I am new to Labview. I need to plot the data (2 cols, one is time, x array, the other one is signal intensity,y array)provided by my instructor in time domain, which I already did. Then I need to convert the data into frequency domain. What should I do? I tried FFT function for my y array. But how can I find out the frequency informaiton? From the x array, I know it's sampling rate is 2500Hz. T= 40s, N=100,000. the effective maxmium frequency should be 1250Hz, right? And I also need to isolate the 1000Hz signal... I am totally confused. Hope you can help me. Thanks in advance. John

Norbert makes some good points- you need to have a play and maybe get a good book on signal processing (not LabVIEW specific). The 'effective' frequency you talk about is I assume the Nyquist frequency=sample rate/2. If this is what you mean, you don't have to do anything about it- just be aware that the maximum frequency your power spectrum will show is sample rate/2. You can isolate a specific time domain signal in the manner you mention, FFT->zero spectral bins you're not interested in->inverse FFT, (use FFT not power spectrum for this). A standard FFT will put out a double sided spectrum, which is symmetrical for time series that do not contain complex numbers, so you will need to blank a given bin on both sides of the spectrum- which is maybe what you were talking about RE: effective frequency. Use the whole thing (not just +ve half) to reconstitute the original signal. The 'blank a bin and inverse FFT' method is not considered a 'good' way of obtaining time series in a reduced band for real signal processing for reasons I won't explain here. You usually just use a filter on the time series, of which LabVIEW provides many.

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