How to make time series

[henryx]

Hello,

I have an array of data sampled by known (500Hz) frequency. If I plot that, I get x-axis in samples. I am able to rescale it by using a scale settings. But the problem is, when I want to do FFT. The axis is in samples or in time, but I need frequency there.

So, the question is how to add to that array another column, which has same length and values corresponding to each sampling step and how to plot everything in right units. I am new to LabVIEW. Thanks

Jindrich 

[Vsh]

Hi Jindrich,

when you connect output of a FFT vi to a waveform indicator, what you see on X-scale is nothing but frequency range. Make sure you have enabled the autoscale X asis to get the FFT peak on the  right frequency on the scale. 

[henryx]

Thanks for your answer.

 Well, the length of my data is 4000 and if I perform FFT I get spectrum range from 0 to 4000 (it corresponds to number of samples). Is there way how to "tell" Labview "my fs is xxx Hz" ?

[henryx · Accepted solution]

I have solved it using "scale" in axis formatting. Half of spectra corresponds to fs/2 so I calculated a multiplier.

[Vsh]

What you just had to do is enable autoscale X axis, and connect the output of the FFT to your waveform graph. Definitely you will see half of the spectrum (real) and imaginary part is not visible.

More, Your frame lenght will always be half of your Scanning Frequency. Simply by Δf=fs/N. where Δf is the frequency resolution, T is the acquisition time, f_s is the sampling frequency, and N is the block size of the FFT.