How can I calculate which is the index (x-value) correspondant to a certain value of a signal?

[Mr_Crack_Spider]

I just want to calculate the quality of the peak in a FFT, defined as you can see in the attached file.

For that, I would need to know how can I calculate in Labview the index or indices (x-value) correspondant to a certain amplitude value of the function.

Kind regards, and thank you for reading,

Jose

[crossrulz]

If you just want the maximum, then use Array Max & Min.  One of the outputs is for the index of the maximum.

[Mr_Crack_Spider]

thanks, but I do not want the maximum, but the index of the half of the maximum

[johnsold]

Look in the Signal Processing palette for pulse measurements, peak detector, and threshold detector. One of those VIs or some combination can probably do what you want.

Lynn

[altenbach · Accepted solution]

If you know the lineshape (e.g. Lorentzian), you can use nonlinear fit. This will give you the most accurate value.

In the most simple case (insignificant offset and noise, no other peaks) you can apply the following code (if there are other peaks, then take a apropriate array subset before applying the code). To get correct units, just scale with df.

[Mr_Crack_Spider]

Thanks a lot!!!!