Complex FFT VI - output units

[jmartyni]

I am currently post processing time domain accel data. I run it through the Complex FFT VI in order to transfer it to the frequency domain, which works wonderfully. Unfortunately, I'm still a bit confused as to what the output units of the plot are...
Any help would be appreciated.

[johnsold]

In LV 7.1 the FFT Spectrum (Real-Im) VI has f0 and df outputs. So the first element of the spectrum is at frequency f0. The freqency for any element can be calculated from fi = f0 + i * df, where fi is the frequency of the ith element of the spectrum and i is the array index. I don't recall what was in older versions, but the algorithm listed in the help should enable you to perform a similar calculation if the VI does not produce the f0 and df outputs.

Lynn

[LocalDSP]

First of all you don't have to use the Complex FFT, the Real FFT would work fine because your input time domain data are real-data. Both Real and Complex FFT VIs return a complex spectrum output.

The complex spectrum data represent the magnitude and the phase of your signal at the different frequencies. So if you use the 'complex to polar' primitive in your Numeric>>Complex palette you can convert your complex spectrum to magnitude and phase (radians) signals. Next you need to scale your magnitude for 'half the length of your FFT' (that is divide by N/2) in order to get the correct 'amplitude' of the different tones. Finally you need to correct your value at 'dc' with a SQRT(2) factor ...

When you are done with all this and optionally also have corrected
for the noise bandwidth of your window, the unit of your magnitude will be the same as your input signal (m/s^2 or g I assume).

So instead of all this I would recommend you to use the high-level VIs that will take care off all the needed conversions and scaling for you. If you build a Waveform using the 'Build Waveform' primitive, you can then pass your signal to one of the fighr-level Power Spectrum, FFT (Mag-Phase) etc... VIs. Check the Analyze>>Waveform Measurements palette.