Re: [SI-LIST] : MECL System Design Handbook

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From: Steve Corey (steve@tdasystems.com)
Date: Mon May 21 2001 - 11:24:23 PDT


Mike -- I agree -- if someone is able to reproduce the results shown in
Fig. 7-17 of the MECL handbook, it probably won't stop anyone's design from
going to fab. I would postulate two reasons why it would be helpful to
answer the original question, though, or at least why I am interested in
finding out the answer.

The first is the technical credibility of the MECL handbook and its
author. If it turned out he drylabbed his TDR measurements, his entire
work gets called into question, not to mention his technical reputation.

The second is that size and frequency tend to offset each other, so that an
effect which is important in a large, low-frequency regime often maintains
its importance in a small, high-frequency regime. This is particularly
true in low-loss situations.

A possible third reason is that it gives some list members the opportunity
to reminisce about the days of black-tape PCB layout... :)

Now for my unsolicited personal opinion about the importance of right-angle
bends in today's typical digital circuit... Assuming the MECL handbook
results are accurate, you won't get enough reflection to matter. The TDR
example in the handbook shows 5% reflection for a 100-mil trace at 30ps
risetime. Since the size of the anomaly becomes smaller with decreasing
trace width, causing its electrical length on a TDR plot to become shorter,
it would end up becoming unobservable for a certain trace width at 30ps.
This is particularly true when skin and dielectric losses are mixed in,
since they fuzz the effects of discontinuities. (Radiation may be an
issue, but I don't consider myself qualified to guess at it. People have
also mentioned skew introduced by the bends. Furthermore, there are
obviously other non-TEM effects caused by the sharp corners.)

If the above drylab hand-waving on my part is correct, then the results
shown in the MECL handbook serve to underscore, rather than undercut, what
the various experts have stated in their seminars, and what several
well-informed people on this list have stated to be their understanding.
In other words, it's possible that everyone is right... If engineers have
been unnecessarily concerned, based on the MECL handbook, about right-angle
bends in their designs, I think it stems from misinterpretation of the
experimental results, rather than blatant inaccuracy of the results. My
primary complaint about the figure is that it would appear, from looking at
the TDR signature, that the relative distances between the four
discontinuities are inaccurate in the drawing.

Ideally, some philanthropic individual would do a careful, thorough
investigation as to where and when right angles do and don't matter,
complete with both measurements and field solutions, and publish it. Since
this probably won't happen any time soon, the only way to know for sure is
to put a good TDR probe to your trace and see what comes back (of course,
filtering the results to your risetime of interest).

  -- Steve

-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Modeling Company."
http://www.tdasystems.com

email: steve@tdasystems.com
phone: (503) 246-2272
fax: (503) 246-2282
-------------------------------------------

"Degerstrom, Michael J." wrote:

> Of course when they were using .1" wide traces back in the 70's
> they didn't have sub-100 ps edges to worry about. I can't see
> the relevance to the sharp corner effect to signal integrity
> concerns for any commercial applications for a couple of decades
> after that work was introduced. Maybe some of us will need to
> be concerned about corner effects now or in the near future.
> Also, my perceptions is that RF designers have to deal with the
> corner effects for their type of work. BTW, I think the handbook
> was and still is to some degree one of the best SI resources
> available.
>
> Mike
>
> > -----Original Message-----
> > From: Ken Cantrell [mailto:Ken.Cantrell@srccomp.com]
> > Sent: Monday, May 21, 2001 9:37 AM
> > To: Ingraham, Andrew; si-list@silab.eng.sun.com
> > Subject: RE: [SI-LIST] : MECL System Design Handbook
> >
> >
> > That was during the westward migration in covered wagons wasn't it?
> >
> > -----Original Message-----
> > From: owner-si-list@silab.eng.sun.com
> > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Ingraham, Andrew
> > Sent: Friday, May 18, 2001 8:53 PM
> > To: si-list@silab.eng.sun.com
> > Subject: RE: [SI-LIST] : MECL System Design Handbook
> >
> >
> > > (Don't forget, the revision date on the hanbook, or at
> > least on the PDF
> > > I downloaded, is 1988.) ...
> >
> > My hardcopy version of the handbook (which I can't find right
> > now) is older
> > than that.
> >
> > Some of the research work is probably from the 1970's. Many
> > ECL boards back
> > then were simple double-sided (2-layer) with ground plane on
> > one side and
> > signals on the other, hand routed using sticky black tape for
> > the traces.
> >
> >
> >
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