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

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From: Fred Balistreri (fred@apsimtech.com)
Date: Fri May 11 2001 - 07:10:38 PDT


Sorry guys, the referfence article mentioned below is in PCB Design magazine
not EDN. It's the featured article. Not much technical details though.

Best Regards,

Fred Balistreri

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Fred Balistreri
Sent: Thursday, May 10, 2001 9:51 AM
To: Michael Nudelman; 'Chris Padilla'; si-list@silab.eng.sun.com
Subject: RE: [SI-LIST] : MECL System Design Handbook

There is an article in this month's EDN magazine about RF and Microwave
design. In it there
is a reference to 90 degree bends versus rounded and why one should use
rounded. The particular
example was not quantified for frequency. However based on the rest of the
article one gets
the impression that the author was talking about designs way above 1Ghz.

Based on theoritical field solvers a single bend cannot be seen by a TDR.
The discontinuity
caused by the bend is very short in duration. A TDR does not have the
bandwidth to detect it.
A lot of TDR equipment will slew 1v/70ps, however one will be luckly to get
100ps to the board.
There are faster and more expensive equipment, but even those are not nearly
fast enough to
catch the discontinuity. I tend to side with Lee's statement that he has
never seen a discontinuity
due to a bend on a TDR, Tom's statement not withstanding.

Best Regards,

Fred Balistreri

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Michael Nudelman
Sent: Thursday, May 10, 2001 10:08 AM
To: 'Chris Padilla'; si-list@silab.eng.sun.com
Subject: RE: [SI-LIST] : MECL System Design Handbook

In Johnson's book there is a chapter "Who's afraid of the big bad bend?" (or
is it in his course?)

There is a capacitive property to a bend, but if you have 1-2-3 of them at
up to 10Gigs, the calculation showed that they don't really matter. All
those curved traces at these frequencies are more for designer's own peace
of mind, if he does not want to see dreams about the "Big Bad Bend" for a
couple of weeks before the final release.

Mike

-----Original Message-----
From: Chris Padilla [mailto: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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