How to choose the points when using fft in Matlab?

matlab fft power of 2

I am trying to calculate the cross-spectral density for a voltage signal using Matlab. I am using the fft function to do the Fourier transform. The fft function is Y=fft(X,n). But I donot know how to pick the points n. My voltage signal Y is 2seconds long. dt is 0.0001s. so sampling frequency is 1/0.0001s=10KHz. I donot know what value I should use for the fft in my case?
Anyone can help? Thanks.

frequency resolution matlab

Just put the whole point into the fft - it should take them and calculate the transform. In fact, if you don't support a number of points, equal to an axact power of 2 (2 powered of n) fft must add some points to receive it (the exact power of 2). You can do it youself, of course.

fft points + freq resolution

Thanks. I am still confused about how to choose the point is best. Can you explain why choose the exact point is OK? In my case, if I use the exact point, n=2second/0.0001s=20000points. While for the default, Matlab uses 2^n=2^20000. That's a huge difference. So which one is better? By what standard, I choose the points when using fft in Matlab?
BTW, two more questions? 1) how to calculate the frequency resolution? In my case, if I use the exact n=20000, freq resoluition is 1?
2) When I plot the psd vs freq? how can I convert back to the frequency for the x-axis? In my case, is it: freq=Fs*(0:n-1)/n=10000*(0 : 19999)/20000? so no matter which n i choose, the freq resolution is always 1?

dmk said:

Just put the whole point into the fft - it should take them and calculate the transform. In fact, if you don't support a number of points, equal to an axact power of 2 (2 powered of n) fft must add some points to receive it (the exact power of 2). You can do it youself, of course.

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using fft in matlab

Well,
It's about the FFT and DFT. The answer is neither easy nor short. Anyway, the FFT is a DFT at power of 2 (so called butterflys). You must read something more about Digital Signal Processing, Discrete Forrier Transform and Fast Furrier Transform.
A good book is Steven Smith's "The Scientist and Engineer's Guide to DSP":
https://www.dspguide.com/
You can download the chapters and read them.
Regards

fft points to frequency

The number of points is how long your vector X is. In your case 20000. You don't have to specify N. If your vector is not a multiple of 2, then the algorithm may not be particularly fast, but should still be accurate.

1 - "How to calculate the frequency resolution"

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- the spacing between your FFT points will be 1/(2 seconds) = 0.5 hz. What you mean by resolution can be a little trickier.... have to think about whether your sample is windowed, whether two frequencies occur close together, whether there is noise.

2) "When I plot the psd vs freq? how can I convert back to the frequency for the x-axis? In my case, is it: freq=Fs*(0:n-1)/n=10000*(0 : 19999)/20000? so no matter which n i choose, the freq resolution is always 1?"

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Always one bin width. If you want a plot vs hz, I would use freq=(0:19999)./2

"While for the default, Matlab uses 2^n=2^20000"

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No, that is wrong. Matlab prefers a power of 2. For example 2^14~16000, But not 2^20000. You can use 20000 as disucssed above.

Note I have posted some example FFTs using MATLAB in the last post in the thread on DSP interview questions.