[SI-LIST] Re: Routing diff pair HDMI signals at 85 Ohms as opposed to 100 Ohms reduces radiation?
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: "Joseph.Schachner@xxxxxxxxxxxxxxxxxx" <Joseph.Schachner@xxxxxxxxxxxxxxxxxx>
- Date: Mon, 17 Dec 2012 22:57:51 +0000
Hello Joe,
I passed that title along hoping it would help - sorry if it confounded more
than clarified (it's been a while since I read it, to tell the truth). I'm
afraid I don't remember off-hand seeing a better articulation of why we use 85
ohms in most of our topologies. Actually, considering how much it's being
used, it seems like there ought to be more... Ah - the Google Gods heard me,
try:
http://www.google.com/url?sa=t&rct=j&q
%20ohm%20differential&source=web&cd=5&ved
EsQFjAE&url=http%3A%2F%2Fportal.fciconnect.com%2FComergent%2F%2Ffci%2Fdocumentation%2Fta4-2_improving_system_performance.pdf&eiæLPULbVNtLWiAKAx4CYBw&usg¯QjCNGDzdmjaKCgWep3R75XVVmSrLAtKw&bvm=bv.1355534169,d.cGE
(and it's free!).
Of course, that URL will get mung'ed. Google "85 ohm differential" and look
for the DesignCon 2007 report w/ Intel and FCI.
I didn't perform the studies, but can offer some hearsay opinion (it looks like
these agree w/ the 2007 report) on why I think 85 ohms has been found to be
advantageous for our PCB applications:
· 85 ohm trace geometries are more friendly
o They can be wider, lowering insertion loss
o They can be more tightly coupled, allowing higher density
§ I believe this also has benefits when transitioning through discontinuities
o Yes, these two are somewhat mutually exclusive, but they offer some things
to trade off. With 100 ohms, in my experience, I'm stuck with very narrow
traces and/or geometries that have no effective coupling.
· The tighter coupling aids when going through discontinuities. Others
may take exception to this assertion, but that is my experience.
· Most discontinuities (vias, pads on series elements, pads going into
connectors, etc.) tend to be capacitive, relative to 50 ohms (100 ohms
differential), so the lower the impedance of the channel, the less impact those
have.
I don't have much help to offer in constructing vias to reach a particular
impedance, though I know there has been some excellent work done in that area.
In my experience, via optimization is not a "knob" I get to tweak, since my
stackups, layer transitions, board thicknesses, etc. vary wildly. Via
discontinuities are a necessary evil I must live with, though I might have some
generic optimizations which I apply.
Regarding the 50/100 ohms universally used in connectors and/or test equipment
- I agree. There are, however, 85 ohm connectors available for instances when
your system is 85 ohms, though clearly they are less readily available than the
50/100 ohm variety.
I apologize for the thrashing and hope this helps.
These aren't any "official Intel" words of wisdom, only some limited insights I
have.
I hope this post actually helps,
Jeff Loyer
From: Joseph.Schachner@xxxxxxxxxxxxxxxxxx
[mailto:Joseph.Schachner@xxxxxxxxxxxxxxxxxx]
Sent: Monday, December 17, 2012 1:27 PM
To: Loyer, Jeff
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Routing diff pair HDMI signals at 85 Ohms as opposed
to 100 Ohms reduces radiation?
Re: "Much of the reasoning for 85 ohms is outlined in the paper "Improve
Storage IO Performance by Using 85Ohm Package and Motherboard Routing"..."
I bought that paper today and read it, after seeing your reference. The paper
seeks to prove that 85 ohms is in fact better, and has a small (one sentence)
explanation in the first section that gives a couple of reasons why that should
be so. I think I am within the license terms if I quote that one sentence:
"Generally speaking, the reasons for 85Ohm design over 100Ohm include less
loss, ease in routing (cost saving), and better overall impedance match (and
hence better interconnect performance) when via on board and vertical path in
package are considered. "
The rest of the paper seems focused on demonstrating, by simulation and
measurement of a particular structure, that 85 ohm design is in fact better.
It does not go further into the reasoning behind why we expect that that is so.
Is there something more that you could say about that? It does not give any
guidance in the way vias should be constructed to yield better impedance, for
example.
---- Joe S.
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