Differentiate Formula

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Hello Everyone,

I am collecting analog electrical data and need to take periodic slopes from my curve. It is fairly noisy but I can easily get rid of that with smoothing. What I really need to be able to do is apply the following equation to my 1D wave.

f'=(f(point + h)-f(point))/h h= width of "box"

The differentiate function does a point by point differentiation. This not appropriate for me, I need to be able to set the width of the "box" and calculate a slope from it. I am not familiar enough with Igor Pro 5 to write the function. Any guidance would be very much appreciated.

Regards,
JBertrand
You need a user function like this:

Function WideSlope(w, h, xx)
    Wave w
    Variable h
    Variable xx
    
    Variable h2= h/2
    Variable val1= w(xx-h2)
    Variable val2= w(xx+h2)
    Variable slope= (val2-val1)/h   // central difference
    
    return slope
End

Then create a wave to hold the slopes and apply the function using x as the index in the wave assignment. For example:
Make/O data=sin(x/8)
Setscale/P x, 1, 0.25, "", data
Duplicate/O data, dataDiff
dataDiff= WideSlope(data,0.5,x)

See the attached graph, too.

--Jim Prouty
Software Engineer, WaveMetrics, Inc.
WideSlopeGraph.png (5.41 KB)

March 8, 2011 at 10:12 pm - Permalink