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Plot single-sided amplitude spectrum.

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mohamad mohamad
mohamad mohamad on 15 Dec 2014
Commented: Adam on 1 Mar 2017
hello .
i tried to find a code for ploting fft,and i found this code which plot single-sided amplitude spectrum according to it's title,is it the same as fft?what is different between single side amplitude spectrum and fft? and is this code ok?
i really appreciate it if you help me.
i have a signal with 135 data point.
Fs = 50; % Sampling frequency T = 1/Fs; % Sample time L = 135; % Length of signal
NFFT = 2^(nextpow2(L)-1);
x=zeros(NFFT,1); x(1:NFFT,1) = rawdata(1:NFFT,1);
YY = fft(x,NFFT)/L; ff = Fs/2*linspace(0,1,NFFT/2+1);
% Plot single-sided amplitude spectrum. plot(ff,2*abs(YY(1:NFFT/2+1))) title('Single-Sided Amplitude Spectrum of tp') xlabel('Frequency (Hz)') ylabel('|Y(f)|')

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sampath kumar kuppa
sampath kumar kuppa on 4 Nov 2016
you got the solution for this code or not. I'm also facing the same problem. if you got the solution means please forward to me.

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Accepted Answer

Adam
Adam on 15 Dec 2014
Edited: Adam on 15 Dec 2014
That code uses the fft so yes, it is the same thing.
The fft is a transform which gives you a complex result spanning negative and positive frequencies ( -nyquist to +nyquist ). The single-side spectrum throws away the negative frequencies which are often not required for things like plotting spectrum to obtain a single-side spectrum.
So the fft is the method used to transform from the time domain to the frequency domain, but its result requires some manipulation afterwards to obtain what is often the desired result, a single-sided power spectrum.
The code:
ff = Fs/2*linspace(0,1,NFFT/2+1);
is taking the positive half of the spectrum (NFFT/2 +1 gives this, including 0 and nyquist, hence the +1) and mapping it onto your real frequencies from 'normalised frequency'.

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Adam
Adam on 16 Dec 2014
I don't know what the signal in your original question is, but it is presumably sampled at 50Hz sample rate, i.e. every 20ms.
If you don't know your sample rate than you cannot get information out of the fft in Hz, all you can really do is either guess at the sample frequency (seems rather pointless) or use normalised frequency - i.e. fs = 1 where your frequency values will be between 0 and 0.5 for the positive have of your spectrum.
Xu LinaM
Xu LinaM on 1 Mar 2017
Hello,after I read your opinion about Single-Sided Amplitude Spectrum,I have a question that where is the negative frequency,why they begin at NFFT/2+2,and what's the order of the size ?Thank you for your reply.Best Wishes!
Adam
Adam on 1 Mar 2017
The negative frequencies begin at NFFT/2 + 2 simply because that is how fft is defined. They have to be somewhere. You can use fftshift if you prefer to have 0 in the centre with negative frequencies before it.

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More Answers (1)

Charanraj
Charanraj on 9 Jul 2015
hey, can a single sided FFT taken directly from the scope by using an absolute block before the scope ?

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