fft and multiple tones extraction: magnitudes mismatch!

[Ale83]

Dear all,

I'm performing some Fourier analysis on a signal coming from an accelerometer. I'm interested in ordering (descending order) the most significant frequencies found in the magnitude spectrum, according to their corresponding magnitudes.
So, I am using  the "Extract Multiple Tone Information" with the "decreasing amplitudes" option.

Have a look at the image*: the exported spectrum (converted from dB to linear) from "Extract Multiple Tone Information" equals the one given by "FFT Spectrum Mag-Phase"...
But the information shown by the output cluster from "Extract Multiple Tone" shows amplitude values that are clearly different from the ones seen in the two spectra!!

After several tries I noticed that the frequency detections always coincide (in graphs and output cluster from "Extract multiple tones") but the associated amplitudes are always different!

How is it possible?

Thanks,
Ale


*   http://img444.imageshack.us/my.php?image=spectrummismatchgp0.jpg


Message Edited by Ale83 on 03-06-2008 11:35 AM

Message Edited by Ale83 on 03-06-2008 11:35 AM

[Simone_NI]

Hi Ale,

       you can see this post where there is a discussion about your question.

I hope this can help you.

Simone

Link:

http://forums.ni.com/ni/board/message?board.id=170&message.id=206256&requireLogin=False