[SI-LIST] Re: R: Re: Question on varying the coupling ratio on a differential pair
- From: "OPREA Dorin" <Dorin.Oprea@xxxxxxxxxxxxxxxxxx>
- To: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- Date: Fri, 19 Jan 2007 12:26:49 -0600
It works when the PCB vendor is known; the problem comes with 'design
anywhere and manufacture everywhere'.
Regards,
Dorin
________________________________
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
Sent: Friday, January 19, 2007 1:06 PM
To: OPREA Dorin
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [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>
<mailto:ryan.satrom@xxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
<mailto: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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