where to find FFT

[Dost]

I am using labview 7.1 i want to use FFT but i could not find any vi for fast fourier transform .which pallete should i search to use FFT related VI .Also is there any matererail avialable for beginnere to start working on FFT using labview.

[Dennis_Knutson]

If you have the full or pro version of LabVIEW, you'll find the FFT on the Analyze>Signal Processing>Time Domain palette. The functions are not available with the base version of LabVIEW. If you do have it, there are a lot of shipping examples. Open the Example Finder and from the Browse tab, open the Analyzing and Processing Signals section.

[Dost]

i checked the analize pallete it does not have signal processing . i also got a toolkit with labview 7.1 i installed signal processing tool kit even than i could not find fft any idea where should i search for FFT vis.

[Dennis_Knutson]

The signal processing add-on is separate from the analysis functions that are part of the full version of LabVIEW. It sounds like you bought the base version and would need to upgrade. What does it say on your LabVIEW installation CD? If it says full or professional, then you should have the FFT functions if the full installation was chosen.

[Dost]

i want to apply FFt on my data that iam acquirng from NI card.I am measuring Pressure and flow rate . i want to apply FFT on these data. i want to  know which  is more suitable way of doing this

1. should i save the data in array and then  pass this array to power spectrum VI of FFt

2. or should i directly connected my incoming signal to input of the Power spectrum VI. I am acquirng data at a rate of 1000 Hz  but during FFT number of samples are also important i want to know if i directly connect the incoming signal than how could i determine what are my number of sample of incoming data.

[PNHorn]

Hi,

It would depend on whether you want to see the FFT updated in real time as you run the code or if you are happy to wait till it stops running and then view the FFT. If you are happy to wait until the code has stopped running then it will have less code to execute and therefore run a little bit faster (this is probably a fairly negligible difference though so I wouldn't sacrifice watching it in real time just for the increase in speed, if you do want to watch it in real time of course).

As you say, you can save the data into an array and do an FFT on it at the end of your code. You can do the same thing as you are running the code, just using a shift register and a build array function on your loop.

Hope this helps, if you have any further questions please feel free to post back.

Best regards,

Peter H.

Applications Engineer.

NIUK.

[Dost]

Thank you for reply  i will like to see FFt in real time in that sense what  should be best option  b/w 1 and 2

1. should i save the data in array and then  pass this array to power spectrum VI of FFt

2. or should i directly connected my incoming signal to input of the Power spectrum VI. I am acquirng data at a rate of 1000 Hz  but during FFT number of samples are also important i want to know if i directly connect the incoming signal than how could i determine what are my number of sample of incoming data.

secondy i found three VI related to FFT.

1. FFt that give real and complex value

power spectrum

power spectrum Express VI

what is the difference b/w these VIs I also checked power spectrum help it has an out put names as magnitude i want to know furhter about it i mean what is this magnitude what doe si tindicate please execuse my beginner level in FFt.

[PNHorn]

Hi Dost,

The simplest way to measure the power spectrum is to use the express VI's. These express VI's have the advantage of a dialog box which allows you to do the configuration in there instead of going through the lower level LabVIEW code yourself. LabVIEW will then build up your sub-vi based on how you configure the dialog box, saving you time and stress.

I would say that for your needs (sampling rates of around 1 KHz) the Express technology will be more than fast enough and will save you a lot of time in the development process.

I have quickly built you an example of a VI which does a power spectrum of a sine wave in real time.

Hope this is what you wanted, if not, please give me a shout back and I'll be more than happy to help.

Best regards,

Peter H.

Applications Engineer.

NIUK.

[Dost]

Thanks for the reply. I am acquiring data with the help of AI start and AI read.AI read gives me a waveform out put that contain both the data value and time inforamtion .for my purpose in order to make the average i took data value out of this wavform out put and calcualte the avergae value for 100 such point.I then displayed this average value on the chart graph.i am doing all this in a while loop at 1 second. I am getting right data on my VI a si can compare it with the actual reading displayed on flow meter.Now i want to ask some question may be silly:) 1.how does waveform chart add timing info.it has two scales Y and X on Y -axis it shows me myparameter value and X -axis it shows me time. 2.My second question is related to FFT.I  want to use Power Spectrum Expres sVI .i want to know in my case should i directly connect the wavform obtain from theAI read to input signal of Power spectrum VI or should i connect the wavform Chart signal to the input signal of the powerspectrm.

[Dost]

Here is my one more question:) I used power spectrun Express Vi. it gives me RMS both in graphic and numeric indicator.on graphic it shows me frequecy b/w 1 to 500. and RMS value is -81. i want to know what is meaning of RMS how would we interprate the result of power spectrum Express Vi. Regards