How can i plot frequency response from 25000 time domain Data?

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mostafa 2015-3-19

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评论： mostafa 2015-3-21

Hello every body, I have near 25000 time domain data, they are from a idustrial plant, I have both input and output data. I want plot frequency response by using this time domain data, please help me how it is possible? Thanks alot.

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dpb 2015-3-19

编辑：dpb 2015-3-20

If you've already sampled at 25 second intervals, you have no valid frequency content above 0.02 Hz by the Nyquist sampling theorem (you do know what that is, at least, I hope?) The problem in determining a "best" sampling time is that you've got to ensure you've sample at a rate at least greater than twice the maximum frequency content possibly of interest. And, of course, you need to ensure that there isn't frequency content above that in the sampled data that is being aliased into that sampling range and that if so, is absolutely not removable by any means whatsoever once the data have been sampled.

I'm reluctant to suggest you just blindly apply FFT techniques to these data until you have a better understanding of the problems; the consequences of doing so are potentially too serious.

Is this an academic or commercial exercise? If academic consult your advisor, if commercial is there some access to consultant in the area you can confer with? This is a Matlab programming forum, not a consulting service, unfortunately.

dpb 2015-3-20

编辑：dpb 2015-3-20

在 MATLAB Online 中打开

I think you're barking up the wrong tree first here; you need to learn something about signal processing and data collection basics first to ensure your data have any meaning before just applying tools. Perhaps the reason the results aren't what you expect is that the data aren't any good...

But, to see what frequency content is contained in the sampled data, there's a very good, concise example of an amplitude spectrum computed via the fft at

doc fft

The Signal Processing toolbox also has the function tfestimate for transfer function estimation and the spectrum function which has multitude of options for estimating spectra.

Again, however, unless there's something actually in the dataset you've collected, none of these can fix corrupted data obtained without adequate protection during that process, whatever it was.

mostafa 2015-3-21

Thanks alot Sir...

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Image Analyst 2015-3-20

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Look up pwelch() in the Signal Processing Toolbox: http://www.mathworks.com/products/signal/features.html#spectral-analysis

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mostafa 2015-3-21

Hello Image analyst, Thank you so much for your attention.

mostafa 2015-3-21

Hello image analyst, I dont know PSD and welch theory, please if it is possible introduce to me quick refrence for it and use it in matlab. Thaks alot.

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