transfer function frequency response

how to get transfer function from this time response plot? Red is input and blue is output....

Answer 1

[J. Webster]

Take the fourier transform of both signals, then the transfer function will be the FFT of the cavity divided by the FFT of the reference.

[Zhe Liu]

Hey! Thanks for the reply. Do you mean for whole time domain or just the later periodic part? I did FFT of both signals in the whole window but the result does not make much sense to me. I want to find the natural frequency(resonance frequency) of a line-cavity system.

Answer 2

[Star Strider]

You are doing ‘system identification’. If you have the Signal Processing Toolbox, use the invfreqz function. (If you have the System identification Toolbox, there are several functions you can use to identify the cavity dynamics.)

You will have to experiment with the number of poles and zeros in the transfer function. I would start with 5 for both, then use the output to define an IIR filter, and then filter the input signal with it, and continue changing the number of poles and zeros (I would specify the same number for each) until you the output reasonably approximates the response to your system.

[Zhe Liu]

Hey! I never had this background knowledge about filter and invfreqz function. Could you elaborate with an example? Mathworks example is a little bit hard for me....

[Star Strider]

Can you post your data as a .mat file? It would be easier to work with it directly, and come up with some directly relevant code.

[Zhe Liu]

Do you want me to put input and output into a single .mat or separate them?

[Star Strider]

A single .mat file, please.

I apologise for the delay in responding. Life intrudes.

[Zhe Liu]

send you shortly...Sorry for the delay...I went home at that time...

[Star Strider]

I’m still here!