How to find the slope of phase data (between two points) which is uneven

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Sriram Guddati
Sriram Guddati 2018-10-12
回答： Greg Dionne 2018-10-26
I have a phase vs frequency plot. I need to find the slope between point 'a' and point 'b' in an automated way instead of looking at the points 'a' and 'b' and calculating the slope. Can anyone help me with this?

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Image Analyst
Image Analyst 2018-10-12
You ask ONLY about the slope, not about finding a and b, so I assume you've found "a" and "b" already. So, assuming your "a" is index1 and "b" is index2, you can do this
coefficients = polyfit(x(index1):x(index2), signal(index1:index2), 1);
slope = coefficients(1);
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Image Analyst
Image Analyst 2018-10-12
OK, that wasn't stated in the original question. So you can use
[peakValues, indexOfPeaks] = findpeaks(signal);
[valleyValues, indexOfValleys] = findpeaks(-signal);
valleyValues = -valleyValues; % Flip again to get right side up.
Of course you will find lots of peaks and lots of valleys so you'll have to decide which to concentrate on.

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jonas
jonas 2018-10-12
编辑：jonas 2018-10-12
I would suggest using either findpeaks to determine points a and b and then calculate the average slope or alternatively using findchangepts. If you want code, then I suggest you upload some data.
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jonas
jonas 2018-10-12
Yes of course. The index is the second output of findpeaks. Note that you need to run the function two times, one for peaks and one for valleys. For the latter, you just turn the plot upside down, i.e.
findpeaks(-y)

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Greg Dionne
Greg Dionne 2018-10-26
You could try a combination of unwrap (to remove the jumps of your array) and sgolay to do the differentiation. See the second example on how to perform differentiation.

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