I want to buy a device without to program a CPU that does the transform FFT for 20 – 500 Hz frequency.
I know that is devices that do FFT for the frequencies of the AUDIO
Where I can buy devices that do FFT for 20 – 500 Hz ​

 

My guess would be that perhaps a modern digital Oscilloscope could do this.

 

So what the heck is FFT?

 

Thanks t06afre, but I don't search after Oscilloscope
I search after electrical device that I can put in the electrical circuit
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So what the heck is FFT?

FFT is short for fast Fourier transform
http://en.wikipedia.org/wiki/Fast_Fourier_transform

 

With all the computation required, a programed device like a DSP chip would be necessary.

 

FFT is an algorithm that give a numerical result as the output. It is not some physical device say like an OPAMP. So I think we need much more information about your project, else we will not be able to help.

 

The Fast Fourier Transform is a digital algorithm. As such it requires a digital device to implement it. To create something remotely similar in the analog domain would require a collection of sharp bandpass filters, say ten of them. Feed the same signal to each one and the output of each filter will be the amount of energy in each passband.

This will be way more trouble and expense than using a DSP chip. Much less flexible as well. I gaurantee you will be disappointed with the results of such a "crude" analog spectrum analyzer.

What is wrong with using a DSP chip anyway?