Curve fitting real functions

When doing my research to see how are real functions fitted in MATLAB, no answer has been found. Only the fitting of complex functions is proposed. Why is that?

Thanks in advance :^)

[Rik]

What do you mean? I don't believe I have seen any complex function being fitted yet (although I don't doubt it is possible).

[wallflower]

Functions written according to s (jwf). Check for example the MATLAB function rationalfit ...

[Rik]

I don't have that toolbox, so I can't comment on why that is only complex-valued.

I'm much more familiar with the standard fit function. Plenty of example of real-valued functions being fitted.

I suspect the answer to your why question is the domain: I would guess the function in RF analysis are simply complex-valued by design or convention.

[wallflower]

I guess I now know what I said wrong. I didn't not express myself correcty. I agree with your answer. What I want is a rational approximation of real functions not polynomial approximations... I hope my english made it safe!

[Rik]

I don't understand what you want well enough to be able to help you.

[wallflower]

I have a frequency dependent resistance R = r(f) as a form of scatter plot. I want to fit the scatter plot using rational functions so I can easily deduce the equivalent electrical circuit that will generate the same frequency behavior as that of R.

[riccardo]

The curve fitting tool seems to have the rational function as an option.

Alternatively, you could always revert to using "fzero" or "fminsearch", by converting your problem to minimising the residuals of:

<YourFunctionR>(f) * <denominator polynomial> - <numerator polynomial> = 0

[wallflower]

Thank you riccardo :D

Answer 1

[Steven Lord]

There is a collection of library models that can be used with functionality in Curve Fitting Toolbox. One type of model included in that collection is 'rational', which contains polynomials of degree up to 5 for both the numerator and denominator. If that's not sufficient, you can also create your own custom model.