[SI-LIST] Re: mode conversion myth
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: "istvan Novak" <Istvan.Novak@xxxxxxx>, "eric bogatin" <eric@xxxxxxxxxxxxxxx>
- Date: Mon, 5 Jan 2009 08:28:13 -0700
Istvan,
At what frequency or rise-time equivalent frequency does local skew become
an issue?
Thanks,
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of istvan Novak
Sent: Tuesday, December 30, 2008 2:59 PM
To: eric bogatin
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: mode conversion myth
Hi Eric,
Yes, in general I agree with your description, I would add one
clarification, though. With a spatial resolution determined by the
bandwidth of signal we want to push through the differential
interconnect, we need to keep the electrical symmetry between the legs
all along the way. In particular, when it comes to propagation delay,
it is very important that not only the end-to-end propagation delay is
kept similar, but we do not introduce any local skew. This becomes
important when we have to decide where and how we compensate for known
skew contributors.
Regards,
Istvan
eric bogatin wrote:
> Hi folks-
>
>
> I give a popular talk on separating myths from reality in signal integrity
> (you can down load a copy here:
> http://www.bethesignal.net/bogatin/index.php?cPath=29)
>
>
>
> By popular demand, I am creating two spin off talks, one specifically on
> myths about PDN design and one specifically on myths about differential
> pairs. In prepping for these, I typically scan some of the questions and
> comments from this list.
>
>
>
> In the last few months, I've seen a number of comments people on this list
> made about mode conversion in differential pairs.
>
>
>
> While it is possible to really screw up signal quality with a bunch of
> differential impedance discontinuities, I have always been under the
> impression that you will only get mode conversion- some of the
differential
> signal converting to common signal- if there is an asymmetry between the
> two lines that make up the differential pair, either in the signal or
return
> paths. This can be a length difference, a time delay difference, a line
> width difference, a return path difference, etc.
>
>
>
> You can have large line width changes, separation changes, even gaps in
> return paths and see no mode conversion, if whatever you do to one line,
you
> do to the other line. Of course, when you go through a connector and the
> return path has a discontinuity, it is sometimes difficult to make the
> discontinuity symmetrical, but it is the symmetry of the discontinuity
that
> determines the amount of mode conversion, not the amount of discontinuity
> that determines the mode conversion.
>
>
>
> Does any one disagree with this comment?
>
>
>
> In case you are curious, the other myths that I am covering in the diff
pair
> talk are:
>
>
>
> - the return current distribution in microstrip and stripline
>
> - when should the common signal be terminated
>
> - the use of differential mode and common mode vs differential
> signals and common signals
>
> - vias are bad and should be avoided
>
> - this one on mode conversion
>
>
>
> Do you have your favorite myth you might want to share? If I use it, I
will
> try to give you credit.
>
>
>
> comments are always welcome.
>
>
>
> --eric
>
>
>
>
>
> *******************************************************
> Dr. Eric Bogatin, Signal Integrity Evangelist
> Bogatin Enterprises
> Setting the Standard for Signal Integrity Training
> 26235 W 110th Terr
> Olathe, KS 66061
> e: <mailto:eric@xxxxxxxxxxxxxxxx> eric@xxxxxxxxxxxxxxx
> v: 913-393-1305 cell: 913-424-4333 f: 913-393-0929
> <http://www.bethesignal.com/> www.BeTheSignal.com
> Blog: <http://www.bethesignal.com/blog> www.beTheSignal.com/blog
> ***********************************************
>
>
>
>
>
>
>
>
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