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

  • From: "OPREA Dorin" <Dorin.Oprea@xxxxxxxxxxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 19 Jan 2007 11:55:37 -0600

 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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