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

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From: Dima Smolyansky (dima@tdasystems.com)
Date: Thu May 10 2001 - 11:31:44 PDT


Fred:

With a little skill, one does not have to be "lucky" to get fast TDR rise
time to the board. If one uses good quality cables low loss and probes, and
delivers the available rise time from TDR to the board, one can observe the
right angle bends in question.

Thanks,

===================
Dima Smolyansky
TDA Systems, Inc.
11140 SW Barbur Blvd., Suite 100
Portland, OR 97219
(503) 246-2272
(503) 246-2282 (fax)
(503) 804-7171 (mobile)
http://www.tdasystems.com
The Interconnect Modeling Company(TM)

----- Original Message -----
From: "Fred Balistreri" <fred@apsimtech.com>
To: "Michael Nudelman" <mnudelman@tellium.com>; "'Chris Padilla'"
<cpad@cisco.com>; <si-list@silab.eng.sun.com>
Sent: Thursday, May 10, 2001 9:51 AM
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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