What reference impedence should be defined for a lumped port
HFSS Online Help says that
'lumped port are similar to traditional wave port, but can be located internally and have a complex user defined impedance'.
I am wondering what reference impedence should be defined.How will the reference impedence affect simulation results?
appreciate anyone who bring more information. :D
jiaping:
The reference impedance simply means the internal impedance you want for the port for S-parameters. Usually, we just use 50 ohms, but HFSS apparently allows you to set any other termination you want. The impedance is usually an ideal lumped impedance that is co-located with the lumped port. Therefore, it is usually on the ideal wire on which the port is located, at the same point as the port.
These internal ports in full 3D simulators only work well for low frequencies. Otherwise, since they are not really wave ports but rather are on ideal wires stretched between two metal bodies, they will exhibit port parasitics that cannot be removed by de-embedding.
For example, if you put an ideal internal port between two metal microstrip lines in a full 3D simulator like HFSS, it is like stretching a bond wire between the lines and putting a small ideal port and an ideal lumped impedance at a point in the middle of the bond wire. You know that this will have extra parasitics in the results, like the extra capacitance/inductance for the junction between the wire and the microstrip line on each side, and the wire itself has a self-inductance. The wider the distance between the two bodies (in this case the microstrip lines) the worse the added inductance becomes.
For your last question, the lumped port impedance will affect the normalization of the data from your port. Also keep in mind that the ground reference for that port is simply the negative terminal of the port. It will not be normalized to the same ground as the waveports unless you explicitly stretch it to a metal body with contact to the outer ground of the simulation.
Be careful with these lumped ports. Make sure you understand their limitations.
--Max