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S-parameter over zero in differential measurements

Question asked by matkok on Mar 11, 2007
Latest reply on Mar 12, 2007 by matkok
Hi,

I'm doing some measurements using an HP8720 and Cascade differential microprobes. I tried several times but the results are strange: a single ended S paramater (S11) goes over zero around 15 GHz (also the differential modes go over zero). The calibration seems good and also the contacts. I'm using Agilent multiport 1.38 with the last service pack. I think I'm making some mistakes with this software: for example during the calibration setting I cannot choose the model of probe I'm using: is it normal? Maybe you know some possible reason for this kind of error.
Thanks,

Matteo  

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