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

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From: David Instone (david_instone@uk.xyratex.com)
Date: Tue May 22 2001 - 04:44:42 PDT


Refering to Waddell's Transmission Line Design Handbook
5.5.2 deals with the optimum MICROSTRIP mitered bend and 5.6.3 deals
with the optimum STRIPLINE mitered bend the diagrams can't be done in
ascci chars so quoting the formulae for then is a bit pointless,
sufficient to say that they are different (quite a bit) and depend on
the width to thickness ratio of the tracks. There is quite a bit of
difference between 0.1"/0.001" and 0.006/0.001! also Figure 5.5.2.2
shows 5 other ways of bend compensation, including one where, as I
mentioned earlier in this thread, the inside of the bend is sharp and
the outside rounded, the default when plotting a track with a circular
laser apperture.
  So if anyone is going to repeat those original measurements please
make sure the OUTSIDE of the bend is also sharp 90deg.

Dave

Steve Corey wrote:
>
> 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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-- 
Regards

Dave Instone. Compliance Engineer Storage Systems Development, MP24/22 Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK. Tel: +44 (0)23-92-496862 (direct line) Fax: +44 (0)23-92-496014 http://www.xyratex.com Tel: +44 (0)23-92-496000

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