danmc
Community Member
Offline
Posts: 35
Boston
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Rather than use calculations to find S11 and then more calculations to find Z and then Leq, why not just directly measure Z in simulation? It is simpler. Either should work, but you're making it harder than it needs to be. Reasons to use s-parameters at all for something like this would be things like, that is what the simulator gives natively (not true in your case), or you are doing real measurements on real components at RF frequencies with a VNA. In that second case, you are again in the case where s-parameters are what the instrument wants to give you so that is what you work with. But for a spice-like simulator, no reason to not directly measure Z=V/I or Y=I/V and work with that.
As to what is wrong, again, make it simpler. Put an inductor in place of that active block in your schematic (not just a calculated result into your spreadsheet). Make sure that you get the correct result. If you don't, then it points to a problem with your netlist (for something simple like this, *look* at the netlist. Does it make sense? Do you have a sign flip somewhere? A control vs output node switch somewhere? ) or a problem in your post processing.
For the example output numbers, it is no wonder your inductance is off because the S11 value you start with is not correct for an inductance. You're completely on the wrong side of the smith chart. For an ideal inductor, you start off with zero impedance at zero frequency which places you at S11=-1. This is on the unit circle on the smith chart, clear over at the left side and on the real axis. As frequency goes up, you move clockwise around the smith chart and don't get to +1 until infinite frequency. Your number places you near +1 but on the lower half which with a real inductor (one with a self resonant frequency) means you're operating past resonance. Or it means you failed to insert the inductor into your test fixture or make good contact and you just have a small capacitance. Again, the troubleshooting is done by simplification. Do open circuit, short circuit and Z0 load standards all give the correct results? If not, fix that problem first (bad cable?). In simulation it is the same. Do elements with exactly known results give the result you expect? If not, then the test setup is broken. Once the test setup is proven out, take it a step towards what you really wanted to check. In the lab, try a different inductor, maybe you had a bad one. In simulation, move from the ideal inductor to the vccs version of the gyrator.
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