Harmonic spectrum of the voltage data

Hi all,

I am new to matlab and have this small questions. So basically I need to plot the harmonic spectrum for the data that I have recorded in a txt file. I managed to import the data into matlab but don't know how to plot the cooresponding hramonic spectrum. The data is in the attachement. Thanks in advances:)

Answer 1

[Star Strider]

Try this:

D = dlmread('voltage_u.txt','\t', 4, 0);
D = D(1:end-1,1:2);                                         % First Two Columns (Third is 0), Elkiminat (0.0) In Last Row
t = D(:,1);                                                 % Time
V = D(:,2);                                                 % Volts
L = size(D,1);                                              % Signal Length

tv = linspace(min(t), max(t), L);                           % Create Regularly-Spaced Time Vector
Ts = tv(2)-tv(1);                                           % Sampling Interval
Fs = 1/Ts;                                                  % Sampling Frequency
Vv = resample(V, t, Fs);                                    % Resample To Constant Sampling Rate

Fn = Fs/2;                                                  % Nyquist Frequency
FVv = fft(Vv)/L;                                            % Discrete Fourier Transform
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                         % Frequency Vector
Iv = 1:numel(Fv);                                           % Index Vector

figure
semilogy(Fv, abs(FVv(Iv))*2)
grid
xlabel('Frequency (Hz)')
ylabel('Amplitude (V)')

Your time-domain data are not regularly-sampled, so it is necessary to use the resample function to provide a uniformly-sampled vector in order for the fft function to produce the correct results.

Explore this at your leisure to understand how it works.

[Zong-Jhen Ye]

Hi Strider,

Thank you so much for the sharing. There is one thing I think we can modeify, which is "Fn = Fs/2; % Nyquist Frequency" in your code. I think it could be "Fn = Fs/3;". The reason is if the data is voltage, the fundamental frequency should be 60 Hz. If we use "Fn = Fs/2", the first peak is at 90 Hz, which do not make sense. But the first peak (i.e. fundamental frequency) will be at 60 Hz if we use "Fn = Fs/3;".

Please let me know if I misunderstanding anything.

[Star Strider]

You are misunderstanding the Nyquist frequency. It is the highest frequency that can uniquely be identified in a sampled signal, and is defined as ‘Fn=Fs/2’ and my defining it that way is not an error. Modern ADC uints all have analog lowpass Bessel fiilters at the input that reject all frequencies greater than the Nyquist frequency of the ADC (the reason that only a llimited number of sampling frequencies are available) in order to prevent aliasing.

See the Wikipedia article on the Nyquist-Shannon sampling theorem for a full discussion.

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