I have an Excel file having sample data and magnitude, I want to convert the time domain to frequency domain using FFT, The file is attached

I have an Excel file having sample data and magnitude, I want to convert the time domain to frequency domain using FFT, The file is attached

The code I used is below, but this one is not giving the desire results

#CODE:

clear all

clc

anum1= xlsread('DSO0001.csv','A4:A20483');

anum2=xlsread('DSO0001.csv','C4:C20483');

anum3=smooth(anum2);

subplot(2, 1, 1);

plot(anum1,anum3)

title("Smooth Data")

xft=fft(anum2,1024);

xabs=abs(xft);

subplot(2, 1, 2)

plot(xabs)

title("FFT")

Answer 1

[Star Strider]

It is probably not giving the desired result because the fft is truncated to 1024. Ths signal is much longer than that, and the ‘correct’ length of the fft is 32768, by my calculations

T1 = readtable('https://www.mathworks.com/matlabcentral/answers/uploaded_files/934554/DSO0001.CSV','VariableNamingRule','preserve')

T1 = 20480×3 table
    Source    CH1      CH2  
    ______    ___    _______

       1      0.1    -0.0052
       2      0.1    -0.0076
       3      0.3    -0.0072
       4      0.1    -0.0072
       5      0.1    -0.0068
       6      0.1    -0.0052
       7      0.1     -0.004
       8      0.1    -0.0056
       9      5.1      0.004
      10      5.1     0.0028
      11      5.1     0.0016
      12      4.9     0.0012
      13      5.1     0.0008
      14      5.1     0.0012
      15      5.1     0.0008
      16      4.9     0.0016

L = size(T1,1);
t = T1{:,1};
s = T1{:,[2,3]};
Ts = mean(diff(t));
% Tsd = std(diff(t))
Fs = 1/Ts;
Fn = Fs/2;

figure
yyaxis left
plot(t, s(:,1))
yyaxis right
plot(t, s(:,2))

NFFT = 2^nextpow2(L)

NFFT = 32768

FTs = fft(s,NFFT)/L;
Fv = linspace(0, 1, NFFT/2+1)*Fn;
Iv = 1:numel(Fv);

figure
subplot(2,1,1)
plot(Fv, abs(FTs(Iv,1))*2)
grid
xlim([0 0.5])
xlabel('Frequency')
ylabel('Amplitude')
subplot(2,1,2)
plot(Fv, abs(FTs(Iv,2))*2)
grid
xlim([0 0.5])
xlabel('Frequency')
ylabel('Amplitude')

figure
subplot(2,1,1)
plot(Fv, abs(FTs(Iv,1))*2)
grid
xlim([0 0.005])
xlabel('Frequency')
ylabel('Amplitude')
subplot(2,1,2)
plot(Fv, abs(FTs(Iv,2))*2)
grid
xlim([0 0.005])
xlabel('Frequency')
ylabel('Amplitude')

The result appears o be appropriate otherwise, since ‘CH1’ is essentially a square wave, and the fft approximates a sinc function, as expected. The ‘CH2’ signal just appears to be noise.

.

[Umair Dildar]

Thanks for the quick response. I have a question

What does this function do "Ts = mean(diff(t));"

it shows error when I run on matlab, it says

[Star Strider]

My pleasure!

That line calculates the sampling interval ‘Ts’ of the time vector.

You are apparently reading it into a cell array. My code does not do that (and does not use xlsread), so I cannot reproduce the error. It would be best to use my code as I wrote it to avoid such problems.

One possibility:

t = cell2mat(t);

Ts = mean(diff(t);

It may be necessary to use cell2mat the entire array.

.