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reterty

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reterty

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DaveE

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In general, you will have to define over what conditions and how you will do the evaluation. The Fourier transform assumes periodicity based on the limits you choose to integrate over. It can not tell you about any periodicity on the order of 1 day, if you only collect data for 1 minute. So, I think just defining your window and looking at the Fourier transform is the only thing we can do. Then for different circumstances, you'll get different spectral data out, which may still be hard to interpret.

In practice, this subject is most commonly described as "jitter" of electronic signals. It is an extremely well studied and hugely important subject. The treatment tends to be statistical in nature. People invariably end up making some (powerful) assumptions about the type of deviation, like "gaussian noise", for example, to allow them to analyze the more general cases. Do some searching about jitter for more information. IRL, we would look at the spectral width of the "almost periodic" frequency out of a Fourier series or spectrum analyzer. This is also called "phase noise".

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