[SI-LIST] Re: Uncoupled differential signalling
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: <johan.lans@xxxxxxxxxxxxxxx>
- Date: Sat, 29 Nov 2014 12:59:53 -0500
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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