How to simulate CPW impedance in ADS/Momentum ?

Hi, so I'm trying to understand this -- using ADS/Momentum and CWP lines, I'm starting in the schematic editor, adding a CPWSubstrate and a CPW line of a certain width/gap/length which is matched to that substrate.

I then simulate this and am trying to see/optimzie the CPW width/gap size(s) in order to obtain 50 ohm matching.

This doesn't work; plotting PortZ(1), and PortZ(2) for the two ports results in 50 Ohm exactly, which is the impedance of the termination ports.

Then I try to use this and dump it into Momentum and do the same simulation; I get a warning about port impedance:

"WARNING: The 2D port solver data is not available. Using default values (50 Ohm for the transmission line parameters".

and

"WARNING: Port Solver failed to compute propagation constant and characteristic impedance. 50 Ohm default impedance will appear in Dataset"

Any thoughts? Can somebody please send me a sample DSN file which shows port impedance calculations?

Thanks!

warnings can be ignored.
Warnings indicate the designer to double check the simulation environment.

Not in this case; in this case it will just give you a crap (i.e. fake) answer on the CPW impedance.

Does anybody know how to simulate (and view) CPW impedance in ADS/Momentum, outside of the Libra line-calc tool?

Thanks!

I think it is not easily possible to directly compute the characteristic impedance in ADS without using linecalc. At least, I don't know any way to do that and all the people I know use LineCalc.

When you use a port in ADS or internal/ground ports in Momentum, you set an impedance which usually is 50 ohm, and this means that you are attaching to your structure a semi-infinite transmission line which has 50 ohm characteristic impedance at any frequency. This is the reason why the port impedance is always 50, this parameter cannot provide you the impedance of your structure.

If you are interested just in the optimization of a 50 ohm cpw, you can use an optimization tool in which your goal will be to minimize dB(S(11)). Then you fix one between cpw width or cpw gap as the parameter to optimize and lunch the simulation in ADS. You can do the same procedure in Momentum.

If you want to know the exact value of the impedance of your cpw at a given frequency (remember that cpw is a planar structure in a not-homogeneus dielectric, so it is dispersive) I do not have any suggestion regarding momentum or ADS.

Yes but at least from my experience if you have microstrip line for example you are able to determine its characteristic impedance using ADS and Momentum. You just have to plot the impedance looking into the line, and *not* the impedance of the 50-ohm standard term (ADS) or port (Momentum) and you can optimize the line to match the linecalc results...

The same cannot be said about CPW; that's why my question...

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