[SI-LIST] Re: Right Angle Bends
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 15 Jul 2011 17:57:21 -0400
Doug
Just to be clear.
* Reduction of return loss by optimal chamfering of corners in RF
designs is supported by good science, experimental evidence, and
is practiced by most RF and Microwave designers world wide.
* With high precision, high bandwidth measurement techniques and PCB
launch design, corner bends, and the associated discontinuity can
be measured, especially on boards with low loss materials.
* For RF applications where traces are 100 mil to 200 mil wide,
chamfering of corners is essential. The effective duration of the
discontinuity is long enough to have an measurable and appreciable
impact on return loss.
* For digital applications where traces are 10 mil or less in width,
there is a discontinuity, but it's duration is so short as to make
it negligible for any application below 25 Gbps, and possibly 50
Gbps, unless one is stupid and places a huge number of corners on
a trace run.
Essentially, this is a numbers game. A bend always causes some increase
in return loss at every frequency ... BUT ... at the same time loss in
the trace sections leading up to a corner causes attenuation of the
reflections, thus mitigating the impact of the bend making return loss
better. Narrowing the trace at 50 ohm constant impedance effectively
causes frequency scaling, pushing up the frequency where return loss
peaking matters. So for narrow traces on medium to high loss materials
it's just about impossible to measure a corner, without getting down to
the impedance resolution of the trace itself. However, with wide traces
on low loss materials, it is quite easy to see the impact and make the
measurements.
As for the idea that there is no experimental confirmation of the
efficacy of bend compensation for low loss Microwave circuits, that's
just hogwash.
R. J. P. Douville and D. S. James, "Experimental study of symmetric
microstrip bends and their compensation," IEEE Trans. Microwave Theory
Tech., vol. MTT-26, pp. 175--181, Mar. 1978.
Abstract---An experimental study of microstrip bends is described. For
right-angle corners, equivalent circuit parameters covering a wide range
of frequencies, linewidths, and permittivities are presented and
compared with the limited theoretical values available. The influence of
mitering or rounding of the corner was investigated, and useful
empirical expressions are given for the optimum miter.
best regards,
Scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
On 7/15/2011 1:56 PM, Doug Brooks wrote:
> The origins go WAAAAAAAY back.
> We wrote our paper
> "90 Degree Corners; The Final Turn" trying to debunk this in 1998. It
> can be found in the articles section at http://www.ultracad.com
>
> The original concern may go back as far as 50 years with microwave
> people, but has been soundly debunked by many people in the last 10-15 years.
>
> Doug
>
>
>
> At 10:43 AM 7/15/2011, Lee Ritchey wrote:
>> None of these calculations are substantiated by actual measurements.
>>
>> There are several published works with actual testing out to 20+ GHz
>> measuring signal traces that show there is no detectable effect on signal
>> quality.
>>
>> I have a half dozen test PCBs which have been used to make the same
>> measurements and I have published the results as well as shown them to 8000+
>> students in my public and private classes.
>>
>> Our industry has a problem with people doing simulations from which
>> conclusions are drawn without validating the simulations. Often, as in the
>> case of right angle bends, much time and money is spent by designers making
>> something go away that is of no consequence.
>>
>> Simulations without validation can be very dangerous.
>>
>> Lee Ritchey
>>
>>
>>
>> --------------------------------------------------
>> From: "Zabinski, Patrick"<zabinski.patrick@xxxxxxxx>
>> Sent: Friday, July 15, 2011 9:46 AM
>> To:<si-list@xxxxxxxxxxxxx>
>> Subject: [SI-LIST] Re: Right Angle Bends
>>
>>> Here are a few of the earlier references on the topic.
>>>
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> TITLE Calculation of Microstrip Bends and Y-Junctions With Arbitrary
>>> Angle
>>> AUTHOR Reza Mehran
>>> SOURCE IEEE Transactions on MTT, MTT-26
>>> DATE June, 1978
>>> PAGES 400-405
>>>
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> TITLE Microstrip - A New Transmission Technique for the Kilomegacycle
>>> Range
>>> AUTHOR D. D. Grieg& H. F. Engelmann
>>> SOURCE Proceedings of the IRE
>>> VOLUME 40
>>> DATE December, 1952
>>> PAGES 1644-1650
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>
>>> These offer related info.
>>>
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> TITLE Equivalent Circuit Modeling of Interconnects From Time-Domain
>>> Measurements
>>> AUTHOR Jyh-Ming Jong, Bozidar Janko, and Vijai Tripathi
>>> SOURCE IEEE Trans. on CHMT
>>> VOLUME 16
>>> NUMBER 1
>>> DATE February, 1993
>>> PAGES 119-126
>>>
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> TITLE Equivalent Circuits for Discontinuities in Balanced Transmission
>>> Strip Lines
>>> AUTHOR Arthur A. Oliner
>>> SOURCE IRE Trans Micorwave Theory& Techniques
>>> VOLUME MTT-3
>>> DATE March, 1955
>>> PAGES 134-143
>>>
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> TITLE Experimental Determination of the Properties of Microstrip
>>> Components
>>> AUTHOR Maurice Ardit
>>> SOURCE Electrical Communication
>>> VOLUME 30
>>> DATE December, 1953
>>> PAGES 283-293
>>>
>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> TITLE Time-Domain Characterization of Interconnect Discontinuities in
>>> High-Speed Circuits
>>> AUTHOR Jyh-Ming Jong and Vijai K. Tripathi
>>> SOURCE IEEE Trans on CHMT
>>> VOLUME 15
>>> NUMBER 4
>>> DATE August, 1992
>>> PAGES 497-504
>>>
>>> ~~~~~~~~~~~~<done>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>
>>> Enjoy,
>>> Pat Zabinski
>>> Mayo Clinic
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>>> On Behalf Of Lee Ritchey
>>> Sent: Friday, July 15, 2011 11:33 AM
>>> 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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> Check out our resources at http://www.ultracad.com
>
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