# LabVIEW

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## FFT Limit testing

Any idea how I can add a limit curve on an FFT plot? The limit I want to use is as follow:

• constant value of 80 dBuA(rms) between 1000 Hz and 10KHz
• a linear ramp down to 40 dBuA(rms) between 10KHz and 100KHz
• constant value of 40 dBuA(rms) between 100KHz and 50MHz

The linear ramp is obviously the tricky part!

Ben64

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## Re: FFT Limit testing

I found a way to do it but I will not mark it as an accepted solution yet. I have a strong feeling that there must be a better way to do it.

Ben64

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## Re: FFT Limit testing

Hi Ben,

I have a strong feeling that there must be a better way to do it.

Well, in the end it should be just (rather) simple math involving some logarithmic functions… 😄

Best regards,
GerdW

using LV2011SP1 + LV2017 (+LV2020 sometimes) on Win10+cRIO
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## Re: FFT Limit testing

@GerdW wrote:

Well, in the end it should be just (rather) simple math involving some logarithmic functions… 😄

Not so simple, the math part might be easy but how to create the limit and integrate it with the limit testing vi is not trivial. You need to define the limit values for the frequencies f0 + n*df of the resulting FFT.

Ben64

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## Re: FFT Limit testing

Are you sure you aren't over-thinking this?  Or maybe I'm under-comprehending...

Why can't limit testing be handled with a lookup table that interpolates between the small # of corner points?   If needed, you can do log-weighted intepolation.  Or you could go ahead and precompute the entire "curve" once you know what dF spacing is needed.

So what am I not following?

-Kevin P

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