Relation between units of real data and its FFT data

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Mon, 03/18/2013 - 03:35 pm

Hello,

I am checking noise of my STM system. From measured data "Voltage (mV) vs Time (s)", I take its FFT by Igor Pro version 6.21 to know the frequencies and amplitudes of the noise.
I used the code

FFT/out=3/dest=Dest_wave Source_wave

Then one can get a plot of "Amplitude vs Frequency".

The frequency is in Hz. But I do not know unit of the Amplitude. I have read for many times in the help menu of the Igor Pro for FFT, but am still stuck.

How is its unit related to mV in the measured data??

It is very essential to know the unit of Amplitude for further analysis. Could you please shed some light on these issues?

Great thanks for your kind help!!!
Best regards,
Bay.
Type this command to see the relevant help for the FFT amplitude scaling:

DisplayHelpTopic "FFT Amplitude Scaling"

Basically, the magnitude result needs to be divided by N/2 (N is the *input* length before FFT transformation), and the result is in mV (same as the input). But there are complications: if the frequency doesn't fall exactly at a multiple of fs/(2*N), the amplitude is smeared into multiple adjacent FFT "bins".

Don't skip reading the above help topic; it explains better than this summary.

--Jim Prouty
Software Engineer, WaveMetrics, Inc.

March 18, 2013 at 04:57 pm - Permalink