How to specify FFT lines in FFT Spectrum (Mag-Phase).vi ?

[Gurdas]

Hi!,

I am acquiring voltage data from a accelerometer using PCI-6023. Scan rate is 40K and input is as waveform data.
I am able to perform Magnitude FFT using the "FFT Spectrum (Mag-Phase).vi"

However some issues remain, namely:

1) Is this the right vi to use for FFT? I have no other choice for hardware (so DSA is out).

2) Where do I specify the number of lines?

Thanks!

Gurdas
gurdas@qagetech.com

[Jeremy Braden]

Whether it is the right VI to used depends on what you want to do. You may want to look in the measurements manual to learn more about ffts. You can find it by opening LabVIEW 6.1 >> help >> search the LabVIEW bookshelf. This launches a pdf. Look for LabVIEW Measurements Manual.

[CoastalMaineBird]

I use "Real FFT" and do my own mag calculations (and phase if needed), but that's a personal choice. I rarely use phase, so I don't calculate it often.

The FFT VIs always produce N output values for N input values. The upper half of the output is usually a mirror of the lower half and can be discarded (you can do tricks with stereo and use one FFT for two input channels, though).

If you want, say 400 lines (from a 1024-point FFT), you have to do it yourself. Use the ARRAY SUBSET function and keep the first 400 (or whatever) results of the FFT output array.