DO sweat the small stuff

My grandma always told me not to sweat the small stuff, and since she just celebrated her 100th birthday, I am inclined to take her advice.  Unfortunately, that advice does not extend well to 10GHz serial links on a PCB, where you really DO need to sweat the small stuff.  With edge rates on the order of 50ps (that’s about 300 mils long on a PCB), 10GHz signals get affected by almost any discontinuity on a board, even those less than 100 mils.  So, special care must be taken to ensure that the entire differential path is free from discontinuities.

The biggest discontinuity faced by a 10GHz signal is a via.  That means your differential via pair must be designed to as close to 100ohms as possible.  This can be achieved by using smaller barrels, removing non-functional pads, and spacing the vias an appropriate distance apart.  The best way to design and simulate a differential via pair is by using a full-wave 3D electromagnetic simulator, like the one integrated into the LineSim GHz via modeler.  Minimizing or preferably eliminating via stubs is also an essential part of the design.  This means selecting the right routing layers, or employing the use of backdrilling, or even using blind and/or buried vias.

I discuss this and other topics related to the successful implementation of 10GHz serial links in my article “Ten Steps to Ten GigHertz“.  Take a look if you are interested in learning more.

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This article first appeared on the Siemens Digital Industries Software blog at https://blogs.sw.siemens.com/hyperlynx/2014/11/06/do-sweat-the-small-stuff/