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

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From: Scott McMorrow ([email protected])
Date: Fri May 11 2001 - 04:47:16 PDT


Everyone,

The best discussion of right angle bend effects is in the
the book, Microstrip Lines and Slot Lines by Gupta.

There is a very good compilation of S-parameter measurements made
of loss versus bend radius and frequency.

I have seen very few boards in recent history where right angle
bends were ever used. CAD tools now automatically chamfer the
edge at 45 degrees and it comes for free. This reduces the
corner effect at high frequencies drastically, such that for digital
applications up to the low gigahertz region it is a non-issue. If you
want better performance, the book shows an optimal chamfer that
gives the best performance. It is used extensively in the Microwave
world, because it will reduce the reflection to the -60dB level with
a little extra CAD effort. Radiused bends can be used, but they
are extremely difficult to perform in most CAD systems. They should
be avoided.

For most cases and most designs on this reflector, as long as
45 degree chamfering is used on the corners (as is normal
practice for most layout tools, anyway) then this is all that is
necessary up to 2.5GBPS. At 10GBPS, more optimal bends
should be investigated.

regards,

scott

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

Steve Corey wrote:

> Not all right-angle bends are created equal, from the TDR's point of view. > Since the length of the discontinuity caused such a bend depends on the width of > the line which is bent, the frequency (or risetime, as the case may be) at which > certain aspects of the discontinuity is observable depends on the width of the > line. Kicking around cutoff frequencies and/or risetimes at which a right-angle > bend is or isn't observable is ambiguous if no mention is made of geometry. I > would hesitate to argue with the two highly qualified time-domain measurement > people who have posted so far that what they have observed is impossible based > on rules of thumb. > > ------------------------------------------- > Steven D. Corey, Ph.D. > Time Domain Analysis Systems, Inc. > "The Interconnect Modeling Company." > http://www.tdasystems.com > > email: [email protected] > phone: (503) 246-2272 > fax: (503) 246-2282 > ------------------------------------------- > > Ken Cantrell wrote: > > > All, > > My understanding is similar to Michael's (I think it was Michael) statement, > > 50 GHz or better. HJ stated 70 GHz or better in his seminar. The microwave > > guys care, we don't (at present). > > > > -----Original Message----- > > From: [email protected] > > [mailto:[email protected]]On Behalf Of Fred Balistreri > > Sent: Thursday, May 10, 2001 10:51 AM > > To: Michael Nudelman; 'Chris Padilla'; [email protected] > > 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: [email protected] > > [mailto:[email protected]]On Behalf Of Michael Nudelman > > Sent: Thursday, May 10, 2001 10:08 AM > > To: 'Chris Padilla'; [email protected] > > 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:[email protected]] > > Sent: Thursday, May 10, 2001 12:46 PM > > To: [email protected] > > 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 > > > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > > [email protected]. In the BODY of message put: UNSUBSCRIBE > > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > > si-list archives are accessible at http://www.qsl.net/wb6tpu > > **** > > > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > > [email protected]. In the BODY of message put: UNSUBSCRIBE > > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > > si-list archives are accessible at http://www.qsl.net/wb6tpu > > **** > > > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > > [email protected]. In the BODY of message put: UNSUBSCRIBE > > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > > si-list archives are accessible at http://www.qsl.net/wb6tpu > > **** > > > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > > [email protected]. In the BODY of message put: UNSUBSCRIBE > > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > > si-list archives are accessible at http://www.qsl.net/wb6tpu > > **** > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > [email protected]. 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