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T = 3*(1/25);
fs = 1000;
t = 0:1/fs:T-1/fs;
x = sawtooth(2*pi*25*t,1/2);
x = (x + 1)*0.5;
subplot(3,1,1)
plot(t,x)
grid on
xlabel('Time t (sec)')
ylabel('Amplitude')

My Signal

This is my code and my signal. I want to find its fifth and seventh harmonics. How can I do that ? I cannot find any source or any example for this. Can anybody help me ?

Output

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3
  • $\begingroup$ Harmonics are usually defined for periodic signals. The usual approach is to find the signal's Fourier Series; each term of the series is a harmonic. Can you clarify your definition of harmonic, and why do you need to do this in Matlab? $\endgroup$
    – MBaz
    Commented Apr 18, 2020 at 20:34
  • $\begingroup$ I am trying to learn Matlab. For example, I want to sum first harmonic and fifth harmonic. After that, I will sum first and ninth harmonic and I will compare these two with my original signal. $\endgroup$
    – Oğuzhan Kırlar
    Commented Apr 18, 2020 at 20:39
  • $\begingroup$ I would use Fourier Series to find the harmonics, then Matlab to generate them and add them. $\endgroup$
    – MBaz
    Commented Apr 18, 2020 at 21:20

1 Answer 1

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Your sawtooth waveform is periodic with a time period of $T = 0.04s$, therefore basic frequency is $\frac{1}{T} = 25Hz$, so to find $5^{th}$ and $7^{th}$ harmonic, project this signal/correlate the signal over any period T with this harmonic. Let $tr(t)$ denote the trinagluar waveform over 1 period. For ex: $$a_5 = \frac{2}{T}\int_{t=0}^T tr(t)Cos(2\pi*5*25t)\,dt$$ similarly $$a_7 = \frac{2}{T}\int_{t=0}^Ttr(t)Cos(2\pi*7*25t)\,dt$$ where $a_5$, $a_7$ denote the coefficients of the $5^{th}$ and $7^{th}$ harmonic.

The $5^{th}$ harmonic would therefore be $a_5Cos(2\pi*5*25*t)$ and $7^{th}$ hamonic is $a_7Cos(2\pi*7*25*t)$

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  • $\begingroup$ f5 = @(t)t.*cos(2.*pi.*5.*25.*t); a5 = integral(f5,0,0.04); a55 = a5.*cos(2.*pi.*5.*25.*t); subplot(3,1,2) plot(t,a55) I write this code and output didn't seem right to me. Am I wrong or is it true ? (I am adding output to the question ) $\endgroup$
    – Oğuzhan Kırlar
    Commented Apr 19, 2020 at 12:18
  • $\begingroup$ Use sawtooth function in Matlab to generate the trinagluar waveform, fs = 1000; t = 0:1/fs:0.04; x = sawtooth(2*pi*25*t,1/2); to generate 1 period. Then define, tra=cos (2*pi*5*25*t), a5 = int(tra."x,0,0.04) $\endgroup$
    – Dsp guy sam
    Commented Apr 19, 2020 at 12:32
  • $\begingroup$ I just add the above code and take this output but okey i will try it now, thank you. $\endgroup$
    – Oğuzhan Kırlar
    Commented Apr 19, 2020 at 12:34
  • $\begingroup$ Just saw your sawtooth wave goes from 0 to 1 so scale the sawtooth function by 1/2 and add 1/2 to each samples i.e.x = 1/2.*sawtooth(2*pi*25*t,1/2) + 1/2.*ones(1, length (t)) $\endgroup$
    – Dsp guy sam
    Commented Apr 19, 2020 at 12:34

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