[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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