Rules of thumb for 28 Gbps

Rules of thumb are probably not going to cut it anymore for high-speed designs with interfaces like USB 3.0, PCIe 3.0 and other serial links (now approaching 28Gbps on the next generation FPGA’s). So here are some rules of thumb anyways 🙂

This comes from Jim Nadolny of Samtec in this recent webinar moderated by Eric Bogatin.

Jim Nadolny: As a good rule of thumb, the eye will probably be sufficiently open with no equalization if the insertion loss at the Nyquist is no more than –7 dB. There should be about 20 dB SNR so the acceptable cross talk should be less than about –25 to –30 dB in this case. Using pre-emphasis only, you can recover an acceptable eye with about –12 dB insertion loss at the Nyquist. And with FIR, CTLE and DFE, you can recover an acceptable eye with about –25 dB insertion loss at the Nyquist.

By “Nyquist” Jim means 14GHz for a 28GBps signal.

So – just as an example – how much is 7dB of loss, really? Link losses in a 4 mil PCB transmission line vary from roughly 0.7 to 1.5dB/in at 14 GHz. This obviously depends on both copper and dielectric, but this is taken over typical types of copper and high-speed dielectrics (Isola FR408HR and better).

With 7dB loss end to end, we have to allocate a bit for the connectors and vias, so we end up with maybe 3dB for the trace losses or 2-4 inches (5-10 cm). As a rule of thumb.

If we allow 25dB loss, and still only 4-5dB for connectors and vias, the acceptable trace length is 13-28 inches (33-72cm). Again as a rule of thumb. Obviously losses for longer traces are a lot more sensitive to the dielectric loss (TanD).

These losses are a combination of skin effect, dielectric losses and copper roughness (which can be seen as a variation of skin effect loss). Dielectric losses dominate, with copper roughness/skin effect also playing a role for the low loss materials at 14GHz. As another one of the high-speed rules of thumb.