# Identification of fundamental frequency and not harmonics in a FFT - using peak locations

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John Navarro on 12 Oct 2021
Commented: Mathieu NOE on 13 Oct 2021
Hello
I alredy got the FFT from my noisy data and use the command findpeaks I have obtain the following
plot(f1,P1,'--b','LineWidth',1.5) % Frequency domain of the signal
Fz=60; % Fundamental frequency
tooth=Fz; %Expected peaks separation
hold on
[pkt,lct] =findpeaks(P1,f1,'SortStr','descend','NPeaks',6,'MinPeakDistance',tooth/5)
% pkt is amplitude, and lct is the corresponding frequency
plot(lct,pkt,'r*','MarkerSize',10)
%%
round([pkt,lct'])
ans =
78 240
18 60
14 180
14 960
13 1050
12 480
You can see, only one of these peak is a non harmonics. I want to modify it so:
1) Identify of the group[pkt,lct], which ones are harmonics of the 60Hz component, label each one as harmonics or nor harmonic.
2) Plot harmonics with one symbols and non harmonic with other symbol.
Warning:Some fft will have more than one non harmonics, or can all the peaks be harmonics, so the group data could be something smaller than 6 items.
Attached one image of the current state of the code but as you may notice, it is not working as expected. Mathieu NOE on 12 Oct 2021
hello
try this
of course I cannot plot the spectrum as this is not provided here
I just let you see how to do the separation between harmonic and non harmonic data.
BTW , the sorting of the data by findpeaks does not bring any added value here - not needed in fact;
here I start from the data : round([pkt,lct'])
% round([pkt,lct'])
data =[
78 240
18 60
14 180
14 960
13 1050
12 480];
pkt = data(:,1);
lct = data(:,2);
% harmonic / non harmonic group segregation
f_base = min(lct);
f_ratio = (lct./f_base);
f_residue = abs(f_ratio-round(f_ratio));
tol = 1e-2; % 1% tolerance
ind_harmo = find(f_residue<=tol);
ind_non_harmo = find(f_residue>tol);
% harmonic group
pkt_harmo = data(ind_harmo,1);
lct_harmo = data(ind_harmo,2);
% non harmonic group
pkt_non_harmo = data(ind_non_harmo,1);
lct_non_harmo = data(ind_non_harmo,2);
figure(1)
plot(lct_harmo,pkt_harmo,'dr',lct_non_harmo,pkt_non_harmo,'*k');
legend('harmo peaks','non harmo peaks');
##### 2 CommentsShowHide 1 older comment
Mathieu NOE on 13 Oct 2021
hello
I think you can do the sorting afterwards , but if you need it anyway in your code , where you do it has little importance

R2021a

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