[SI-LIST] Re: R: Re: Question on varying the coupling ratio on a differential pair

George,

Thanks for the clarifications.

Lee


> [Original Message]
> From: George Dudnikov <George.Dudnikov@xxxxxxxxxxxxxxx>
> To: <scott@xxxxxxxxxxxxx>; <Dorin.Oprea@xxxxxxxxxxxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 1/19/2007 12:36:56 PM
> Subject: [SI-LIST] Re: R: Re: Question on varying the coupling ratio on a
differential pair
>
> Scott, Lee, Dorin and others interested in Dk discussion,
>   I have been following this thread on Dk measurement with interest and
> wanted to add some input from a PCB fabrication standpoint. All of your
> points and concerns are valid. Please consider the following:
> 1. No two laminate material manufacturers  measure or report electrical
> properties Dk or Df the same way. Some use Beraskin some use Split Post
> Cavity, others Full Sheet Resonance  etc. In many cases, laminators
> prefer to use methods which make the electricals for their materials
> look better. No big surprise. Or they stick with methods that they are
> accustomed to.
> 2. When they do measure, it's usually done on a specific core thickness
> and at several, usually lower discrete frequencies. Higher frequency
> data is usually extrapolated. Again, samples are picked to make the
> electricals look good.  Along with concerns regarding resin/glass ratio
> and  pre-preg Dk effects, there are other variables that haven't been
> mentioned.. like copper foil roughness, glass weave etc. 
>
> The reality of life is that laminate vendor Dk data in most cases is
> only useful for rough calculations or benchmarking comparisons. Any
> serious modeling or simulation , specifically at higher frequencies ,
> requires empirically validated test data representative of the specific
> resin and stackup being evaluated. 
>
> Here at SSCI, we were forced years ago to measure laminate properties
> ourselves,in order to obtain data we could reliably use in our impedance
> models and SI simulations which would then correlate with actual VNA and
> TDR measurements once we built the PCB. We adopted a method we call
> Modified Full Sheet Resonance . There is a DesignCon 2003 presentation
> on our website co-authored with Istvan Novak at Sun which discusses some
> of the early work utilizing this method. Try this link to access
> http://www.sanmina-sci.com/Solutions/pdfs/pcbres/SANM_Resonance_Test.pdf
>
>
> We found this method effective up to about 4GHZ. We have migrated
> towards developing microstrip and stripline based test structures for
> extracting Dk/Df up to 20 GHz using a high frequency VNA. Although we
> prefer frequency domain analysis, we have also benchmarked high speed
> time domain methods such as those published by Alina Deutsch et al at
> IBM and feel both domains have merit. We now have a pretty comprehensive
> data base on all the leading materials from FR-4 to the low Dk/Df
> materials which allows us to accurately select and substitute different
> materials and stackups. 
> Bottom line...you have to measure. My SI folks will be at DesignCon if
> anyone wants to compare notes.
>
> Regards
> George Dudnikov
> Sr.VP & CTO
> PCB Division
> Sanmina-SCI
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Scott McMorrow
> Sent: Friday, January 19, 2007 10:06 AM
> To: Dorin.Oprea@xxxxxxxxxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: R: Re: Question on varying the coupling ratio on
> a differential pair
>
> Dorin
> If you want to accurately know what your Er is, the only way to be sure
> is to measure it yourself, or have a lab measure it for you.  All
> measurements of FR4 Er have errors inherent in the measurement technique
> and the physical size of the sample. You just have to choose a method
> that is as close as possible to your board stackup and traces of
> interest.  Some manufacturer's data only gets you in the ballpark.
>
> 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(r) is the registered service mark of Teraspeed Consulting
> Group LLC
>
>
>
> OPREA Dorin wrote:
> >  Lee,
> >
> > Can I ask if you get also the data for the prepeg and how do you take 
> > in to account for the change in the dielectric properties due to the 
> > PCB manufacturing process?
> >
> > Thanks,
> > Dorin
> >
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx 
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > On Behalf Of Lee Ritchey
> > Sent: Friday, January 19, 2007 12:09 PM
> > To: ryansatrom; si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: R: Re: Question on varying the coupling ratio 
> > on a differential pair
> >
> > Ryan,
> >
> > Most laminate manufacturers have a dielectric constant vs. frequency 
> > curve for their materials.  This is the place to start.  In most 
> > cases, the er starts at some level for 1 MHz and goes down until about
>
> > 2 GHz at which point it flattens out.  This is true for all "FR-4" 
> > class materials as well as Rogers RO 4350.
> >
> > I'd get that data from the laminate manufacturer as a starting point.
> >
> > If there is any good news in this as far as logic is concerned, the 
> > edges are so fast with modern logic that the frequencies of concern 
> > are above 1 GHz, where er is approaching a constant value.
> >
> > It's from 1 MHz to 500 MHz where the greatest slope is for the er 
> > curves.=20 Not all that much a problem for most logic, but a potential
>
> > issue for RF engineers.
> >
> > Lee Ritchey =20
> >
> >
> >   
> >> [Original Message]
> >> From: ryansatrom <ryan.satrom@xxxxxxxxxxx>
> >> To: <si-list@xxxxxxxxxxxxx>
> >> Date: 1/17/2007 9:44:10 AM
> >> Subject: [SI-LIST] R: Re: Question on varying the coupling ratio on a
> >>     
> > differential pair
> >   
> >> Thanks for your feedback.  Sometimes, living in the simulation 
> >> world,=20 you forget the real-world effects of boards.
> >>
> >> Lee:  Is there any way to predict this frequency dependency of er?
> >>
> >> Fred:  Is the frequency dependence of R and G caused by the same=20 
> >> issues Lee is discussing (dielectric constant over frequency)?  
> >> Are=20 there any other causes of this?
> >>
> >> (As a side note, the equation suggests R and G to be fixed in=20 
> >> frequency, while L and C are the frequency dependent values.  I 
> >> guess=20 that's why the lumped element model breaks down and models 
> >> are more=20 accurately viewed as distributed full-wave EM models)
> >>
> >> Thanks again for the comments.
> >>
> >> Ryan
> >>
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