FFT trouble

[Dachenday2]

Hello again! I was hopeing that somebody could help me out with the use of FFT in labview. I'm doing something wrong, but I have no idea what it is. This time I have permission to attach a file so hopefully someone can tell me what I'm doing wrong.

the vi is basically nothing more than a dozen attempts to perform an fft, all done at once. After I couldn't get the FFT vi to work, I started tested other ways to get frequency data, such as power spectrum measurements. None of them worked.

I was testing the vi by reading in 3 files. I had 2 artificially generated sin waves, one at 60 hertz and one at 120 hertz (I generated them in excel). I also recorded data from my emg without electrodes attached with the idea that the leads would become antennas and I would pick up 60 hertz "noise" from the electrical wiring.

For some reason, my vi keeps insisting that my artificial 60 hertz sin wave is actually 10 hertz, that my artificial 120 hertz sin wave is actually 20 hertz, and that my raw "emg" data is mostly 120 hertz.

several places in the vi I have an "auto correct" that will multiple something by a factor of six, in an attempt to change the 10 hertz to 60 hertz and the 20 hertz to 120 hertz. That wouldn't do anything about the "emg" data somehow being 120 hertz, but I was pretty desperate by that point.

Also, I am of course Dachenday, I just forgot my password and so I simply made a new account.

I would really appreciate the help. I have googled "labview FFT" (or something worded slightly different) many times now, and I find almost no reports of difficulty with this vi. This leads me to suspect that I'm simply doing something stupid, but I can't seem to figure out what it is.

1 second of fft every 5 seconds mk2.3.vi

[Tim_S]

I'm guessing your dt is incorrect by a factor of 6.

As a suggestion, try a simpler VI and use the diagram cleanup tool to make the code more readable.

Tim

[Dachenday2]

Thanks. Over the weekend I figured out that it was off by a factor of 2 pi, and that that was due to excel working in radians. (I still don't know why excel works in radians by default).

[asbo]

Thanks. Over the weekend I figured out that it was off by a factor of 2 pi, and that that was due to excel working in radians. (I still don't know why excel works in radians by default).

This is pretty standard - every calculator or software I've ever used only used degrees if you explicitly asked it to. Radians are less intuitive for us meatbags, but I imagine there's a technical reason our silicon friends default to them.