Hi didac,
Just a comment. If you are using Spectre as the source for your analysis, you might as well use the PZ analysis to get all the poles and zeros of interest directly.
You may have to get rid of some of the high frequency poles/zeros, but this is easy.
I once had to do transfer function estimation based on data from a PAC analysis. I did it the old fashioned way - by eyeballing the major poles/zeros from the
phase plot and from the magnitude. I compared not only the AC response but also the step response of the resulting system in MATLAB to the response of the
original system. That way, you can see if you caught some pole/zero doublets lurking at low frequencies. These would matter as they would determine the
settling time constants. In my case, the responses matched almost perfectly even though I had just taken rough estimates. The method was also very fast. But
maybe it would be less effective for another system.
If you do make a nice program for estimating the TF, be so kind as to send me a copy
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Regards
Vivek