FreeLists / si-list / [SI-LIST] Re: Question about differental to common mode conversion

["tao xu" <helen.tao.xu@xxxxxxxxx>]

Hi, Allan
If your goal is to control EMI, differential to common mode conversion ratio
is more direct indication.

Tao ( Helen )



On 6/2/07, Allan Wang <libran2008@xxxxxxxxx> wrote:
>
>
> Well 1 mil is aobut 0.1ps ... ...
>
> I have a releated question and need opinion from
> expert. Any information is appreciated.
>
> For high speed differential cable such as SATA, hdmi,
> display port, infiniband, etc, what parameters need to
> be specified at cable assembly component level to
> control EMI?
>
> For example, which one is better? intra-pair skew of
> the cable or differential to common mode conversion
> ratio SCD21.
>
> Thanks in advance.
>
>
>
> --- Scott McMorrow <scott@xxxxxxxxxxxxx> wrote:
>
> > Several points I'd like to make.  Even though the
> > software says that
> > there is a 1 mil delta, that does not mean there is
> > really a 1 mil
> > delta.  In every PCB layout tool that I am aware of,
> > measurements are
> > made from the origin point of one pad to the origin
> > point of the other
> > pad, through the centerline of the trace.  If all
> > parts are centered on
> > the pads, this can be reasonably accurate.  However,
> > many surface mount
> > devices with rectangular pads will contact the pads
> > at various location
> > off-origin.  This results in significant error.  An
> > example of this
> > might be TSOP memory devices.
> > Traces are measured from the center of the track.
> > However, the true
> > path delay can be shorter (or longer) going around a
> > bend, depending on
> > whether the bend is predominantly inductive or
> > capacitive.  If the
> > number of bends are not equal, and the direction of
> > the bends between
> > two traces are not equal, their delays will not be
> > equal and
> > significantly larger than 1 mil ( 170 fs in FR4).
> >
> > Another example might be traces crossing over or
> > near antipads, where
> > delay is altered.
> >
> > Another example is with serpentine traces.
> > Crosscoupling between trace
> > loopbacks can introduce significant delta in
> > end-to-end delay.
> >
> > Ultimate, look at the numbers.  1 mil of stripline
> > trace in FR4 has a
> > delay of 170 femtoseconds.  Does it make sense in
> > any design to match
> > something to accuracy that is beyond the propagation
> > velocity uniformity
> > of the underlying materials?  Ultimately your FR4
> > has a variation of
> > somewhere around 3 to 10 ps/inch.  For my money, if
> > traces are matched
> > to within 50% of the underlying material
> > uncertainty, I feel pretty
> > good. That amounts to about a 50 mil trace matching
> > requirement at the
> > smallest (8.5 ps).  If you're worried about EMI,
> > don't be. Even if this
> > was the maximum skew in a differential pair the  8.5
> > ps common mode
> > pulse width would have maximum energy around 85 GHz.
> >  I'm pretty sure
> > that most dielectrics will absorb that.
> >
> > Go ask your silicon designers just how well matched
> > the outputs of their
> > drivers are, and what the skew in the internal clock
> > trees are.  Ask the
> > IC package designers how closely they matched the
> > trace delays in their
> > package, and how accurately they report those
> > numbers back to you.  And
> > ask your material vendors what the lower bound of
> > material Er variation is.
> >
> > Then justify the pain and suffering that a layout
> > designer has to go
> > through to match to within 1 mil or even 10 mils of
> > trace length.  I'm
> > quite tired of lazy engineers not running the
> > numbers and placing the
> > burden on someone else.  In most cases, if your
> > design cannot withstand
> > even 100 mils of trace length mismatch (17 ps of
> > delay mismatch), you
> > have a real serious problem with your design margins
> > elsewhere, which
> > you are not going to eliminate with ultra silly
> > trace length matching.
> > If you were a mechanical designer, designing
> > components with a material
> > that had the dimensional stability of rubber, and
> > ask the machinists to
> > mil it to within 1 mil tolerance, you'd be laughed
> > out of a shop.
> >
> > respectfully,
> >
> >
> > Scott
> >
> > Scott McMorrow
> > Teraspeed Consulting Group LLC
> > 121 North River Drive
> > Narragansett, RI 02882
> > (401) 284-1827 Business
> > (401) 284-1840 Fax
> >
> > http://www.teraspeed.com
> >
> > Teraspeed?is the registered service mark of
> > Teraspeed Consulting Group LLC
> >
> >
> >
> > Bill Owsley wrote:
> > > Jeff,
> > >   You said it much better than I was inclined to.
> > >   Thanks.
> > >
> > >   As for leading edge designs using an
> > auto-router, the guys spend nearly as much time
> > working the constraint table as it would take to lay
> > it out by hand and then it can be done over and
> > over.  But they still get those annoying leftover
> > nets that auto-router chokes on, reducing them to a
> > miniumum is their objective.
> > >
> > > "Loyer, Jeff" <jeff.loyer@xxxxxxxxx> wrote:
> > >   I think folks are missing Bill's point. I don't
> > think he's saying that
> > > the design is going to break at 2 mils routing
> > difference, only that he
> > > can easily attain a maximum 1 mil delta, so he
> > uses that as his
> > > constraint. If his CAD folks balked at that
> > number, he'd probably
> > > adjust accordingly and, if it got close to having
> > significant impact,
> > > would do a more detailed analysis.
> > >
> > > This is very similar to what we've done for
> > common-clock FSB signals:
> > > tell our CAD folks to match them within 10 mils
> > (or some number), even
> > > though we know we can absorb more. But, 10 mils is
> > easily attainable,
> > > and guarantees essentially no skew due to length
> > differences. Granted,
> > > it's a little more work on CAD's part, but the
> > tradeoff is I don't have
> > > to do a grand analysis to figure out exactly how
> > close they HAVE to be
> > > matched, and then worry about whether my analysis
> > was correct (and
> > > require subsequent validation in the lab,
> > etc.).=20
> > >
> > > This is about compromises, and where you put your
> > energy into in-depth
> > > analysis. If you can easily do 1 mil spacing, no
> > problem. It's a
> > > little tighter than I'd generally specify, but
> > it's not absurd, either.
> > > If you're using an auto-router (that's surprising
> > to me, I didn't think
> > > leading-edge designs could use them), you probably
> > have different
> > > constraints.
> > >
> > > Jeff Loyer
> > >
> >
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