getting X domain as Frequency when applying FFT
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hi everyone,
i have data of Volt Vs Second of some measurment i did. i plotted this data and made dft(fft) on this data in order to get into the frequency domain and to filter some noise by a bandstop filter.
the problem is that when i make fft, the x axis is not Frequency. how can i invert it to Frequency so i can know in what Hz i have nosie.
tnanks a lot
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Answers (2)
Wayne King
on 31 Dec 2013
Edited: Wayne King
on 31 Dec 2013
You have to know the sampling frequency, which is the reciprocal of the difference between your time increments. I'm assuming that your time instants are evenly spaced. I'll assume you know your sampling frequency, but you can get if from your time vector if you do not.
Here is how to create a frequency vector.
t = 0:1/1000:1-1/1000;
x = cos(2*pi*100*t);
xdft = fft(x);
xdft = xdft(1:length(x)/2+1); % even length signal
Fs = 1000;
df = Fs/length(x);
freqvec = 0:df:Fs/2;
plot(freqvec,abs(xdft))
If you have the Signal Processing Toolbox, use periodogram() that will output a meaningful frequency vector for you.
1 Comment
HS Kim
on 21 Nov 2016
I think you uesd "xdft(1:length(x)/2+1)" for deleting mirror image that will be happended by 'abs(xdft)' in plot, right?? I learned that plot(freq, abs(dtft)) then mirror image is happened if do plot without dtft/2. Please let me know exactly. Thank you :D
kobi
on 31 Dec 2013
1 Comment
Wayne King
on 31 Dec 2013
You are not wrong. If you want to filter your signal, then do it that way -- using filter()
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