## qucs-devel

 [Qucs-devel] vector fitting From: Stefan Jahn - 2007-09-29 13:21:36 ```hello there, what do you think about a synthesis tool in qucs performing "vector fitting" in order to derive passive circuits (RLC) for frequency dependent transfer functions? does anyone have heard of it, or experiences with it? i recently heard about this topic regarding the ability to model frequency dependent transmission line losses using simple passive networks in the time domain... and i think this is a nice idea in fact. any comment is welcome. cheers, stefan. ```
 Re: [Qucs-devel] vector fitting From: roucaries bastien - 2007-09-29 18:05:46 ```Reply to all this time On 9/29/07, Stefan Jahn wrote: > hello there, > > what do you think about a synthesis tool in qucs performing "vector > fitting" in order to derive passive circuits (RLC) for frequency > dependent transfer functions? does anyone have heard of it, or > experiences with it? I have heard of it. My thesis domain is non destructive control, and therefore I need to fit data. Waht I can say is, in the general case this problem is mathematically ill defined and very instable. In my case I know by hand that is the model, and therefore I try to fit parameter from measurement to my model. Therefore, you need a really good measurement precision . For RLC circuit a simple non linear least square fitting work pretty well if Q is not high (<100), I used it during my scholarity. I used the non linear least square fitting found in gnuplot :-) By the way vector fitting is the algorithm described in http://www.energy.sintef.no/produkt/VECTFIT/Files/vector_fitting.pdf http://www.energy.sintef.no/produkt/VECTFIT/index.asp In the other hand this algorithm do not try to fit a model to measurement,and could be really useful but in all the case keep in mind that: - real pole (high Q) are difficult to approximate due to finite precision measurement - large imaginary pole (low Q) are difficult to approximate because magnitude effect is quite low. In conclusion: this algorithm is good for finding empirical models, or fitting well behaved model. Regards Bastien > i recently heard about this topic regarding the ability to model > frequency dependent transmission line losses using simple passive > networks in the time domain... and i think this is a nice idea in fact. > > any comment is welcome. > > cheers, stefan. > > ```