Scattering matrix cascading : simple case with Ansoft Designer & HFSS

1/2*abs((0.5+0.3i)*(0.9678-0.25103i))^2=0.1699

Hello,


Anyone who master scattering parameters definition in HFSS & Designer ? Even the HFSS support don't want to help me !


Best regards,

The definition of the scattering parameters in HFSS and Designer is common, not so special.

Hello,
What do you mean ?

I agree with you on the fact that the HFSS (& Designer) definition of the S parameters is common. However, the link between S-parameters and 'Scaling Factor' is not clear (to me for sure). Using basic S-matrix algebra, I can't find the same results with a simple model.

For 1W input power, a1 = sqrt(2) + 0*j (and not 1)
This is because:
P_incident = |V+|2 / (2*Zo) (V is peak value)
and
an = Vn+ / sqrt(Zo)

This still does not give the same values as HFSS.
Are all of your ports (in HFSS and designer) normalized to the same impedance (e.g. 50 Ohm)?

Best regards

Hello, and thank you for your answer,

Did you took these definitions in Designer or HFSS's help ?

HFSS's support gives me the following explanation, which should come from the help (but I didn't managed to find it!)


In Designer, in the "port definition", I've checked the "one port data" option of the HFSS model, i.e. Designer should use the characteristic impedance of the HFSS model. So, if I'm right, the characteristic impedances should be the same.

Both formulas are from this lecture note:
http://whites.sdsmt.edu/classes/ee48...1Lecture17.pdf

OK.

Because the definition of general scattering parameters may vary from a software to an other.

In HFSS's help, S parameters are defined in a modal way :

Digging in the help, I can only find that concerning the scaling factor in the Edit Source menu:
what is really unclear to me, is relationship between the unitless complex amplitudes of the incident (and reflected) modal fields am (and bm) and the scaling factor. And finally, how the scalling factor are calculated from Designer ?

I have found a way to calculate ScaleFactor / OffsetPhase, using Qucs:
Qucs project: Quite Universal Circuit Simulator
It works for my project.

Thank's a lot for digging this problem. Actually, I've drop this problem but I didn't found the solution. I'll recontact the HFSS support.

I didn't know this software. I'll look at it, thank you !

Looking at your results, I've a few comments/questions :
- The frequency used in my test case is 3.7 GHz (in your file, the power source was @1GHz, but the results is the same : what mean this parameter ?)
- The offset phase sign in HFSS is defined by the sign of the port field line. So a +/- 180° difference is understable.

Thus, I've also found a scaling factor of 0.34, exactly half of the scaling factor defined in the HFSS doc... That would means that there is something wrong the HFSS doc. Anyway, this is not the result that Designer find... So I still do not understand...

Some extra comment:
- In fact, the port impedance may also be defined as the characteritic impedance of a rectangular waveguide ? (for the fundamental mode here). This characteristic impedance is Zc=455.7152 Ohm. (in your file, this was 50Ohm, the same in the S-parameters description files). The final result (traveling waves or scaling factor) is the same, since it only a matter of normalization.

OK, I've found in the Qucs documentation that the quantities are defined in RMS, not peak. This explain the factor 2 of the scaling factor.

But this still do not explain the Designer result.

PS : I've made the simulation directly with a rectangular waveguide instead of the 2port Smatrix : results are the same.

The frequency property of the power source can be ignored. It is overridden by the ac simulation block
If you define emf in the other direction, than the 180° offset disappears.

Do you mean the paragraph
"Sometimes waves are defined with peak voltages and peak currents. The only difference that appears then
is the relation to power "
in the "technical papers? (link: http://qucs.sourceforge.net/docs/technical.pdf )
This does not refer to the measured U and I quantities as I understand.

The qucs "Workbook" (link: http://qucs.sourceforge.net/docs/workbook.pdf )
says:
"Please note that all voltages and currents are peak values and all noise voltages are RMS values at 1Hz bandwidth."

Yes, this is what I understood from this doc. Am I wrong ?


I've now understood how HFSS's scaling factor are calculated from Designer. The scaling factor in HFSS are:

OK, so finally I managed to find how to achieve the same results in Qucs than in Designer/HFSS.

Attached, there is 3 similar Qucs simulation file and the associated S-matrices of the waveguide and the load.

If I well understood, in order to get the same results than in HFSS/Designer, I have to:

1. The Scattering matrix of waveguide, which is a touchstone the 2-port file, must be renormalized to the characteristic impedance of the waveguide (ie adding R 265.49 in the file). Otherwise, I get SF=0.34 instead of SF=0.26.
2. The scattering matrix of the load, which is a 1-port touchstone file, must be normalized to 50 Ohm

• In Qucs, the waveguide element is the same than the 2port file renormalized to 265.49 Ohm reference impedance (the reference impedance is the characteristic impedance of the waveguide, which is quite logical)
• In Qucs, a equation-based component defined by its S parameters assumes a 50 Ohm reference impedance (and it can't be changed to another value).

Hi,

I've finally understood the problem: in Designer, when creating a N-port bloc-device (my load in my example), Designer assumes that the S-parameters of this N-port device are normalized to 50 Ohm. It seems that it cannot be changed.

A workaround solution is to create a touchstone file with the correct characteristic impedance of the N-port defined inside, and import it into Designer. Then Designer will normalize the S-parameters of the device with the correct characteristic impedance.


Best regards,