Waveform FFT

[JackAlthon1]

Hello,

Anybody, please help with data. I want to do FFT for data, but it seems that I don't understand how to get FTT, because if I connect like as shown in figure I see only one line, it doesnt look like FFT spectra. How to do FFT from Waveform? I dont know how to do sampling. 

Thank you

[mikeporter]

First, you should spend some time going through the tutorials for LabVIEW.

Second, you should spend some time reading up on FFTs so you know what they are and how they work. There are FFT examples that ship with LabVIEW, they can be helpful here.

Third, you should post some actual code -- the picture you posted is clearly one piece of something larger.

Fourth, you should show some pictures of you inputs and the output you are getting.

Mike...

[JackAlthon1]

Yes, I checked all samples, but you see it is easy to get FFT spectrum from generating signal, but how to get FFT spectrum when you read file? How to do sampling? Thank you

[mikeporter]

See my first point. You need to learn LabVIEW.

Mike...

[mikeporter]

When reading from a file, there is no sampling to be done.

Mike...

[JackAlthon1]

No sampling? Frequency? I dont get it. I am writting, but it does not work

[RavensFan]

You are reading a waveform so it has a dT as part of its data.

Attach your VI and your data file.

[JackAlthon1]

OK, please look at code, it is something wrong. How to get FFT? I want to add data, but the size is ~ 700mb. Please look at code and send it back. Thank you

[JackAlthon1]

I know how to get deltaT, but how to connect that get a correct spectrum? 

[Bob_Schor]

You are reading a file (raw_02.dat) using Read Waveform from File.  Do you understand this function?  [I suspect not, as you use the "All Waveforms" output, then use Index Array to get the first one, when the function has explicitly a First Waveform in Record output].  Do you know what a Waveform is?  Have you looked at the Waveform?  Do you know how many points it has?  Do you know the frequency at which its data were acquired?  [Hint -- the answers to the last two questions are contained in the Waveform, itself].

Rather than flailing around, why not write a small VI where you do the following:  Open a single file, save the first record in a named Indicator, write a little code to get N (number of points) and dt (sampling time interval), plot the waveform on one graph, do a single FFT (you choose which you want to do), and plot those results.

Now, if you look at your Front Panel, you should have indicators showing (among other things) the plot of your waveform and the value(s) in the Waveform, itself.  Go to the Edit menu and click "Make Current Values Default".  Post this little VI, which will send us (in your named Indicator) a sample of your data.  We can then change it to a Control and test your code, modifying it if necessary to get it to work, or explaining to you why you might not have the kind of data that you need to do an FFT.

Bob Schor