FFT from a .CSV not giving the correct results

%Below is the code. The file is a simple sine wave recorded on an ocilloscope. I do not know what I am doing wrong. Any help with what is wrong would be very helpful.

t10k = readtable('Scope_10kHz.csv','NumHeaderLines',11);% 11 skips the first 11 rows of data 
T10k = table2cell(t10k); %converts the table to cell
xv = cell2mat(T10k(:,1));
yv = cell2mat(T10k(:,2));
S = [xv,yv];


L = height(xv);             % Length of signal
T = xv(L,:)-xv(1,:);  % Sampling period 
Fs = 1/T;            % Sampling frequency                    
t = (0:L-1)*T;        % Time vector
Y = fft(S); %compute the Fast fouier transform

P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);

f = Fs*(0:(L/2))/L;

figure 
plot(S)
xlabel('Time (s)')
ylabel('Voltage (V)')
title('Ocilloscope Measurment')

figure
plot(Y)
xlabel('Frequency')
ylabel('Magnitude')
title('FFT raw')

figure
semilogy(f,P1) 
title('Single-Sided Amplitude Spectrum of X(t)')
xlabel('f (Hz)')
ylabel('|P1(f)|')

Answer 1

[Star Strider]

It works correctly. However it is also taking the Fourier ttransform of the time vector, so restricting it to only the data vector likely produces the desired result —

t10k = readtable('https://www.mathworks.com/matlabcentral/answers/uploaded_files/750984/Scope_10kHz.csv','NumHeaderLines',11); % 11 skips the first 11 rows of data 
T10k = table2cell(t10k); %converts the table to cell
xv = t10k{:,1};
yv = t10k{:,2};
S = [xv,yv];


L = height(T10k);             % Length of signal
T = xv(2)-xv(1);  % Sampling period 
Fs = 1/T            % Sampling frequency                    

Fs = 1.4286e+07

Fn = Fs/2;              % Nyquist Frequency
t = (0:L-1)*T;        % Time vector
Y = fft(S(:,2)); %compute the Fast fouier transform

P2 = abs(Y/L);
P1 = P2(1:L/2);
P1(2:end-1) = 2*P1(2:end-1);

f = Fn*(0:(L/2)-1)/L;

figure 
plot(S)
xlabel('Time (s)')
ylabel('Voltage (V)')
title('Ocilloscope Measurment')

figure
plot(Y)
xlabel('Real')
ylabel('Imaginary')
title('FFT raw')

[pks,locs] = findpeaks(P1, 'MinPeakProminence',0.01)

pks = 2×1

0.1945 0.0523

locs = 2×1

7 13

figure
loglog(f,P1) 
title('Single-Sided Amplitude Spectrum of X(t)')
xlabel('f (Hz)')
ylabel('|P1(f)|')
text(f(locs),pks, compose('Frequency: %.1f  Amplitude: %.3f', [f(locs).' pks]), 'Vert','bottom', 'Horiz','left')

.

[Nicholas Calvano]

That was what I needed! Thank you. I thought it was something simple.

[Star Strider]

As always, my pleasure!

.