[SI-LIST] intra-pair skew and jitter
- From: Jim Nadolny <jim.nadolny@xxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 8 May 2012 15:26:45 +0000
We all know skew is the bane of differential signaling...at least I always
thought so. But some simulations have me re-thinking this a bit.
First - this is a test application with phase matched coax cables sampling a 10
Gb/s signal. The question is "How tightly phase matched should these cables
be?". Conventional wisdom says that they should be phase matched within a few
ps. In PCB design we match trace lengths to within a few mills or less for
EMI/crosstalk reasons. This design practice is transferred into coax cable
specs (for test applications) which have very tight phase match requirements
and adds test cost.
I wanted to look into this a bit deeper so I ran some sims in ADS. I'm looking
at the ideal case with a simple timing shift in an uncoupled lossless system.
I'm working at 10 Gbps but let's normalize everything. The risetime is 0.2UI
(20-80%). The results were surprising to me in that jitter was not affected by
even gross level of intra-pair skew.
With 0 UI skew we have 0 UI of total jitter, again a lossless ideal system is
the focus
With 0.05UI skew, we have 0 UI of jitter and the risetime degrades 0.203 UI
With 0.1UI skew (10 ps) we have 0 UI of jitter and the risetime degrades to
0.0.21 UI
With 0.4UI skew we have 0 UI jitter and the risetime degrades to 0.46 UI (this
is "ludicrous" intra-pair skew and still no jitter)
Once we get to 0.5UI of skew we get "huge" jitter because of a shelving in the
transitions.
Clearly as intra-pair skew increases the differential risetime degrades
(increases). This is consistent with increased differential insertion loss due
to mode conversion as skew increases. But the eye pattern does not show any
increased jitter which is counter intuitive.
Before we get all giddy about these conclusions let's bear in mind a couple
things"
* EMI/crosstalk is sensitive to mode conversion and is a good motivator
to keep things matched
* Coupled systems (twisted pairs, twinax) are a bit of a different
animal than this coax cable test application. Mode conversion and in
re-conversion is a different effect that does impact jitter.
Have others observed this lack of jitter with increasing intra-pair skew?
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts: