[SI-LIST] Re: Antwort: Re: 90 degree turn in PCB tracks

Scott,

good points.


> [Original Message]
> From: Scott McMorrow <scott@xxxxxxxxxxxxx>
> To: <leeritchey@xxxxxxxxxxxxx>
> Cc: Matthias Bergmann <MBergmann@xxxxxxxxxxxxxxxxxx>;
<si-list@xxxxxxxxxxxxx>
> Date: 7/25/2006 10:01:27 AM
> Subject: [SI-LIST] Re: Antwort: Re: 90 degree turn in PCB tracks
>
> Lee
> The experimental design used in Ultracad corner test is quite 
> sub-optimal, given the incredible launch discontinuity.  I'd be 
> surprised if better than a 35 to 50 ps rise time edge actually makes it 
> into the board.  Then, given the length of the traces involved, I 
> suspect that the actual bandwidth seen at the corners is somewhere 
> around 3 GHz, at best.  Unfortunately, there do not appear to be any 
> structures available to determine the actual measurement bandwidth.  
> Just because Todd Hubing was responsible for the measurements, does not 
> mean that the experimental design is as good as an 18ps launch and 20 
> GHz makes it sound.  If you can get a fast launch into a board, and can 
> use low loss materials, it is possible to see corners.  Gus Panella and 
> I did so about 8 years ago on a test vehicle that we designed using 
> Rogers 4350 material.
>
> Having said that, 3GHz bandwidth is typical of the fastest standard 
> logic signals on most conventional PCBs.  Your assertion that corners do 
> not matter for digital designs is true.
>
> The original source of the corner information comes from the Microwave 
> literature.  Gupta, et. al., Microstrip Lines and Slotlines, is a good 
> reference.  It has been well known that, in Microwave design, right 
> angle bends are not a good thing.  If you're trying to decrease return 
> loss in a microwave design, you will most assuredly use radiused corners 
> or the optimal chamfer.  Why?  Because microwave boards use low loss 
> materials, like PTFE, and very wide trace width on thick substrates to 
> reduce total power loss.  The corner discontinuity on a 5 mil wide logic 
> signal trace on a conventional high density PCB may not be an issue, but 
> on a low loss microwave board that uses 50 to 200 mil wide microstrip 
> traces, that silly little corner is a killer.
>
> The magnitude of the corner discontinuity is proportional to it's 
> duration.  A corner discontinuity lasts for approximately 85 x sqrt(2 x 
> Er(eff)) x w ps:
>
> Where
> Er(eff) = is the effective Er of the material
> w = the width of the trace in inches.
>
> If we run the numbers,
>
>     For a 5 mil stripline trace on FR4 with and Er(eff) of 4, the
>     discontinuity lasts for 1.2 ps.
>     For a 100 mil microstrip trace on Duroid with an Er(eff) of 2, the
>     discontinuity lasts for 17ps.
>
> If we then use the rule of thumb that a discontinuity is important only 
> when it approaches 1/10th of the risetime, then our little 5 mil corner 
> has an effective operating bandwidth of .35/(1.2 e-12 x 10) = 29 GHz.  
> Clearly out of the region where we are interested for digital logic.
>
> But for the 100 mil trace, we have an operating bandwidth of .35(17e-12 
> x 10) = 2 GHz.  This is quite frankly not a very good microwave design.
>
>
> 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
>
>
>
> Lee Ritchey wrote:
> > The paper published by Ultracad tests the effects of right angle bends
in a
> > logic signal trace with an 18 pSec edge which is equivalent to about 20
> > GHz.  There is no detectable effect.  These tests were conducted by Todd
> > Hubing of UMR in a well equipped lab and are to be trusted.  There have
> > been amy other similar tests done with the same result.
> >
> > I believe the notion that right angle bends are a source of problems
stems
> > from an error in the Motorola ECL handbook published in 1974 and still
in
> > print with the error.  Now O Semiconductor publishes it.
> >
> > This may be one of those cases where simulation shows a change in the
field
> > distribution around the right angle bend, which we expect.  The
question is
> > whether the change is significant.  One of our jobs is to distinguish
> > between visible and significant.
> >
> > Once again, we have lies, damn lies and simulations.  Simulations
without
> > validation may well be more dangerous than no simulations at all in some
> > cases. 
> >
> >
> >   
> >> [Original Message]
> >> From: Matthias Bergmann <MBergmann@xxxxxxxxxxxxxxxxxx>
> >> To: <si-list@xxxxxxxxxxxxx>
> >> Date: 7/25/2006 1:42:10 AM
> >> Subject: [SI-LIST] Antwort: Re: 90 degree turn in PCB tracks
> >>
> >> Hello,
> >>
> >> Following the advices I read the article about 90 degree bends on
> >> ultracad.com as well as the appropriate chapter in Eric Bogatins book.
> >> I am quite surprised about the conclusion that 90 degree corners don't
> >> matter. I remember that once in a simulation of a 50 ohm
microstrip-lin=
> >> e
> >> with
> >> a chamfered 90=B0 corner in ADS Momentum, I never got a S11 better
than=
> >>  15 dB
> >> at frequencies higher than 15 GHz, even 10 GHz made problems.
> >> The mentioned articles don't consider the frequency respectively just
> >> consider designs where the right-angle bend is electrically smaller
tha=
> >> n a
> >> rising edge.
> >> Would be interesting to know how two 45 degree corners behave at higer
> >> frequencies.
> >>
> >> Regards, Matthias
> >>
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