extract points from the fft spectrum

[Water]

Hi,

Just a query. I have just converted a time signal into the frequency domain.I have two specific signals of interest. One is at a particular frequency at f while the other is the value at DC. If I want to take measurement of the amplitude at this two frequency points or regions, is there any simple way I can do it? Can I just simply use tone measurement.vi in the waveform analysis and set the two frequencies of interest?

Also if I need to record down the whole frequency spectrum that I have produced. Is there a fast vi to do it?

Thanks for your help!

[Paul M.]

Water,

With the waveform in a frequency domain, retreiving the amplitude at a specific frequency is as simple as indexing into the array of points at the frequency you're interested in.  Point 0 will correspond to DC, and point X * bin size will return the amplitude at the frequency you're interested in.

Use the Index Array VI.  Use the Write Mesurement File VI to save your data to a text file that you can read in a text editor, or a program like Excel.

Let us know if you need more details.

[Water]

Hi,

I wonder if it is correct to use the extract single tone information.VI to do this? I have set the approximate search frequency and search width. For the signal input, I have used the pre-FFT signal.

Thanks for your help!

[Paul M.]

Water,

That'll work fine as well and would probably be an easier implementation.  Just remember that the input signal for the extract tone VI needs to be in the time domain as it performs the FFT for you.

--Paul
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Paul Mandeltort
Applications Engineering
National Instruments, Inc
http://www.ni.com/support