How to set port impedance for S-parameter in co-simulation

Hi all,

I need helps on how to set reference impedance in a co-simulation between circuit/EM simulation (ADS/CST). I have a complex circuit including conventional lumped components such as inductor, capacitor, transmission lines, etc. and a complex EM structure (in CST) which cannot be easily treated as an equivalent circuit.

My purpose is to run this circuit in ADS using Harmonic Balance simulation. My idea to run this circuit is first extracting S-parameters of the EM structure using CST, then inserting this S-parameter data into ADS. This is what we call co-simulation.

The problem is because the extracted S-parameter depends on port impedance (or reference impedance), how I can set correct port impedance in CST to obtain a correct result of the co-simulation in ADS? In think the port impedance can not be set to 50 Ohm because in real circuit the EM model is placed between non-50 Ohm components. This issue has really made me a big trouble so far.

I think this question is similar to how we can use S-parameter model of lumped components in Harmonic balance simulation.

I would highly appreciate if someone could help!

Thanks for your consideration.

Kind regards,

Fortunately, it is simpler than you expect. You can use 50 Ohm S-parameters, or any other well defined reference impedance. The "black box" behaviour is fully defined then, and the simulator can calculate the terminal voltage and currents for any external load, regardsless of the S-parameter reference impedance. To keep it simple, we use 50 Ohm.

Dear Dr/Mr. volker@muehlhaus,

Thanks so much for your prompt response and clear explanation. Actually so far I don't know that the "black box" behaviour can calculate the terminal voltage and current regardless of the S-parameter reference impedance.

Regards,

Maybe it is more intuitive if you think of Y or Z parameters. These can be calculated from the S-parameters (reference impedance must be known, of course).

Thanks for your quick response! Because previously I thought that voltage and current of the "black box" in ADS depends on the port impedance of the S-parameter, when converting this S-parameter to Y- or Z-parameter, these parameters also depend on the port impedance.
From your answer I now understand that the voltage and current at the terminals of these parameters (S, Y and Z) block in ADS don't depend on their reference impedance.

I would appreciate your time for the discussions.