Learning Forum

[M.Moba24]

Why does changing Lumped port impedance in Ansys HFSS change the Z-parameters of my 2-port network? I have put 2 lumped ports, one for input, one for output. Changing the impedance of my output port impedance for example to 100 Ohms or 150 Ohms changes my Z-parameters, but shouldn't Z-parameters be independent of those impedances? if not, how is that impedance shown in an equivalent circuit? Thank you.

[Gia]

This is the expected behavior. You can consider HFSS to provide equivilent measurement data as a Vector Network Analyzer (VNA). All S, Y, and Z parameters also assume a specified refrence characteristic impedance, 'Z0'. For lumped ports in HFSS, the user can set the chacteristic impedance of the port to any arbitrary value and the resulting S,Y, and Z parameters will be scaled accordingly. For example, if you compute the Z parameters with a lumped port set to Z0 = 50 ohm, and then you change this value to Z0 = 75 ohm, then the change in Z parameters that you observe will be the same that you would observe if you were to measure the Z Parameters using a VNA with Z0 = 50 ohm ports, and then compare to the same measurement but using a different VNA with Z0 = 75 ohm ports.

[M.Moba24]

Thank you very much for your response. I had 2 more questions: 

1. Imagine I want to put a 180 Ohm Load resistance at the output port in real world experiments. What should I put for the output lumped port impedance in HFSS so that it matches the real world experiments? should I put 180 Ohms?
2. If I want to export an s2p file from HFSS to another software and measure the output voltage for a 180 Ohm resistor, should I put the resistor at the output or is it considered in the snp file by itself? (I am using a balanced differential signal) for example is the image that I attached the correct way to put it?

[Gia]

 

1) Consider how you would make the connection of the physical device to a physical VNA. – A lumped port is essentially a connection to a virtual-VNA where the characteristic impedance of the virtual-VNA is the same characteristic impedance that is defined in the lumped port settings. – This is why the lumped ports default to 50 ohms. – If the physical VNA is not connected to the 2nd port when performing the physical measurement, and it is simply terminated with a 180 ohm load, then to make a direct comparison of measurement vs. simulation of S11, you can replace port 2 in HFSS with a ‘LumpedRLC’ boundary set to 180 ohms and export as a 1 port touchstone file.

2) In a similar way as the lumped ports in HFSS, the symbols used to represent your ports in your circuit model should also have a setting for their reference characteristic impedance. You should see similar results for your S11 for the above case as if you were to use a 180 ohm port impedance for your 2nd lumped port while keeping the default 50 ohm impedance for the virtual ports in the circuit schematic.

There is also a built-in Circuit tool with the Ansys Electronics Desktop that is similar to ADS, here are some YouTube videos on the topic:

Steps to Co-simulation Using Ansys HFSS and Circuit

PIN Diode simulation in Ansys HFSS + Circuit co-simulation | Part 1