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

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From: Ritchey Lee (leeritchey@earthlink.net)
Date: Fri Jun 01 2001 - 09:13:44 PDT


The context is which the MECL handbook was written was for logic signals that
have edge rates on the order of 0.5 nSEC. It in that context that I encounter
engineers avoiding right angle bends, often at no small costs to their
projects. It in that context that I called the right angle rule a myth, and it
is clearly not a problem there.

There have been many tests at edge rates as fast as 100 pSEC that demonstrate
this.

My objective was to clear up the problem for logic designers, the readers of the
MECL handbook.

It is possible that at very high freequencies, 20 GHz and above, that a right
angle corner might affect signal quality. If I or anyone else were working at
those frequencies, it would be good engineering to retest to demonstrate whether
or not a right angle corner might be problematical.

A significant problem our profession has is applying blanket rules when they
might not be appropriate. The right angle bend rule for logic is one of those.

As an aside, there are often many right angle bends in the IC packages we all
use every day. Odd that no one worries about those!

Lee

Dan Swanson wrote:

> Chris, Lee and all:
>
> Here is some info on return loss (reflection coefficient)
> for bends and mitered bends. This happens to be for
> a ceramic substrate. Relative improvement will be the
> same for PC board materials. PC board bend may start
> out worse, w/h ratio will be greater for the lower dielectric
> constant.
>
> If someone wants to send me some typical dimensions of
> interest, I might have time to run it on the field-solver.
>
> <<bends.pdf>>
> PDF file is two pages from my course notes.
>
> Dan
>
> Dan Swanson EMAIL: d.swanson@ieee.org
> Bartley RF Systems, Inc. TEL: 978-834-4085
> 37 South Hunt Road FAX: 978-388-7077
> Amesbury, MA 01913
>
> > -----Original Message-----
> > From: Chris Padilla [SMTP:cpad@cisco.com]
> > Sent: Thursday, May 10, 2001 12:46 PM
> > To: si-list@silab.eng.sun.com
> > Subject: Re: [SI-LIST] : MECL System Design Handbook
> >
> > Lee,
> >
> > It is not a myth, there is "some kind of effect" from right angle bends.
> >
> > The real question is, I believe, "Do I care, given the speed and/or
> > frequency content of the signals I am dealing with?"
> >
> > For most of us right now, on this list, we probably *should not* care as
> > you point out.
> >
> > If one is designing a 50 GHz antenna, then it might make a world of
> > difference!
> >
> > Out of curiosity, what kind of rise time were you pushing through that
> > right angle bend? 100 ps? I am farily certain that I've seen some small
> > dips (aka capacitance) through right angle bends but I cannot honestly say
> >
> > that it was directly attributed to the bend.
> >
> > Thanks----->Chris
> >
> > >Any of you who want to know how the myth about right angle bends got
> > >started, look
> > >at figure 7.17 on page 155. This alleges that right angle bends can be
> > >seen by a
> > >TDR. I've done this measurement dozens of times and coiuld never see a
> > right
> > >angle bend.
> > >
> > >A few years ago, I called Mr. Blood the author of the book and asked
> > >about the
> > >diagram. His reply was that he knew the diagram was flawed, but there
> > >wasn't time
> > >to fix it before the book went to press.
> > >
> > >As a result, thousands of engineers have spend countless time worrying
> > >about right
> > >angle bends.
> > >
> > >When we publish technical information such as this, it is important to
> > >insure it
> > >is accurate.
> > >This applies especially to applications notes, whic often contain
> > entirely
> > >false
> > >data.
> > >
> > >Lee
> >
> >
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> ------------------------------------------------------------------------
>
> Name: bends.pdf
> bends.pdf Type: Acrobat (application/pdf)
> Encoding: base64

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