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Johanson provides S-parameter measurements for their RF capacitors in touchstone file format (see here). Specifically, looking at the S-Series files, they provide series and shunt type measurements.

I downloaded the files R07S100-H-SER.s2p and R07S100_SNT.s2p as an example and performed an S-parameter simulation using QUCS to visualize the different S11s.

Usually, when simulating e.g. matching networks in low frequency ranges (up to 3GHz) I was only using series files and thought I could just as well use them in shunt configuration. But looking at the simulation of the Johanson components I see quite different S11 behaviour for higher frequencies.

I would like to know why these two situations are so different or if there is a mistake in my assumptions or simulation.

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    \$\begingroup\$ If you swap ports 1 & 2 on the shunt file do you see any difference in the S11p3 simulation results ? If you leave port 2 open-circuit on the shunt file, do you see any difference in S11p3 results ? \$\endgroup\$
    – citizen
    Commented Oct 18, 2022 at 11:28

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Generally speaking, S-parameter models are small-signal linear behavioral models of a component or circuit with any number of ports. They can easily capture frequency dependencies. In this cases S-parameters properly capture the decay of capacitance, the complete variations of effective series resistance, and the lowering of effective inductance, as frequency increases. They can even capture secondary internal resonances, and all this in shunt and/or series configurations of the components being characterized. In the case of these capacitors, one will expect a variation in these parasitic second order behavior at higher frequencies between shunt and series configurations. Most useful for RF circuits analysis will be the series measurement as you can use it in series and in shunt. From my understanding the shunt files will be more useful and probably more accurate for PDN analysis.

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