LabVIEW_USB_DAQ_FFT_instructions - LabVIEW - Instructions...

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LabVIEW - Instructions for Modifying a Virtual Instrument: Generate a Frequency Spectrum from Sampled Data Author: John M. Cimbala, Penn State University Latest revision: 28 February 2008 Introduction This module assumes that you have already created a simple virtual instrument that samples and plots voltage data. In this exercise, you will modify the existing virtual instrument so that it also calculates and plots a frequency spectrum using the fast Fourier transform (FFT). Initial setup 1. Turn on the function generator and the USB data acquisition system (remember that the power switch is in the back of the device). 2. Start LabVIEW (Start >> All Programs >> National Instruments LabVIEW ). Continue . 3. Open the virtual instrument that you previously created: File >> Open >> filename, where filename is the name of the vi file (you may need to browse to find it). 4. As always, you may need to move or close the Controls window, if it pops up blocking your view. Add spectral analysis to the existing VI 1. You will modify the existing VI to add spectral analysis functionality. Specifically, you will add an Express VI (a kind of sub-VI) to perform an FFT of the acquired signal. To do this, you must first make room for the new sub-VI by expanding the while loop. Go to the Block Diagram window. Then, click o the upper right-hand corner of the while loop, and drag it acr the screen (shown to the right). This will give you a little more room to add another sub-vi. Note : You may first also need to enlarge the Block Diagram window itself. n oss 2. In the empty space you just created, open the ‘ Functions Palette ’ by R (right click)-Express >> Signal Analysis >> Spectral , and place this sub- VI inside the while loop. Doing this will launch a configuration window that allows you to configure several properties of the spectral measurement. 3. In the configuration window, select Magnitude
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(peak) under Spectral Measurement . Also, select Linear under Result . Finally, select None for the Window type. Note that there are several different types o window functions available here – the m common being the Hanning window, which we discuss in class. Finally, click OK f ost to close the window. 4.
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This note was uploaded on 07/23/2008 for the course ME 345 taught by Professor Staff during the Spring '08 term at Pennsylvania State University, University Park.

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LabVIEW_USB_DAQ_FFT_instructions - LabVIEW - Instructions...

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