mixed mode format touchstone file

I'm trying to generate 2-port mixed mode S-parameters from a 4-port S-parameter file loaded in IE3D/MODUA 8.0. I define the frequencies, simulate and then the results come up and I plot them.

The problem is that if I do the conversion manually, the results turn out completely different than in MODUA.

As far as I can see, I'm doing all the right things, reading the touchstone file, converting to the complex format from the MA format by doing r*exp(i*rho*(pi/180)), where r and rho are the polar form of the complex number.

Then I add the complex values for S11, S13, S31, S33 in the following manner and plot the :

SDD11(dB)=20*log10(abs(0.5*(S11-S13-S31+S33)))

Is this the differential conversion that IE3D implements as well??

Hi, Plammox:

The implementation on IE3D/MODUA is a general implementation for N-port. It has been there for years and used extensively and it should be correct.

I have not seen your formula. Is it for the special case of single-ended 4-ports converted to differential 2-ports? From what I see, the resulting 2-port differential s-paramters should be related to all the elements of the single-ended S(i,j), where i, j=1,2,3,4, if both the original and the resulting s-parameters are normalized to 50-ohms. Can you let me know where the formual is from? Thanks!

Hello Jian,

I'm using the formulation of the mixed mode scattering parameters as described in David E. Bockelman and William R. Eisenstadt, ?Combined Differential and Common-Mode Scattering Parameters: Theory and Simulation,? IEEE Trans. On MTT, vol. 43, pp.1530?1539, July 1995

They are used in the XFP optical module standard which is freely accessible on http://www.xfpmsa.org (click on MSA on the menu bar to access the standard itself) The MSA contains a cookbook for the differential s-parameters in one of the appendices.

I successfully extract the single ended s-parameters from the touchstone file, and there is good agreement btw my calculation and MODUA for them. The problems start when I try to generate the mixed mode s-parameters. Then I can't find an agreement. (the differential return loss and insertion loss are what I'm interested in for this application)

Ultimately, I'd prefer using MODUA for this instead of MS Excel... I just need to understand what the differential conversion in MODUA actually implements....

Can you point me to an explanation or article that explains this?

Hi, Plammox: The implementation in IE3D is a general one for any number of ports with combination of differential ports and single-ended ports. It is not from any book and we developed it internally here. If you can send me (jian@zeland.com) the s-parameters with description and your results based upon the formula you have, I can check it for you. Happy Thanksgiving Day!