Electronics > PCB/EDA/CAD

Calculating FR4 permittivity for another frequency

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lindworm:
Hi guys,

I've designed a pcb for an 868 MHz RF project and need the permittivity (aka dielectric constant) for the frequency I'm using it at.
The datasheets of the FR4 manufacturers usually only contain the permittivity at 1MHz, is there any way to calculate the permittivity for another frequency?

If you have a rule of thumb instead of a formula thats fine too.

Kind regards

lindworm

free_electron:
Thats like saying i want to calculate the speed of my car on gravel, instead of on asphalt...

First of all : what car ? There is no such thing as FR4 ... FR4 is a class of fire retardant materials. There are hundreds of different FR4 substrates. Even Rogers, Nelco or Isola RF substrates are FR4...

You need to know specifically what your board house is using and you need to know the prepreg thickness and layer stack. Once you have that info you can dig up the data for what you are going to build.

lindworm:
Hi free_electron,

I do have the data I need (it's SHANGHAI NANYA SN-L4 FR4 NY1140) and I know that FR4 differs a lot between manufacturers, products and even batches.
My problem is that I know the specific Material used and it's dielectric constant at 1 MHz.
However, I couldn't find a way to figure out the dieelectric constant in the frequency range I design for.

Some datasheets for other base materials have three or four values, depending on the frequency, but most datasheets only list the permittivity at 1 MHz.

Kind regards

lindworm

free_electron:
Ouch, then it becomes difficult... Call them ?
Or, have a proto run done with a coupon for impedance co trol and test it

andersendr:
Have you looked at the Wiki?  http://en.wikipedia.org/wiki/FR-4  It looks as if it does not change a hole lot with frequency.  The 1GHz is listed.

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