[SI-LIST] Re: Right Angle Bends

Dear Mr. Ritchey,


--------------(1)-------------------

Edwards says the following about right-angle bends in his 1st edition on
page 109, "Capacitance data has been determined theorectically by Silvester
and Benedek[13] and inductance data by Thomson and Gopinath[5]. Fairly
extensive measurements have also been conducted....  "

The book continues to discuss small parasitic effects that happen at other
discontinuities, such as the apparent slight elongation of an open-circuited
endpoint due to the parasitic capacitance at the end of an open-circuited
trace, discontinuity effects at step changes of impedance (above and beyond
the expected reflection due to the change in impedance), and other details
of great interest to microwave designers, for which I assume this material
is standard practice. 

[13] Silvester and and Benedek, "Microstrip discontinuity capactiances for
right-angle bends, T-juntions and crossings," IEEE Trans., MTT-21, No. 5,
May 1973, 341-346

[5] Thompson and Gopinath, "Calculation of microstrip discontinuity
inductance", IEEE Trans., MTT-23, No. 8, August 1975, 648-655.


The book by K.C. Gupta, "Microstrip Lines and Slotlines", Artech House, 2nd.
ed. 1996. Page 194 shows a comparison of theoretical results from Thomson
and Gopinath with experimental measurements by Easter[33].

[33] Easter, "The Equivalent Circuit of some Microstrip Discontinuities",
IEEE Trans., Vol. MTT-25, 1975, pp. 655-660, a paper based, I believe, on
his earlier work, "Resonant Techniques for the Accurate Measurement of
Microstrip Properties and Equivalent Circuits," Proc. 1973 European
Microwave Conf., paper B. 7.5.


I can't imagine what better data one could ask for than Gupta's comparison
of theoretical models with experimental measurements.


------------------(2)----------------------

Regarding differential microstrip lines, the coupling between the lines
introduces an additional complication.  That has to do with mode conversion
between the differential and common modes of propagation that happens when a
differential pair passes through a corner.  The outer trace being slightly
longer than the inner trace, some of the differential mode is converted to
common-mode, and vice-versa.  If the next bend breaks to the opposite
direction, all is restored.  In an uncoupled (or loosely coupled, meaning
widely-spaced) differential pair, the mode conversion is completely
reversible. A tightly-coupled pair does not work that way.  The even and
odd-mode impedances of the tightly coupled pair are not the same, and
crucially, the even and odd-mode impedances may not be the same, so it seems
to me obvious that, were any power converted to a mode with a different
impedance than the original, some small reflection must be generated.  

I've not personally heard of any measurements corroborating such
differential reflections, however I am certain that if one were to fabricate
enormous traces as was common in the microwave industry in 1973, perhaps
100-mils wide on a 50-mil thick substrate, the small reflections so
generated would rise to a level of concern in a high-quality microwave
application (such as radar). 

I restricted my earlier remarks to the case of single-ended traces precisely
because there exists a wealth of well-established literature documenting
their performance. 


Best regards,
Dr. Howard Johnson, Signal Consulting Inc.,
tel +1 509-997-0505,  howie03@xxxxxxxxxx
www.sigcon.com -- High-Speed Digital Design seminars, publications and films
 



-----Original Message-----
From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx] 
Sent: Saturday, July 16, 2011 9:56 AM
To: Howard Johnson; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Right Angle Bends

Has any of this been verified with measurements?  Doesn't sound like it from
what I can read out of this.  If so, can we see them?  If not, the proper
approach is to assume the conclusions are not valid.

Why would it matter whether a trace is single ended or differential?  A
transmission line is a transmission line either way.

--------------------------------------------------
From: "Howard Johnson" <howie03@xxxxxxxxxx>
Sent: Saturday, July 16, 2011 9:35 AM
To: <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Right Angle Bends

> Hi Guys,
>
> I assume we are talking about right-angle bends in ordinary (not
> differential) traces.
>
> In that case, I am in agreement that a right-angle bend, on a trace as 
> typically fabricated with today's (2011) small geometry, at the speeds 
> where we operate (2-20 Gbps) does not present much of a signal 
> integrity issue.
>
> There is, however, a perfectly valid engineering reason behind the old 
> adage that right-angle bends should be avoided.  It is explained in 
> the following short editorial article, the complete text of which is 
> copied below.
>
> H. Johnson, "Who's Afraid of the Big, Bad Bend?", EDN, 5/11/2000
>
> Best regards,
> Dr. Howard Johnson, Signal Consulting Inc., tel +1 509-997-0505,  
> howie03@xxxxxxxxxx www.sigcon.com -- High-Speed Digital Design 
> seminars, publications and films
>
> --------------- TEXT OF ARTICLE -------------------
>
> Who's afraid of the big, bad bend?
> (Originally published in EDN Magazine, May, 2000)
>
> Right-angle bends in PC-board traces perform perfectly well in digital 
> designs in speeds as fast as 2 Gbps.
>
> In most digital designs, the right-angle bend is electrically smaller 
> than a rising edge. For example,the delay through a right-angle bend 
> in an 8-mil-wide, 50-ohm microstrip trace in FR-4 is on the order of 1 
> psec.
> That's less than 1% of a 100-psec rise time. For any object of this 
> tiny physical scale, a lumped-element model should suffice. Years ago, 
> TC Edwards presented good lumped-element models for right-angle bends 
> for the microwave industry (Reference 1). These models indicate that a 
> right-angle bend has two primary effects: a slight delay plus some 
> excess lumped capacitance.
> You
> might imagine that, as a signal traverses a right-angle corner, the 
> trace appears to grow wider at the corner. This simple idea explains 
> why you see an excess capacitance (lower impedance) near the corner.
>
>     [Ed. note: Edwards goes on to show a three-element L-C-L 
> equivalent circuit for
>      the bend, and discusses means of correctly chamfering the corner 
> to mitigate
>      the effect.]
>
> For an 8-mil-wide, 50-ohm microstrip transmission line in FR-4, the 
> excess lumped capacitance works out to 0.012 pF. Assuming that you are 
> using 100-psec rise and fall times, the size of the reflected signal 
> that bounces off this capacitive discontinuity is 0.30% (that's 0.003) 
> of the incoming step amplitude. I conclude from this analysis that the 
> reflection from a single corner is too small to worry about. (The 
> reflected signal size scales in proportion to the trace width and 
> inversely with rise and fall times.)
>
> Some people worry that conduction electrons are traveling so fast that 
> they won't be able to make it around a square corner. Perhaps they 
> might reflect back or fly off into space. Such arguments are 
> ridiculous. Sure, individual electrons move at high speeds, but their 
> aggregate drift velocity is less than 1 in./sec as they bounce from 
> atom to atom. Your average electron smacks into something and changes 
> directions billions of times in a length of 10 mils. Electrons don't 
> have any trouble banging around a corner.
>
> Might the electric-field concentration at a sharp, pointy corner 
> create a lot of radiation? Hogwash. As a trace rounds a corner, it 
> stays a constant distance from the underlying reference plane the 
> whole way. The electric field intensity from trace to plane doesn't 
> radically vary at any point along this track except for a modest 
> perturbation in the vicinity of the actual pointy tip of the corner. 
> It's true that a microscopic electric-field probe directly adjacent to 
> the corner would detect this field concentration.
> However, measurements taken from farther away account for the average 
> of everything that happens along the whole trace, not just at the 
> corner. The corner, because it's so small, cannot noticeably affect 
> the far-field radiation.
>
> Layout professionals often point out that modern layout systems 
> already round off all the outside corners, assuming that this rounding 
> eliminates the square-corner effect. It doesn't. Rounding the corners 
> removes 21% of the copper in the corner. Edwards shows that you must 
> remove 70 to 90% of the copper from a right-angle bend to neutralize 
> (to first order) the excess capacitance. Rounding removes only a small 
> fraction of the required amount of copper. Rounded-corner right-angle 
> bends work well in digital designs not because they are rounded, but 
> because the corners are too tiny to cause significant problems in the 
> first place.
>
> Today (circa 2000), only microwave designers need to worry about 
> right-angle bends. At microwave speeds, roughly 10 times the rate of 
> most digital designs, parasitic capacitance presents 10 times more of 
> a problem. Also, microwave designers often use big, fat, 100-mil 
> traces to reduce skin-effect losses, so their corners appear 
> electrically 10 times bigger. They also tend to linearly cascade 
> multiple stages. Cascading sums the imperfections in each stage, 
> making microwave designs about 10 times more sensitive to tiny 
> imperfections. Overall, contemporary microwave designs can be 1000 
> times more sensitive to right-angle bends than are digital designs.
>
> As digital designs push toward higher speeds, you may eventually reach 
> a point where the right-angle bends begin to matter. For example, 
> corners are just beginning to affect the design of 10-Gbps serial 
> connections, and they also contribute perceptibly to skew in certain 
> poorly routed differential pairs. If you accumulate a lot of corners, 
> as in a serpentine delay structure, you may begin to see a little 
> extra delay. Other than these extreme applications, right-angle bends 
> remain electrically transparent.
>
> Some manufacturing engineers complain about the use of right-angle 
> bends when using wave-soldering equipment. They worry that wayward 
> solder balls or solder flux will get trapped in the inside corners. 
> With reflow soldering and good solder masking, neither is a problem. I 
> have heard no other credible negative comments about the 
> manufacturability of right-angle bends, but I am always happy to hear 
> from others whose experience may differ.
> Please write.
>
>
> REFERENCE
> 1. Edwards, Terry, Foundations for Microstrip Circuit Design, John 
> Wiley & Sons, 1981,1992.
>
> -------------------- END ARTICLE ---------------------
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx 
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On
> Behalf Of Lee Ritchey
> Sent: Friday, July 15, 2011 12:15 PM
> To: Vinu Arumugham
> Cc: Moreira, Jose; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Right Angle Bends
>
> That is another urban legend.  Never been true.  I've seen fabricators 
> say this and then etch outer layers with all sorts of surface mount 
> pads that have traces entering them that result in right angle corners 
> and never complain!
>
> We'll probably all go to our graves before we flush out all this 
> misinformation!
>
> --------------------------------------------------
> From: "Vinu Arumugham" <vinu@xxxxxxxxx>
> Sent: Friday, July 15, 2011 11:28 AM
> To: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
> Cc: "Moreira, Jose" <jose.moreira@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Re: Right Angle Bends
>
>> Are 90 degree turns not an "acid trap" problem especially for 3-4 mil 
>> wide traces?
>> Lee Ritchey wrote:
>>> There is a paper on my web site authored by Doug Brooks using lab 
>>> measurements performed by Todd Hubing when he was at UMR as part of 
>>> the EMI department that clearly shows that right angle bends in 
>>> signal traces are not detectable out to something like 20 GHz.
>>>
>>> You can download it from www.speedingedge.com.  It is titled "90 
>>> Degree Corners, The Final Turn".
>>>
>>> This article has the info you are looking for.  I'm sure Doug would 
>>> not object to you sending it around.  Anything to stop the crazy 
>>> rumors about this topic.
>>>
>>> It is truly amazing the stories I hear about how evil things happen 
>>> at right angle bends.  In every case, the author of the rumor has no 
>>> data to back it up or makes a statement such as "I measured this at 
>>> my last company and
>>> cannot share it without their permission".   Doesn't that sound a little
>>> fishy?
>>>
>>>
>>> --------------------------------------------------
>>> From: "Moreira, Jose" <jose.moreira@xxxxxxxxxx>
>>> Sent: Friday, July 15, 2011 9:48 AM
>>> To: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>; 
>>> <si-list@xxxxxxxxxxxxx>
>>> Subject: [SI-LIST] Re: Right Angle Bends
>>>
>>>
>>>> From my experience (I also get that statement from my customers) it 
>>>> comes from the thinking that on a right angle bend the signal would 
>>>> "see" a larger trace width when crossing "the bend" and because of 
>>>> this different trace width it will have a different impedance at 
>>>> that point so there is a discontinuity that is bad for the signal.
>>>>
>>>> I remember an old conference paper showing no measureable effects 
>>>> to 10 Ghz but I cannot find it again.
>>>>
>>>> I would be interested if anyone know of a good up to date reference 
>>>> (with
>>>> measurements) that I can also send to my customers when they have 
>>>> the same statement.
>>>>
>>>> Jose
>>>>
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: si-list-bounce@xxxxxxxxxxxxx
>>>> [mailto:si-list-bounce@xxxxxxxxxxxxx]
>>>> On Behalf Of Lee Ritchey
>>>> Sent: Friday, July 15, 2011 6:33 PM
>>>> To: si-list@xxxxxxxxxxxxx
>>>> Subject: [SI-LIST] Re: Right Angle Bends
>>>>
>>>> Does anyone know the origin of the myth that right angle bends 
>>>> cause signal integrity problems?
>>>>
>>>> I though this myth had died a long time ago, but had a conversation 
>>>> last week with a well known fabricator who was told by a customer 
>>>> that right angle bends were a no no.
>>>>
>>>> This was a customer who should know better.
>>>>
>>>> Lee Ritchey
>>>> Speeding Edge
>>>>
>>>>
>>>>
>>>>>>>> ---------------------------------------------------------------
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