# Find fourier series of my signal

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')


I want to take DFT(Discrete Fourier Transform) of my signal using FFT and FFTShift and find frequency spectrum. However,the graphics I found did not seem right to me. Is there anyone who can help? If there are errors in what I do, I want to find and fix it.

Following code for second graph:

y = fft(x);
n = length(x);
f = (0:n-1)*(fs/n);
power = abs(y).^2/n;
subplot(3,1,2)
plot(f,power)
xlabel('Frequency')
ylabel('Power')

• "the graphics I found did not seem right to me" Please edit your question to add what you expected to see, and why what you got seems wrong.
– MBaz
Apr 19, 2020 at 23:51
• I'm actually new and trying to learn. I couldnt find any example for triangle waves. I tried to create the chart with my own interpretation. However, but the graphics that came up didn't seem right to me. I asked to find out if I am wrong because I do not have enough information to make sure what I do Apr 20, 2020 at 0:06
• @Oguzhan You have posted the same question some hours back, why not use that?instead of posting the same question? dsp.stackexchange.com/questions/66618/harmonics-in-matlab/… Apr 20, 2020 at 6:16

First, your period is $$T = 1/25s=0.04s$$ which means you do not need to take integral of 3 periods for getting fourier series. A plot of the DFT is given below when I took only first $$T=0.04s$$ is given by
As you can see, all even multiples of $$25\text{Hz}$$ is zero, while odd multiples of $$25\text{Hz}$$ is decreasing with increasing frequency. From the plot $$|X[5]|^2 = 0.002914$$, while $$|X[7]|^2=0.0008386$$. Even multiples of $$25\text{Hz}$$ have very low values ($$\lt 10^{-12}$$). Taking DFT is not the correct method, so read on.
The above values are expected because fourier series of your even symmetric signal with non-zero DC has all $$b_k =0$$. For computing $$a_k$$, your signal $$x(t)=2t/T$$ for $$0 \le t \le T/2$$ and $$x(t)=-2t/T-2$$ for $$T/2 \le t \le T$$. I am not going into details of how to compute fourier series coefficients but I will give hints $$a_0=\frac{2}{T}\int_0^T x(t)dt\\ a_k=\frac{2}{T}\int_0^T x(t)\cos(2\pi kt/T)dt \\ =\frac{2}{T}\int_0^{T/2} 2t/T\cos(2\pi kt/T)dt + \frac{2}{T}\int_{T/2}^T (-2t/T-2)\cos(2\pi kt/T)dt\\ =\frac{2((-1)^k-1)}{\pi^2k^2}\\ a_k=\frac{-4}{\pi^2k^2}\text{ for k odd}\\ a_k=0\text{ for k even}$$ So as you can see, the even harmonics are all zeros, while odd harmonics decrease by a factor of $$1/k^2$$. So $$a_5=\frac{-4}{\pi^2 25}\\ a_7=\frac{-4}{\pi^2 49}$$