scan rate and samples for FFT affecting dt

[johnsold · Accepted solution]

Mike,

You can get 1 Hz resolution with a 5 Hz update.  What I described earlier was a way to do the FFT once per second.

If you want to update the FFT 5 times per second, just do the FFT after each acquisition.  You still need to accumulate 1 second of data in the circular buffer.

Suppose you have 0.2 second acquisitions numbered 1 through 5.  An FFT on {1,2,3,4,5} will give the 1 Hz resolution.  0.2 seconds later you have acquisition 6.  Then do FFT on {2,3,4,5,6}.  Some of the data overlaps so rapid changes will not show up in the transformed result immediately, but that is a consequence of the resolution you have selected.  This is called a "sliding window" calculation.

Lynn 

[M.Warlow]

Hi again,

Many thanks for your time and effort you have shown in helping me out here.

One final question as i now understand what you have said so far, but how would i create a circular buffer for the smaller samples.

Any chance you could put a small pic up showing how its done?

Again , thanks for your help

Mike

[johnsold]

Mike,

I have been pretty busy today.  I will try to post something tomorrow.

Lynn 

[M.Warlow]

Thanks Lynn, look forward to getting this working 🙂

Cheers

Mike

[Coq_Rouge]

If you want to update with 5 Hz and show 1 second of data, you could use a waveform chart, and pass the content of this chart to the FFT function

[M.Warlow]

Hi, i've tried that, but most probably incorrectly as it still did not work, could you provide a quick diagram if you have time

Many thanks

Mike

[M.Warlow]

Hi all (again!) 🙂

I think i may have got it working using the chart as suggested, below is what i have done , all seems to work fine - is this what you guys meant?

This was for 10hz and a history of 10 samples

Cheers

Mike

[Coq_Rouge]

I made this a quick example. The chart is strip-chart. The chart history length is set to 5000 so it will always contain the last second of data. As note this method is OK then you are using a relatively small chart history length, and do not update to often. A large history length and high update rate will hog system recourses.  

Message Edited by Coq Rouge on 12-03-2009 12:45 PM

[M.Warlow]

hi, not sure if there is supposed to be an image attachment?

Just looking at my example too..i think i probably have to add from earliest to latest time wise  not from latest to earliest as i have done?, or does it not really matter?

[Coq_Rouge]

I posted a full VI not a picture