Integrating a frequency domain signal

Question

Hi
I am trying to extract the response of a frequency dependent signal :
1. First, I use the SignalFFT vi to get into the frequency domain.
2. Then I plan to integrate the FFT signal around a certain frequency and take a ratio of that wrt to the integral over the entire frequency spectrum to get a measure of how strong the signal is around that frequency.
I am choosing a range of frequencies above because the physical phenomenon producing the time domain signal produces a certain lifetime width. I need a numerical integration VI that can do step 2 above. The numerical integration VI that I found in Labview (using ver. 7.1) integrates the whole signal, and there is no way to define limits between which the numerical integration should be carried out.
Any ideas ?
Thanks.

Answer 1

That a cool question!
This has to do with the sample rate, when you adquire the signal, you get set of samples, that hava a sample rate, and when you do an FFT on those samples, every sample corresponds to frecuency bin (kinda of frecuency samples). Example:
If you get 100 samples at 1kHz, you will get a 100 windows, this is 100 samples to represent 1kHz, however, only the half of the spectrum is valid, because of the Nyquist teorem, so this mean that from 0 to 49, you have a correct representation, from 50 to 99 you have a reflection of the spectrum. So in the first 50 "frequency samples" you have 0-500Hz
Tania Lozoya