linear scale fft chart

[Nishant Koti]

Hello,

I am doing the analysis on the signals obtained from a unltrasonic transducer, using a PCI--4452 DAQ board. I have the FFT spectrum and power spectrum of the signal. I need to find the value of the voltage at particular frequency. what is the method to do that?

 thank you

Nishant Koti

[JLS]

Hello,

I think you are asking how to obtain the value of your FFT at a given frequency.  Well, your data is likely in an array, so that comes down to indexing the array at the point which represents your FFT.  Underneath the LabVIEW functions (you can see this if you bury down deep enough), you'll find that df is a function of dt as:

df = 1/(dt*block size), where the block size is the length of of your data.

See the example Amplitude Spectrum (sim).vi in the example finder.  To do this, click Help -> Find Examples... and search the keyword spectrum in the search tab... double click the keywords after your search to populate the examples list.  You'll see there the plot of the FFT along with the labeled x-axis, which is constructed based on the expression noted above.

I hope this helps!

Best Regards,

JLS