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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!

  • redwire

    I ran this in 16.6 and got 122 ohms.  I  noticed you have an etch factor of 90 in Cadence which is impossible.  Try 45.

    I cross-checked in Polar and got an impedance near Cadence.  Saturn is good but there are some hidden factors that you may not be getting told about.  All that said -- you do know that this is a loosely coupled differential so it's essentially a moot point trying to do this as a coupled pair -- look at the single ended impedance and the gap you have specified...

  • jmadsenee
    in reply to redwire

    Thanks, redwire.  That cleared up some things for me.  The paper steve referenced suggested "Trapezoid traces are specified by setting the trapezoidal_angle_in_degrees variable in the "Signal Analysis" category of the User Preferences dialog. I used "74" as suggested in SourceLink solution 11064512."  I had looked, but didn't have trapezoidal_angle_in_degrees.  Now I understand that that is "Etch Factor."  I guess Saturn must assume some etch factor...

    This is my first time trying to use the calculation for a differential pair; can you please explain what you mean by "loosely coupled?"  When is it good enough to just look at the single ended impedance?

  • redwire
    in reply to jmadsenee

    Hmmm...I left a detailed explanation a couple of hours ago but it seems to have disappeared.  Maybe I'll write the explanation another day...

  • jmadsenee
    in reply to redwire

    That's happened to me...  I would appreciate that explanation; brief would be fine...

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