how to get the Datapoints of an FFT spectrum in Labview

[Nishant Koti]

Hello,

I am getting the FFT spectrum by using LabVIEW software with the support of Sound and Vibration tool kit. I need the list of data points of that spectrum (FFT Chart or Power Spectrum Chart) to analyze the peak value. I am using PCI-4452 DAQ board and BNC 2140 switch, and I am getting the signals from a Ultrasonic transducers. Thank you.

Nishant Koti

[Dennis_Knutson]

I'm not sure I understand your problem. The output of power spectrum or FFT functions has a list of data points. For example, the output of FFT Power Spectrum is a cluster that contains f0, df, and a magnitude array. If you any or all of the elements for futher analysis, you can use the Unbundle by Name to get all or some of the elements.

[Nishant Koti]

My problem is getting the data points like we get in an MS excel sheet for a graph/chart. I ahve the power spectrum and the FFT for a signal from ultrasonic transducer which is conditioned using a switch BNC 2140. I want all the values of X, Y in the graph/chart of FFT and power spectrum. Fromt hese values of data points I will be analysing the peak point in the spectrum.

Thank you

[Dennis_Knutson]

Like I said, you have all of the Y points already. When you wire the output to a LabVIEW graph, it automatically creates the x-axis based on t0, fo, and the size of the Y array. LabVIEW is smarter than Excel in this case. I suspect you really don't need to create a separate x-array but you can do that like in the attachment. The reason I said that I suspect you don't need to calculate an x array is that you can find the peak magnitude with the Array Max & Min function and get the freq at the max magnitude with some simple math. That is also in the attachment. If you need additional help, please provide details on the exact type of post-processing you need.

[Nishant Koti]

Thank you for your reply. I am a novice to the LabVIEW software, I have no idea what you are talking about. Tell me in brief without involving much of technical background, how to get the data points of the FFT Spectrum/ Power Spectrum. I am getting these spectra by running the exmples(FFT Spectrum of Traditional DAQ) given in the Sound and Vibration tool kit. I just need the list of data points to see the variation in the spectrum. Thank you.

[Dennis_Knutson]

What exactly don't you understand about the picture I posted? Since you didn't post your code, I used the FFT Power Spectrum on the Waveform Measurements palette. I then used an Unbundle by Name to get the various components of the output. This is the same output that you would wire to a graph. The output is what is called a cluster. A cluster can contain any number of different data types. In this case, the cluster consists of a numeric called f0, (frequency 0), a numeric called df (delta frequency), and a numeric array called magnitude. The magnitude array is all of the y values in the graph. A normal graph in LabVIEW does not require an x array because it automatically spaces the y values based on f0 and df. Any specific x value can be determined for a given y value by a little array manipulation and arithmetic. This is what I showed in the bottom of the picture. If you do require a full x array, then that too can be created. That's what I showed in the top of the picture. A lot of the analysis functions in LabVIEW do not require a separate x array since they will do all of the calculations for you to determine an x value. Some functions do require a separate x array and that creation of one is showed in the top part of the picture. You might also want to check into some of the links at LabVIEW Learning Center.

Message Edited by Dennis Knutson on 10-19-2005 10:25 AM