Connection of ports to ground

[LorenzoMarcellino]

Hi! I want to ask a question about the correct positioning of the ports and their connections. I'm simulating the propagation of a signal in a strip on a simple PCB. That strip is floating on the PCB and then goes to a connector that transports the signal to an ASIC. I just want to simulate how the signal from the starting point of the strip arrives to the ASIC connectors and see how much effect has the path of the strip. In particular I want to extract the S-Parameters matrix of my system. What I'm concerned about is that in my real PCB the strip is not connected to the ground layer under the PCB, so if I put a port to inject an input signal, and this ports as in many videos an tutorials is putted from the ground to the trace, I would create a trace-ground connection that in reality doesn't exist. This should modify the behaviour of the simulation and in theory produce a different result respect to what I have, right? Am I missing something? Can I create a port that isn't connected to the ground but instead it is "incapsulated" in the trace (so same transverse section and long some mesh cells along the direction of the strip) to inject the input signal (and receive the output one)? In that way I wouldn't create that connection to the ground that doesn't exists. My point is that I'm not sure that what I saw in the tutorials and examples would simulate properly the real connections of my trace.

[VolkerMuehlhaus]

Can I create a port that isn't connected to the ground but instead it is "incapsulated" in the trace

No.

Port requires two terminal, signal and ground. There is no other solution. This is the same for all RF EM solvers, because S-parameters are only defined with ports that have signal and ground.

[LorenzoMarcellino]

For the ASIC-related part I see your point and yes it's working. For the "input" part of my strip it's a bit different because the strip isn't physically connected to anything. The signal is induced from the movement of the charges in the active part of the detector and the induced signal on the floating strip then moves to the ASIC.
Since this is the mechanism, does it make sense to put the input ports at infinite resistance and the output ones to the ASIC impedance? Or should I put 50 Ohms and have the same impedance between input and output ports?

[VolkerMuehlhaus]

For the "input" part of my strip it's a bit different because the strip isn't physically connected to anything.

I don't understand this concept. For me coming from electronics, no current can flow through an open ended wire.

From what you describe, I have no idea where you want to put the "input" port then, i.e. what is the ground reference? S-parameters for port with infinite resistance make no sense, results are undefined, so I still don't know what your are trying to do?

But I'm curious now, and would like to help you, so feel free to contact me so that we can arrange a short video chat and look into your model together. No cost, no promises. https://muehlhaus.com/

Best regards
Volker

[LorenzoMarcellino]

The physical process that generates the signal is the current induction by a moving charge . The current signal is induced in the strip (that in the active area of the detector is floating on the PCB) and goes to the electronic readout. Under the pcb there is a ground plane that is the reference plane but in the initial part the strip isn't connected to it. The current path passes by the ASIC.

In any case I think that I'm gonna put the ports from the ground to the strip as you suggested because I don't want to see how the real current signal is propagated but just to extract the S-Parameters matrix of my model in the frequency range of interest, and then use it in post processing with a circuit simulator.

Thank you so much for your help! I will contact you for sure!

Best regards
Lorenzo

[VolkerMuehlhaus]

but just to extract the S-Parameters matrix of my model in the frequency range of interest, and then use it in post processing with a circuit simulator

Great, then you can use default 50 Ohm ports. When using S-parameter data in a circuit simulator, this will automatically re-normalize the data, and the port impedance value used during EM simulation doesn't make a difference.

Best regards
Volker