Hi,
I'm using FFT algorithm on a vector data of 4096 elements, so i get a vector of 2048. I need to cut high frequencies, but i cannot understand where i should cut the vector. Time step of original data is 0.004 secs (Camp Freq=250Hz).
Can someone explain to me how can i do the cut? (i.e. at 30Hz)
Thanks in advice.
You need to better explain "vector data of 4096 elements, so i get a vector of 2048" becuase the FFT of N complex value results in N trasnformed complex values. I assume that your 4096 values are 4096 real values and that this results in 4096 complex values but since the data is pure real the value at FFT(N-k) is the complex conjugate of the value at FFT(k) so you only need 2048 complex values.
The length of your data is 4096 * 0.004 = 16.384 seconds so the frequency resulution is 1/(4096*0.004) = 0.061 Hz. To filter out every thing after 30 Hz then you set to zero all the trasnform values after index 30 / 0.061 = (approx)492 .