Hello
When tracking a PCIe plug-in card, is it best to: a) Try to make each lane the same length with tromboning, or b) Make the traces as short as possible?
I have set the track and gap parameters (and the layer build) to get a 100 Ohm differential impedance, but wonder if the tromboning (adding zig-zags to the tracks) is detrimental.
Ta.
These guys might know something:
Have Fun! Rich
All of the pairs should match each other in length. And short as possible is also good too.
That is not the way to do it. Differential impedance does not matter (contrary to popular belief), take a look at this for starters:
Dave.
Wow! That is contrary to popular belief and what we've been told by other academics.
Maybe I've missed something, but I was under the impression that the impedance should be controlled as impedance discontinuities will cause signal reflection. As the impedance is normally set to 100 Ohm differential in all other parts of a PCIe system, then I should maintain this on my PCIe card (not my first PCIe design, but the most complex one I'm working on).
I think that linked 1st article is a tad misleading. It does conclude by saying that you need two signals of equal length and of a specified characteristic impedance. By far the easiest way to achieve this is to use closely coupled signals. And I'm not sure saying 12.5 mm for trace length differences is ideal as this is 1/10 of a bit flight time.
I thank you for the link(s) but will likely continue to place my traces in the 'accepted' way.
messagenews: snipped-for-privacy@n58g2000hsf.googlegroups.com...
Correct. But the point being made in the article is that they can be treated as two separate terminated lines, therefore any differential impedance becomes irrelevant. Even for a seemingly "differential termination" like that in the LVDS example.
Yes, I agree. But the focus on differential impedance is usually unwarranted, and that's really the main point. Getting correct differential impedance can often be quite touchy, so it can be easier to simply ignore it and work with single trace impedance. Mine usually end up within 10% for the differential impedance anyway.
Yes, 12.5mm is pretty big. I try to keep mine within 100thou, just because I'm a bit fussy about those sorts of things :->
Sure, no harm in that.
Dave.
Thanks. We paid for their base spec a while back. It just seems to be full of targets but no actual suggestions.
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