January 27, 2010 |Estimated reading time: 1 minute
Vias are a necessary evil in all circuit boards. They enable transitions for signals between layers. But if not designed correctly, the vias can also be the weakest reliability link and the weakest signal integrity link.
When vias are electrically short, and rise times are long, they can be invisible to the signal. But, with 6-10 Gbps signals and vias as long as 5 mm, these tiny structures can sometimes dominate the performance of a 1 meter interconnect.
In this application note, Dr. Yuriy Shelpnev, the Electromagnetics Guru, walks you through a new way of thinking about the electrical performance of vias.
Yuriy points out that the return path for the signal is just as important as the signal via. For single-ended signals, adding adjacent return vias will keep the return current "localized" to the signal via. This design feature not only makes the analysis possible, it can dramatically improve the performance of the signal via.
S-parameters are the most complete description of interconnects above 1 GHz. Return loss is a measure of the reflected signal and insertion loss is a measure of the transmitted signal. Yuriy shows how a 3D field solver can accurately predict the insertion loss and return loss of via structures by comparing his simulations to measurements. He shows that at 5 GHz, not adding the return vias to localize the return current can increase the return loss by more than 15 dB.
The measurements from a specially designed test board are used as a reference to compare with the simulations. The agreement between the measurement and the simulation for single ended and differential vias is excellent, even as high as 20 GHz.
To learn these secrets from the Electromagnetics Guru, read his app note here.
Yuriy Shlepnev is president and CEO of Simberian Inc., which will be exhibiting in Booth 915 at DesignCon 2010. He will be presenting during the conference portion of the show.