Recently I designed a couple of PCIe Bifurcation risers to solve some difficult build problems I was faced with, namely fitting a second PCIe card into a case which was only designed for one.
There are more pictures in this thread on Hardforums.
They work good for PCIe Gen 3, all I needed. They wouldn’t work for Gen 4.
D.I.Y. solutions for Gen 4 are largely out of reach for all but the most deep pocketed, determined enthusiasts. If you could design something like this, you certainly don’t need my help 😉
I figured there’s no good them sitting around in my archive, so I’ve uploaded the KiCad design files to GitHub, in the event anyone is considering attempting the same thing, and is looking for a starting point.
Note that I (unfortunately) ended up using a ZL40264 as a clock buffer which isn’t an ideal solution because it doesn’t have output termination, which added more complexity to the design. Why? Because of the damn chip shortage. I couldn’t get my hands on anything else. Unfortunately these chips are used in PCs and Laptops which we’re apparently buying rather a lot of at the moment. Ideally I would have used the 9DBL0442 from Renesas, but, 6 months since I built this, it’s still not obtainable.
Designing a PCB like this requires an understanding of high speed PCB design, which is a big subject area. I had attempted started writing up an easy-to-digest summary but it got too difficult so gave up on it.
Just the other day I came across a series of newly released YouTube videos by Zach Peterson from Altium. He says it better than I ever could. Pay more attention to the videos than my design. I did cut a number of corners.
Relevant clips (Definitely watch if attempting something like this):
- Length Tuning and Differential Pairs
- Advanced Differential Pairs Concepts
- Differential Impedance Deep Dive
- Alternative 4-layer Boards for High Speed PCBs
- To Pour or Not To Pour | Copper Pour in PCB Design
There are of course many others from other people. Check them out too.
PCBWay’s Impedance Calculator is another valuable resource.
Also worth a look if you’re not familiar with the more fundamental theories around character impedance:
(Yes that is a lot of stuff about impedance, but it helps to fully understand when you are trying to work out the design parameters of a high speed transmission line on a PCB).
Some more PCIe specific material:
I have also design an OCuLink to PCIe adapter. More about that here.