[SI-LIST] AW: Re: Uncoupled differential signalling
- From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
- To: "scott@xxxxxxxxxxxxx" <scott@xxxxxxxxxxxxx>, "johan.lans@xxxxxxxxxxxxxxx" <johan.lans@xxxxxxxxxxxxxxx>
- Date: Mon, 1 Dec 2014 08:12:41 +0000
Johan,
routing differential pairs as single ended traces takes much more space than
routing differential, because you need distance to isolate the traces. Routing
on different Layers might look like a nice alternative as you can obviously
copy and paste the routing, but that’s not true for high speed signaling. You
will have different via barrel length, and via stub length along with the
potential difference in Material properties, and that leads to timing problems
if net well compensated. You also loose much of the noise immunity that
differential signaling has been invented for because the noise will be very
different on these different Layers.
If the differential traces are too wide, just lower the substrate thickness
until you reach a good routability. This is especially helpful when you are
forced to route thru many dense via fields. The additional Losses are
acceptable.
BR
Gert
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-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von Scott McMorrow
Gesendet: Samstag, 29. November 2014 19:00
An: johan.lans@xxxxxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] Re: Uncoupled differential signalling
Johan,
There are 2 issues. One that you have brought up yourself, which is that the
noise environment in different planar cavities will be different, causing a
noise imbalance on the pair. I imagine that this could be managed with Gnd-Gnd
cavities and appropriate stitching. The second issue is a difference in the
dielectric environment, due to the anisotropic properties of the laminate,
which will cause unpredictable skew. This is probably the more difficult
problem to resolve.
Scott
Scott McMorrow
Teraspeed® Consulting - A Division of Samtec
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
On Sat, Nov 29, 2014 at 2:42 AM, Johan Lans <johan.lans@xxxxxxxxxxxxxxx>
wrote:
> On a recent design I tried out routing loosely coupled differential pairs.
> Previous tests I've have done showed that loosely coupled pairs are
> more consistent in differential impedance and easier to simulate
> accurately, so I think that there are good reasons to try this out.
> However, routing proved kind of tricky since the total width of the
> pair was large. A colleague suggested routing the traces as uncoupled
> 50ohm traces. If this is done on layers that are separated by one or
> more planes, one could route the traces almost exactly identical, over
> each other (perhaps compensating for the two signals traversing
> different lengths in vias).
> Would there be any negative effects to doing this? My thought is that
> between different planes in the stackup, there could be different
> noise affecting the transmission lines so that the noise cancelling
> effect of the differential signalling is without effect.
> /Johan
>
>
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