Evaluating output using input value and transfer function

[lucy123]

Hi all,

I have used matlab to fit frequency data to a transfer function. The result is a rather complex transfer function shown below. The best fit value is 96%. Can you please guide me on how I can calcuate the output in the frequency domain given a single input value?

Thank you

0.1966 (+/- 7.971e08) s^6 + 3.751e04 (+/- 3.237e10) s^5+ 0.07548 (+/- 3.478e12) s^4+ 1.566e08 (+/- 1.471e14) s^3+ 251.6 (+/- 2.501e12) s^2- 9.345e09 ( +/- 2.047e12) s+ 29.2 (+/- 3.822e10)

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s^7 + 842 (+/- 3.926e08) s^6+ 2.307e05 (+/- 2.435e11) s^5+ 2.253e07 (+/- 8.408e12) s^4 + 7.164e08 (+/- 7.952e12) s^3+ 6.962e07 (+/- 1.986e11) s^2+ 2.212e09 (+/- 5.062e11) s+ 7.066 (+/- 2.519e10)

 

[albbg]

You can replace "s" with "jw" to find the complex transfer function, then multiply it by the, in general, complex input value. "w" is the angular frequency
However I don't understand your notation. What you exactly mean with, for instance, 0.1966 (+/- 7.971e08) s^6 ?

 

[lucy123]

This is the output I obtained in Matlab. I'll give it a try and I'll see what I'll obtain.

Thanks

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It seems my tf is completly wrong or I am doing the wrong calculations? Why is the calculated output is complex when it should be real?

 

[FvM]

Can you please explain what "fit frequency data to a transfer function" excatly means for you? What's given, in which form? What do you want to obtain?

 

[lucy123]

so I have used ident in Matlab to obtain a transfer function between my input (real) and Output(real) of a system which is frequency dependent. That is for a particular frequency and an input I would like to calculate the output. I hope this makes it abit clearer.