How to use FFT in matlab using imported data in time domain excel file

I have used a multimeter to measure my phototransistor and I got one excel file with time versus Magnitude. But I got some noise, so I want to use Matlab to do FFT from time domain to frequency domain(frequency versus magnitude) called Photorespone. I attach my file as below. I wanna make this kind of figure, freq versus amplitude.

[Adam]

You didn't attach a file or tell us which part of it you are having a problem with. Reading an excel file into Matlab and running FFT analysis are two completely independent tasks, which one do you need help with?

[Linda Yakoubi]

I have a similar task. I already imported the data from excel in Matlab but I don't know what to do next and how to use the function "fft()". Any ideas ?

[Adam]

doc fft

is the best place to start!

Answer 1

[Star Strider]

Use the examples in the documentation on fft (link) to calculate the Fourier transform.

Example Code For Your Signal —

[d,s] = xlsread('1.5V.csv');
t = d(:,1)*1E-3;                                        % Convert To ‘seconds’ From ‘milliseconds’
v = d(:,2);                                             % Voltage (?)
L = length(t);
Ts = mean(diff(t));                                     % Sampling Interval (sec)
Fs = 1/Ts;                                              % Sampling Frequency
Fn = Fs/2;                                              % Nyquist Frequency

vc = v - mean(v);                                       % Subtract Mean (‘0 Hz’) Component

FTv = fft(vc)/L;                                        % Fourier Transform
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                     % Frequency Vector (Hz)
Iv = 1:length(Fv);                                      % Index Vector

figure(1)
plot(Fv, abs(FTv(Iv))*2)
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude (V?)')

I did not post the plot, since my code will plot the Fourier transform of your data.

[Javis Chiu]

Thank you so much I can run this right now !!!!

[Star Strider]

As always, my pleasure!

[Javis Chiu]

Sorry for bothering again.This time I wanna export the result after FFT to excel, can you help me ? thanks !!!

[Star Strider]

You’re not bothering!

Use the xlswrite (link) function. (I have no recent experience with it. You will need to experiment.)

[Javis Chiu]

Yes I made it !! But I have one question I want to adjust my frequency, I'm using your code now and export the FFT raw data to excel and it's 2Hz each point, I want that will be 0.5Hz each point, do you know how to adjust it ? Thank you

[Star Strider]

The only possibility is to lengthen the fft to be 4 times the original length, to increase the frequency resolution. You can tweak the size of the fft to be precisely 0.5 Hz, or to be whatever frequency resolution you want.

This comes acceptably close:

FTv = fft(vc, L*4)/L;                                   % Fourier Transform
Fv = linspace(0, 1, fix(length(FTv)/2)+1)*Fn;           % Frequency Vector (Hz)

The rest of my code remains the same.

[Javis Chiu]

I've tried it, but it says Error using xlswrite (line 220) Excel returned: Error: Object returned error code: 0x800A03EC.

Error in FFT (line 20) xlswrite('frequency.xls',Fv)

[Star Strider]

I don’t use xlswrite often. I have no idea what that error is, since I’ve never seen it.

I would combine the frequency vector and the Fourier transform in one (Nx2) array.

For example:

A = [Fv(:)  FTv(:)];

convert them to column vectors first and concatenate them in ‘A’, then write ‘A’ to your Excel file:

xlswrite('frequency.xls',A)

See if that works.

[Jonathan James-Gunn]

@Star Strider I am doing a similar thing, my FFT plot shows upto 12Mhz but most of the area of interest is lower than 2Mhz, can you show me how to limit the FFT plot and horizontally zoom so that the data is not squashed; I've attached my .CSV file.

[Star Strider]

This should be a new question. I assime the file has time-domain information, although it looks really strange.

That aside —

T1 = readtable('NewFile0.csv')

T1 = 10240x3 table
       Var1        Var2     Var3 
    ___________    _____    _____

     -0.0002048    0.048    {' '}
    -0.00020476    0.048    {' '}
    -0.00020472    0.048    {' '}
    -0.00020468    0.048    {' '}
    -0.00020464    0.048    {' '}
     -0.0002046    0.044    {' '}
    -0.00020456    0.044    {' '}
    -0.00020452     0.04    {' '}
    -0.00020448    0.036    {' '}
    -0.00020444    0.036    {' '}
     -0.0002044    0.032    {' '}
    -0.00020436    0.028    {' '}
    -0.00020432    0.024    {' '}
    -0.00020428     0.02    {' '}
    -0.00020424     0.02    {' '}
     -0.0002042    0.016    {' '}

figure
plot(T1{:,1}, T1{:,2})
grid

% NMT = nnz(~isempty(T1.Var3))
% Item = T1.Var3(~isempty(T1.Var3))

format longG
TimeStats = [mean(diff(T1.Var1)); std(diff(T1.Var1)); std(diff(T1.Var1))/mean(diff(T1.Var1)); 1/mean(diff(T1.Var1))]

TimeStats = 4x1

1.0e+00 * 4.00000000000003e-08 8.22772071399427e-21 2.05693017849855e-13 24999999.9999998

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[FTs1,Fv] = FFT1(T1.Var2, T1.Var1);
[pks,frqs] = findpeaks(abs(FTs1)*2, Fv, 'MinPeakProminence',0.009);

Fourier_Results = table(frqs(:), pks, 'VariableNames',{'Frequency','Peak Magnitude'})

Fourier_Results = 5x2 table
       Frequency          Peak Magnitude   
    ________________    ___________________

    9155.27343749992    0.00956967305983158
    192260.742187498      0.012785867656168
    207519.531249998     0.0146608779233046
    259399.414062498      0.014515383456828
    315856.933593747     0.0130072594204464

figure
plot(Fv, abs(FTs1)*2)
grid
xlabel('Frequency')
ylabel('Magnitude')
title('Original')

figure
plot(Fv, abs(FTs1)*2)
grid
xlabel('Frequency')
ylabel('Magnitude')
title('Use ‘xlim’')
xlim([0 1.5E+6])

figure
plot(Fv, abs(FTs1)*2)
grid
set(gca, 'XScale','log')
xlabel('Frequency')
ylabel('Magnitude')
title('Use Logarithmic Frequency Axis')

function [FTs1,Fv] = FFT1(s,t)
    t = t(:);
    L = numel(t);
    if size(s,2) == L
        s = s.';
    end
    Fs = 1/mean(diff(t));
    Fn = Fs/2;
    NFFT = 2^nextpow2(L);
    FTs = fft((s - mean(s)) .* hann(L).*ones(1,size(s,2)), NFFT)/sum(hann(L));
    Fv = Fs*(0:(NFFT/2))/NFFT;
%     Fv = linspace(0, 1, NFFT/2+1)*Fn;
    Iv = 1:numel(Fv);
    FTs1 = FTs(Iv,:);
end

.

[Jonathan James-Gunn]

@Star Strider Thank you! I really appreciate the help.

[Star Strider]

My pleasure!

A Vote would be appreciated!