How to see the frequencies present in a measured signal.

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Hello,
I measured a pulse signal sampled with a frequency of 64Hz and want to see the fre-quencies responsible for the signal. Can some explain how to plot the power spectrum using FFT function to see the fre-quecy components responsible for the pulse.The pulse data of 10secs duration is at-tached below.
  4 Comments
Joseph Cheng
Joseph Cheng on 7 Apr 2014
the documentation for fft has examples of how to do it. http://www.mathworks.com/help/matlab/ref/fft.html
The first example can be adapted with the information you gave above.
Gova ReDDy
Gova ReDDy on 7 Apr 2014
Yes,I tried like this and resultant was
a1=data(1,1:640);
plot(a1)
a1_fft = abs(fft(a1));
a1_fft = a1_fft(1:length(a1)/2);
plot(a1_fft);
But from the output pulse signal(posted in the above) it can be understood that it only has 1Hz frequency component but the FFT output shows 1Hz components in larger and also other frequencies of 11 ,21,25,31Hz.
Can someone explain why the other frequency components are also in larger magnitude.

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Accepted Answer

Image Analyst
Image Analyst on 7 Apr 2014
What make you think a square wave has only one frequency? A sine wave does , but a square wave is made up of an infinite number of sine waves . Hence you get power at multiple frequencies. The FFT of a single square pulse is a sinc function, but when you convolve with a comb function to get a square wave train, you'll get tons of sinc's all adding together. Here are some links with basic info on FFT theory:
  8 Comments
dpb
dpb on 8 Apr 2014
What was your sampling frequency and length? The relationship is
df=1/T
where T is n*dt and n is number of samples and dt the sample rate. From that you can set the frequency axis appropriately for your sample case.
Again, I commend the example at
doc fft
to your attention -- it has exactly the right steps illustrated for a sample case; simply plug your numbers in in the appropriate places. The only difference is they chose to use the power of 2 length FFT, but the same is true if you just use the signal length.
Gova ReDDy
Gova ReDDy on 9 Apr 2014
I am confused.Can you please help me out to plot this.

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