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Fast Fourier Transform - FFT - CFFT - arm_cfft

Question asked by RMS-69 on Aug 9, 2017
Latest reply on Aug 10, 2017 by RMS-69



I try to use the fft code provided in the arm library. For the moment i only generate a sinus to test these codes but i already have an issue. Here is the code that i use to generate the sin :


int freqHz = 100;
int fsanpling = 4000;
float32_t result[512];


for(int j=0;j<256;j++){


Then i apply the fft :


  arm_cfft_radix2_instance_f32 S[1];


The problem is that when i plot (with excel or Matlab) the output of the fft, i get the first spike at f=200Hz instead of 100Hz. And this for differents values of fsampling (Always over 2*fmax). However when i process the fft on Matlab with the sinus values obtained above (with the first code), the output is fine and i get the spike at the right position. So i guess the problem comes from the way i use the fft or configurate it. I tried with the radix4 and get the same problem.


If anybody has already used these codes, thanks in advance for your help