plot the fundamental harmonic wave

Hello everyone. I have exported data to an Excel spreadsheet where the x-axis is represented by theta (in radians). How can I plot the fundamental harmonic wave from this wave data?

[Mathieu NOE]

hello

you can work either in time / angular domain , by applying a band pass filter centered at your target frequency (fundamental)

or you can work in the frequency domain (fft your data, then keep only the frequency(ies) of interest and do ifft)

[hamid k]

Hello, my issue is that I can't execute an FFT on discrete points since Matlab recognizes either the time or frequency of a waveform, while my data is plotted against theta (degrees).

[Mathieu NOE]

well, show me your data (and code) and I will answer you

[Star Strider]

The fft function only wants ‘theta’ to be sampled at a constant interval. It does not know or care what ‘theta’ actually is (seconds, years, metres, parsecs, radians, degrees, or anything else). The frequency unit will then be the inverse of that (cycles/second, cycles/metre, cycles/parsec, cycles/radian, cycles/degree).

[hamid k]

You can find the attached data and code files here.

[Mathieu NOE]

see my answer below

Answer 1

[Mathieu NOE]

hello again

so this is it, we do a single frequency DFT (aka order extraction) at the second harmonic (as we have two period of signal for theta ranging from 0 to 2*pi

result :

code :

%Load Excel file
data = readmatrix('Book1.xlsx'); % theta(Rad)	Br(T)

%Extract theta and y columns
theta = data(:,1); % theta (0 - 2pi)
y = data(:,2);  % y data

%%%%%%%%%%%%% main code %%%%%%%%%%%%%%%%%
% 2nd order extraction (DFT)
order = 2;
% model fit : X = A*cos(order*theta) + B*sin(order*theta) + C
C = mean(y);
y = y-C;
n = numel(theta);
A = 2/n*trapz(y.*cos(order*theta));
B = 2/n*trapz(y.*sin(order*theta));

yfit = A*cos(order*theta) + B*sin(order*theta) + C;

% plot
figure(1),
plot(theta, y, 'b',theta, yfit, 'r')
legend('data','model fit');

[Abdullah]

Hello Mathieu,

i tried to use your code with my data, but it does not work, do you have an idea?

https://de.mathworks.com/matlabcentral/answers/2110271-how-to-plot-the-fundamental-harmonic-wave-from-given-data?s_tid=srchtitle

Answer 2

[Sulaymon Eshkabilov]

Here are two MATLAB fcns (bandpass() and fft()) which can be applied to solve your exercise.