[SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz frequency
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: <alexh1@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 25 Nov 2003 08:52:04 -0700
Andy,
Eric Bogatin's new book does a great job of explaining the issues involved,
Sec 9.4 - 9.11.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Alex Horvath
Sent: Monday, November 24, 2003 4:05 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz
frequency
Exactly, it seems to be fairly common misconception that skin effect losses
always dominate. Dielectric losses are proportional to frequency, although
they start out lower they will always exceed skin effect losses between 500
and 1Ghz or so. The biggest factor affecting this seems to the resin content
of the PCB.
I'd like to hear more about the properties of the resin, if it can be
manipulated for less loss and what are the tradeoffs. I wasn't able to find
out much about resin properties on the web.
Thanks
steve weir <weirsp@xxxxxxxxxx> wrote:
Stuart, for common dielectrics, somewhere in the high hundreds of MHz
dielectric loss overtakes loss from skin effect.
Steve.
At 05:16 PM 11/24/2003 -0500, Stuart Brorson wrote:
>True enough, but aren't the dielectric losses in FR-4 more or less
>frequency independent (i.e. to the <10% level that the dielectric
>function is constant)?
>
>Anyway, Tom is right: you need to consider the frequency variation of
>both Dk (the dielectric function) and Df (the dielectric loss
>function). However, both are typically dominated by skin effect
>losses, particular in the GHz range.
>
>Stuart
>
>
> > To make the answer complete you need also to include dielectric losses
> which
> > also are frequency dependent.
> >
> > Tom Dagostino
> > Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC
> > 2926 SE Yamhill St. Device Modeling Division
> > Portland, OR 97214 13610 SW Harness Lane
> > Beaverton, OR 97008
> > http://www.teraspeed.com 503-430-1065
> > tom@xxxxxxxxxxxxx
> >
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Stuart Brorson
> > Sent: Monday, November 24, 2003 1:22 PM
> > To: andyk@xxxxxxxxxx
> > Cc: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz
> > frequency
> >
> >
> > If I am not mistaken, your question is equivalent to this one: Is the
> > dielectric function constant over the 1 MHz -- 1 GHz range? The
> > answer is: It is not constant, but varies by several percent or more
> > over that range for many FR-4s. (FR-4 is not a single material, but
> > is a family of materials, each different depending upon your PCB
> > vendor, and whatever batch of materials he was shipped on any
> > particular day.)
> >
> > However, the bigger problem for you to think about is the loss due to
> > the skin effect in your copper traces. The skin effect losses vary as
> > the square root of the frequency, IIRC. The frequency-dependent loss
> > due to the skin effect dominates over the frequency variation of the
> > FR-4's dielectric constant.
> >
> > Here are some references:
> >
> > http://www.tdasystems.com/library/appnotes/siml0803.pdf
> > http://signalintegrity.com/Pubs%5Cnews%5C4_5.htm
> >
> > Stuart
> >
> >
> > > Hi all,
> > >
> > > I am designing a high speed PCB at over 1Ghz frequency using FR4
> material=
> > > .
> > > I am wondering how much the dielectric constant will be affected by
the
> > > frequency over 1Ghz as compared to 1Mhz?
> > >
> > > Thank you
> > >
> > > Regards,
> > > Andy
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