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Cross Section Editor Impedance Calculator

jmadsenee

Hi All,

I am trying to use the impedance calculation in the Cross Section Editor.  I have compared the results to other impedance calculators.  Three different calculators give very consistent results, while the results from the Cross Section Editor differ greatly.  Attached are screen shots from the Editor and Saturn PCB Design's Toolkit.  The Saturn Toolkit results match the other two calculators I tried.  Looking at the TOP layer, the single ended impedances are different, and the differential impedances are very different.

Should the dielectric constant for the top layer be 4.5?  I read in another post that it should be 4.5 for the internal layers because the prepreg squishes around them, but what about the top?  Dropping it to 3.33 brings the single ended impedance close to the other calculators, but not the differential.  Why should that dielectric constant matter at all?

So, the question is:  Why should I use/trust the CSE calculator when 3 others give consistent different results? 

Thanks or any insight you can give!

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  • steve

    Someone wrote a paper on this back in 2006 which might help explainCDN2006_Users_Paper_Cross_Section.pdf

  • jmadsenee
    in reply to steve

    Thanks, Steve.  Interesting paper, but it just muddied the waters a bit more for me...  I asked my PCB manufacturer for the dielectric constant of the soldermask; I do not have those layers there and it seems they should be taken into account.  Maybe that will bring the calculator results closer together.  I also wrote to the calculator people and asked them about their assumptions.  Even after I add the soldermask layers to the CSE, I still wonder - can I trust the CSE results?

Reply
  • jmadsenee
    in reply to steve

    Thanks, Steve.  Interesting paper, but it just muddied the waters a bit more for me...  I asked my PCB manufacturer for the dielectric constant of the soldermask; I do not have those layers there and it seems they should be taken into account.  Maybe that will bring the calculator results closer together.  I also wrote to the calculator people and asked them about their assumptions.  Even after I add the soldermask layers to the CSE, I still wonder - can I trust the CSE results?

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