[SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz frequency
- From: sdb@xxxxxxxxxx (Stuart Brorson)
- To: tom@xxxxxxxxxxxxx
- Date: Mon, 24 Nov 2003 17:16:09 -0500 (EST)
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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