simulation of S-paramaters in CST and stability analysis

[tjuby]

Greeting, this is my first post here on this board. I tried to find the most fitting forum for my question so I hope this will be the correct one.

Before I ask my question I would just like to pay my gratitude to this board which has given me a lot of knowledge about RF design and such. Nevertheless, I couldn't find anything specifically regarding my problem.

I am currently working on bettering the design of a total effect radiometer. A previous realization of this resulted in oscillation between three LNAs in cascade. There was not done any specific stability analysis, so this is now my task. To analyse the stability and come up with a better design.

I use CST Design Studio to import touchstone files for the different components. These include 3 LNAs and a cascade of two lowpass filters and two highpass filters.

The design frequency is 3.5 GHz.

Performing an S-paramter analysis in CST gives me the possibility to view magnitude in dB for the elements of the S-matrix, or to view the phase, polar plot and smith chart. (If you are familiar with CST this is probably common knowledge..)

I was wondering if anyone know if there is any way to perform a stability analysis, calculating Rollets condition and the determinant of the S-matrix within CST and also to draw stability circles. Or do I have to use another software for this?

Also, I was wondering if I could just export the data from the plots into matlab and perform some calculations there, but I am unsure how to do this.

If anyone has any good suggestions I would be very grateful

If there's any ambiguity about my problem please ask for more detailed information.

best regards,
Alexander

 

[mamali]

Probably exporting to Matlab is the best way, your task fits Matlab. To do so, simply right click on the S parameter you want the analysis on (say, S11), copy, and then past to your Matlab Mfile, or notepad; you'll get a two column table of S11, the first is the frequency, and the second is the S11 in that frequency (in dB if you chose so).
There is also another way. The Visual Basic Scripting is quiet powerful in CST, you can use it to write your own codes very efficiently. You may consult the CST comprehensive help for this matter.
And, as far as I know, you can use "Simulation -> New Task -> Amplifier ... " in Design Studio to define amplifier tasks, n which you will find a check box for Stability Factor calculation, and you also can use Genetic Algorith, the new Simpleton Method, and Simulated Annealing for optimization of your circuit. You should better know that you can mix microwave studio and design studio elements into one circuit.
I hope this can help you, and have fun with you task

.m

 

[tjuby]

Thanks for the answer mamali.

I have done the following:

S parameter analysis in CST Design Studio.

Exported (real and imaginary) data for all S parameter of the system to ascii files.

Loaded them into matlab and merged the vectors to look like this:
S11 = 0.23 + j*0.342 etc for all frequencies.

This allows me to compute for example Rollet's stability factor K with auxiliary conditions or the \mu-condition for stability for specific frequencies or for all frequencies. But I'm having trouble drawing staility circles along with smith charts as there is no built in function to do this. I have tried some user-made toolboxes available for this such as the one supplied here, https://www.ece.rutgers.edu/~orfanidi/ewa/
But I can't quite make it work. Guess I'll just have to try some more.

I am not familiar with visual basic at all, so I think given the time limit (15. december) it'll be a tough one to write any code sufficient for my use .. But I might have to look into it when I have time.

My CST licence does not support amplifier analysis unfortunately Just as unfortunately my MatLab licence does not support the RF toolbox, which does all of the things I am having trouble with.. Typical that..

Anyway, thanks for the answer and I sure do have fun with my task.. I just wish I had more time