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I have conflict between THD from the scope and from normal calculation for the following signal

0 votes
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I have read the answer of the question "What is the method for calculating THD in PLECS? Why does the THD value in PLECS look like not right" but  I still have a doubt for the value I got from the scope. I got around 111% but the calculated one is around 90%. is it possible to get THD greater than 100%?

Thank you

 

asked Feb 17 by mhalqa01 (12 points)
Hello,

Are you able to post an example model?

Thank you,

Kris

Thank you for your reply. 

I thought I have attached some pictures for my question but I don't know what is going on.

anyway, picture 1 shows the THD calculated from Plecs scope based on fundamental frequency 60Hz. the value is 1.09(109%). is that right?

Also I have export the data as excel sheet and calculate THD based on the equation from this website: https://en.wikipedia.org/wiki/Total_harmonic_distortion which give me different value (95%) which for me is more resnonable than THD from the scope.

Do you have idea which one is correct or I did something wrong?

Thank you

 

 

1 Answer

0 votes

 is it possible to get THD greater than 100%?

Yes, it is.

As explained here already, the THD is calculated as  sqrt(sum(Vn^2))/V1 for n>=2. This ratio can have any value, because Vn and V1 are totally independent from each other.

A better analysis of your data is only possible if you post your model. Mathematical deductions based on partial screenshots are hard to do.

Kind regards,

Oliver Schwartz

 

answered Mar 4 by Oliver Schwartz (273 points)
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