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Matlab and fft resolution

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vaka85

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Hi,

I have a signal witch spectrum (calculated with matlab fft) is 70 Hz wide.
But I really need only the first 5 Hz. Is there any way to "concentrate" the fft only on that part of the spectrum? In this way (maybe) I can use the 1024 fft points only in the first 5 Hz, instead of 70 Hz, increasing a lot the resolution..

Thanks,
bye,

Valerio
 

1) what is the frequency of sampling (fs)?
2) how many points of data do you have? i mean how many samples do you have?
3) Take a look at any DSP books that talk about FFT. using the above data you can normally select proper beans for FFT,

As I know, by using 1024 points FFT, fs will divide by 1024. so if fs is 1Khz then you have bins (divisions) approximately near 1 Hz (1000/1024), then you can select first five bins.
 
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I am not aware of DFT algorithms that can "focus" only on a small frequency range. On the other hand, if you know which frequency you are interested, you might want to look at a "single-bin DFT" over a frequency range, like the sliding DFT using the Goertzel algorithm.
 
You have to use windowing function. For example, hamming window to extract frequency of your interest, like 5 Hz in your case.
 

thank you for your advices.

@ehsanica: fs=71 Hz, and I have 1000 points of data.

@jack: maybe I wasn't so clear.. I don't need only to "zoom in" the spectrum, because for that purpose I can use the matlab zoom :)
I have to re-calculate the fft, but only on the first range of frequencies, to take advantage from the high number of points concentrated there.

@Joannes: I'll make some searches on this matter.

If you have other ideas you're welcome :)

Thanks
 

Hi,

I've tried some "single bin dft" function, for example the Goertzel algorithm.. But the problem is that all of these methods require integer points where to calculate the DFT!
Instead of this, I need very little frequency resolution (0.01 Hz or so..) and 1 Hz of resolution isn't very nice...

Do you know how to calculate the dft over a frequency range, or only single-bin dft, where the single bin isn't an integer?

thanks
 

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