I have a question about FFT

I have a vector of discrete data( they show the positions in single time), I am using fft to see how this data behaves in terms of periodicity? all example I found in matlab talk about the sample frequency and sample length , I know that the sample length can be the length of my vector ,but how the sample frequency plays role here ? will you please give me some helps by answering my question?

Answer 1

[Adam]

The sample frequency will be determined by how long your signal is in e.g. seconds. This then gives a hard link between frequency in Hz and your sample length.

If you don't know what the length of your signal is in time then you can still just work with normalised frequency, but it depends what you want to interpret from it. If you want to look at, for example, how much power is in your signal between 20Hz and 30Hz then you will need the sample frequency to map from normalised frequencies onto the Hz range as

frequencyHz = normFrequency * sampleRate;

Where normFrequency runs from 0 to 1, 1 being your sample frequency, 0.5 being the Nyquist frequency. Or -0.5 to 0.5 if you are going to mess with negative frequencies too I guess.

[Saee]

Thank you Adam for your answer, but I don't have a time dependent signal , I only have a vector in space domain which shows the positions of my deformed line. I want to see if this deformation looks like an periodic wave or not? I need to do FFT and then from FFT I should be able to see my wavelengths,right? now I am confused by this sample frequency and sample length. If I don't defined them correct then I am not able to get the right number for my wavelength.

[Adam]

It would probably help if you post the formulae you are trying to use here. I always get confused by where I need to plug in some of the parameters to get a properly scaled FFT, but I have embedded my usages in classes I have written that now does all that for me. Right now I don't have time to plough through all the Matlab help myself to refresh my memory on what their examples do with the sample frequency.

If you have no known sample frequency then you can probably just set it at 1

Answer 2

[Saee]

Thank you Adam for your answer, but I don't have a time dependent signal , I only have a vector in space domain which shows the positions of my deformed line. I want to see if this deformation looks like an periodic wave or not? I need to do FFT and then from FFT I should be able to see my wavelengths,right? now I am confused by this sample frequency and sample length. If I don't defined them correct then I am not able to get the right number for my wavelength.