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

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From: Hassan Ali (hali@lanterncom.com)
Date: Thu May 10 2001 - 11:15:51 PDT


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 <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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