FFt amplitude

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Vincent G.
Newbie
Posts: 0
Joined: Mon Mar 14, 2016 3:53 pm

FFt amplitude

Post by Vincent G. » Mon Mar 14, 2016 4:07 pm

Hello,

I would like to know why the fft of a perfect 1 kHz sinus of 2V amplitude gives a peak at 1 kHz of only 1.3V.
Is there a numerical explanation ?
The sinus was generated with the demo setup (when no device is plugged) and the spectral mode was set on linear amplitude and with a Blackman frame.
Maybe I am wrong but I thought that the amplitude given in the spectrum for a given frequency was the amplitude of the sinus compound of the function for that frequency.

Thanks for you help.
Vincent G.
PHD student

AndrewA
PICO STAFF
PICO STAFF
Posts: 199
Joined: Tue Oct 21, 2014 3:07 pm

Re: FFt amplitude

Post by AndrewA » Tue Mar 22, 2016 12:42 pm

Hi Vincent,

The amplitude in FFT is the Root Mean Square (RMS) value of the waveform.
The FFT settings will also affect the results, number of bins and the where the bins fall in relation to the signals frequency. As well as the window function used.

For a 2V peak (4v peak to peak) sine wave the RMS value is 1.414v. This is close to your measured value of 1.3v.

The Flat Top function should have the best amplitude accuracy, according to wikipedia-
https://en.wikipedia.org/wiki/Window_function
Regards Andrew
Technical Specialist

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