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plotting Smith charts

dzakharian

Hello,

I'm running an .sp analysis with a simple setup--port set to 100ohm, with 100ohm load.  I'm plotting results on a Z-Smith, which i thought should give me Re(Z)=1 (normalized)... but instead I'm getting something like 2e-12 as the prime center.  I don't understand why this is the case.  I'm relatively new to Smith charts, but I'm guessing the plot actually shows reflection coefficient, not the impedance?  If this is the case, how can I request an impedance plot?

I'm on version 6.1.8.

Many thanks in advance for your help,
David

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  • ZoltanT

    Hi dzakharian,

    It seems good to me. You have a 100 Ohm reference system loaded with 100 Ohm. No reflection, so S11=0. The 2p is probably numerical error. 2e-15 is very good accuracy!

    The Z- and Y-Smiths returns the same reflection coefficient plane, just with different grids. Clearly this is not how one works with Smith charts on paper, but once the server does all the calculations it is great, or even better.

    Regards,
    Zoltan

  • dzakharian
    in reply to ZoltanT

    Hi Zoltan,

    Yes, the matching at S11=0 is perfect, in terms of scattering parameters.  But I'm interested in the actual impedance, not scattering parameters (so Z11, not S11). 

  • Andrew Beckett
    in reply to dzakharian

    Why not plot Z11 then? You could plot zpm('sp 1 1) on a polar chart (not sure it makes much sense to plot Z11 on a Smith Chart, but I'm not a microwave engineer so I might be talking rubbish). You can also plot real(zpm('sp 1 1)) and imag(zpm('sp 1 1)) on a rectangular graph to see the complex impedance that way. Or plot one versus the other with waveVsWave.

    Andrew.

  • ZoltanT
    in reply to Andrew Beckett

    Andrew, a stereotypical typical microwave engineer is more likely to think in terms of normalized waves, than fuddling with impedances or dealing with the differences between a current or voltage signal.

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  • ZoltanT
    in reply to Andrew Beckett

    Andrew, a stereotypical typical microwave engineer is more likely to think in terms of normalized waves, than fuddling with impedances or dealing with the differences between a current or voltage signal.

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