Scope acquisition then FFT

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viral
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Scope acquisition then FFT

Post by viral » Fri Apr 22, 2016 12:29 pm

Hi everyone,

It is possible to do a scope acquisition for one channel ( time acquisition of 50 seconds) and then do a FFT to see the spectrum of the signal ?

I hope you can help me and sorry for my english, i got the 5442A
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hexamer
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Re: Scope acquisition then FFT

Post by hexamer » Sat Apr 23, 2016 3:58 pm

It's definitely possible to do a single FFT on a single capture. The main trick to that is to just setup the trigger mode to be Single (control at bottom left portion of the screen). To get an unpadded and untruncated FFT with the exact time window you want (50s) takes some extra steps.

Here's what I did to try out your example:
  • The properties window is really handy to confirm what's going on with sampling, so start by opening it: View -> View Properties
  • Setup the AWG to output a 5Hz sine
  • If necessary, setup input channel (A) to appropriate voltage range (so it's not over-voltage)
  • Setup the input channel (A) to be 5 sec/div and enter a specific sample count in the sample control (about the middle control in the top row of controls near the menu) "16384 S"
  • Change triggering to single
  • Open a separate view to the spectrum mode: Views -> Add View -> Spectrum
  • Open the Spectrum Options and set Spectrum bins to 16384
  • Click the Go button
Once the run is completed, the FFT will be computed and shown:
SingleFFT.png
I did this all with the demo mode, so it may differ with specific scopes' sampling properties. But the overall principle should be the same.
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viral
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Re: Scope acquisition then FFT

Post by viral » Sun Apr 24, 2016 9:05 am

Thanks you so much !

I will try this tomorrow with my 5442A and I will tell you the result !
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viral
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Re: Scope acquisition then FFT

Post by viral » Fri Apr 29, 2016 12:06 pm

Thanks you for your answer, It works very good but I want to know if I can change the frequency range ?

I just want to see the spectrum between 0 and 40 Hz but I don't know how to do it :(

Thanks guys :D
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hexamer
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Re: Scope acquisition then FFT

Post by hexamer » Fri Apr 29, 2016 2:27 pm

viral wrote:Thanks you for your answer, It works very good but I want to know if I can change the frequency range ?

I just want to see the spectrum between 0 and 40 Hz but I don't know how to do it :(

Thanks guys :D
If you just want to see that range of the existing plot, you can zoom in to that range.

If you want the FFT to be over that range, you'll have to change the number of samples (try dividing it by 4). It will not be exactly 40 Hz, but would be close.
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