[SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz frequency
- From: sdb@xxxxxxxxxx (Stuart Brorson)
- To: andyk@xxxxxxxxxx
- Date: Mon, 24 Nov 2003 16:22:02 -0500 (EST)
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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