How can I measure the full width half max pulse-width of a gaussian function?

[Stysley]

Hi I am measuring a laser pulse using a Tektronix scope and feeding that data into my labview (v 7.1) based data system on a Mac OS X computer. I want to measure the full width pulse at half the maximum peak. The included measurement VIs that come with labview and the tek scope dont seem to do the job (even though the scope itself can do the measurement). I've attached my example VI. If anyone has any suggestions that would be great.
Thanks,
Paul

[Brett_B.]

Stysley,

Have you looked in the manual of your scope to see if there is a command that will return the measurement you want via GPIB/RS232? That would be the easiest way to do it. If you cannot do it that way I would need to know how the data was getting from your scope to the LV program. What format is it in?

-Brett

[Stysley]

Hi Brett, thanks for the reply. I am using a GPIB-ENET/100 to get the scope signal to the computer. I've attached the library that allows me to talk to the scope but I dont see anything that will give me a pulse-width.

[DFGray]

If you can display a trace on the screen, you can get the full width at half maximum. LabVIEW gives you several options:

1. Use the Pulse Parameters VI to determine the width directly. This will be a width based solely on the data with no assumption as to an underlying model.


2. Use the Levenberg Marquardt VI to fit a gaussian to your curve, then calculate the full width at half maximum from the coefficients. Levenberg Marquardt is a least-squares/gaussian algorithm, so is somewhat noise sensitive.


3. Use one of the minimizer functions (e.g. Downhill Simplex) to construct your own fit using a different assumed error distribution. If you are unsure how to do this, check out a copy of Numerical Recipes in C by Press et. al. and read the section on data fitting.


4. Do your own data based calculation. Find the peak with Peak Detector. This will give you the magnitude, but you may want to verify it with a polynomial fit (parabola is usually good enough) across the top of the peak to avoid noise problems. Then find the positions of the half-maxima on either side of the peak. The Interpolate 1D Array will probably prove useful for this.

Choose your method depending on your pain threshold and how accurate and robust you need the result to be. Good luck!