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Negative impedance error when using 2 port s-paramter block.

Question asked by jhinri on Apr 28, 2004
I have some measured 2-port s-paramter data for a filter, and I want to use this filter in a spectrasys simulation.  The s-paramter block works fine as long as there are no other elements in the schematic, but as soon as I starting adding anything else (even a resistive pi-pad in front of the 2 port block) I get negative impedance errors.  The error says that there is a negative impedance looking into the output node of whatever is connected to the 2-port block's input (e.g. a resistor.)  What is causing this?  The ability to use measured data in my simulation is very important.