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#73 Ngspice-42 or 43 Cannot calculate minimum noise figure (NFmin) if S2P file is used
Ngspice 42 or 43
Qucs-s 24.3
I am trying to design an LNA at 170 GHz frequency using SG13G2. I tried to build the circuit with transmission lines, using the S2P files of transmission lines. I faced the problem of 'nan' in the minimum noise figure (NFmin). It seems that the simulator cannot calculate NFmin.
if I use lumped elements, the NFmin gives value. However, when I use S2P files as the transmission lines, the NFmin does not provide any value. It seems that the simulator cannot calculate noise figure when there are S2P files.
I am sharing the test case and the 4 s2p files in zip format. The S2P file names are marked in red within the schematic.
@dwarning could you please share your concerns here?
This is the previous discussion on github
I think before we try to solve the original, more complex issue we should investigate the simplified testcase as attached.
We see NaN for resistances lower the 50 Ohm.
Last edit: Dietmar Warning 2024-12-31
@h_vogt @nicolati @pnenzi could you please help me with this problem? I am actually stuck here. I am trying to catch the deadline of IHP open design tapeout.
Last edit: AAli 2025-01-03
My latest commit solves the problem with NaN for low resistances between the ports.
The original problem is actual not solvable because the XSPICE xfer model doesn't support noise analysis. Additional the DC behavior of the xfer model can follow in problems to find operating points in particular circuits.
And XSPICE does not support noise analysis! That might be fixable with a little extra code, but I also have trouble understanding the Touchstone noise parameters. We have a noise figure, but what is the reference level? Some fixed standard, or perhaps the Johnson noise of the reference impedance? But I would think transmission lines have no thermal noise. And what to do with the other two parameters?
Converting the s-parameter files with vector fitting approach from python package scikit-rf can be not only a work around.
See here