modify fft frecuency wide

[Hernan Gelburt]

Hi, 

First of all sorry for my english, I ll try to do my best.

I am analyzing an acquired data with a FFT but I want to refine or reduce the frecuency wide of each "window" (I only want to analyze the 0-350 Hz region)

in order to be able to view better where my maximum in the frecuency domain is.

If you have any idea where I can modify this frecuency wide so as to refine the plot, please tell me!

Thank you very much!

[CJ_Butler__DASYLab_Pro · Accepted solution]

The sample rate determines the range of the FFT computation.

Using the Nyquist theorem, if you sample a 1000 samples/second, the FFT will compute the range to 1000/2 = 500 samples/second.

You MUST sample at least two times the highest expected frequency to be able to use the FFT module to find that frequency. 

The resolution of the FFT module is determined by the block size. For the default amplitude spectrum, the FFT will output a block that is also 1/2 of the incoming block. Block size = 512, FFT block size is 256.

You can increase the resolution by increasing the block size, and also by changing the block size to a number that can divide the sample rate. For example, if the

sample rate is 1000, and the block size is 500, the output FFT will have a resolution of 250 over a range of 500.

You can use the Data Window module to change the block size -- increase it to use more data and increase the resolution of the FFT computation. Decrease it to look at a smaller time interval, and with lower FFT resolution.

I hope that this helps. 

[Hernan Gelburt]

Thank you very much!

It has really been of great helpful!