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

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From: Rich Peyton (p2rich@testprobe.com)
Date: Thu May 10 2001 - 11:56:08 PDT


Although you cannot see the reflection on the TDR, the figure in the
MECL book demonstrates that the 90 deg. bend will create a greater
reflection at higher speeds.

Rich

Hassan Ali wrote:

> Tom,The argument is that this Zo (and the resulting voltage
> amplitude) dip for typical today's digital systems is just too small
> to worry about. But this 'game' of using right-angle bends if used
> irresponsibly may cause havoc in a high-speed printed circuit board.
> The cause of havoc is not from the corner itself, but from what the
> corner gives rise to. If you have two signal layers on a printed
> circuit board where the tracks are routed orthogonal to each other
> (and there is no plane separating the two signal layers), right-angle
> corners can eliminate the orthogonality and cause tracks (in the two
> layers) to run parallel to each other - probably at long lengths - and
> give rise to cross-talk problems. In today's dense boards, it's not
> uncommon to see such signal layers separated by 5 mils or less. With
> tracks running parallel and separated by that much, and worse still,
> coupling in a broadside sense (as opposed to edge coupling),
> unforeseen crosstalk problems can occur. And not just crosstalk
> problems, a tight coupling of that sort also gives rise to an
> impedance discontinuity causing further signal amplitude loss due to
> reflection.So, I see this great distaste for right-angle bends, as
> just like those childhood myths that are basically untrue, but
> nevertheless providing a useful end result. Regards.Hassan.
>
> -----Original Message-----
> From: Dagostino, Tom [mailto:tom_dagostino@mentorg.com]
> Sent: Thursday, May 10, 2001 1:09 PM
> To: 'Chris Padilla'; si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : MECL System Design Handbook
>
> If you look at the cross section of the trace at the bend it
> gets wider. A wider trace has more C per unit length. More
> C will look like a lower Zo. I've seen dips in impedance
> TDR'ing right angles.
>
> Tom Dagostino
> IBIS and Tau Modeling Manager
> SDD
> Mentor Graphics Corp.
> 503-685-1613
> tom_dagostino@mentor.com
>
> -----Original Message-----
> From: Chris Padilla [mailto:cpad@cisco.com]
> Sent: Thursday, May 10, 2001 9:46 AM
> 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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