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

0 votes

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, 2019 by mhalqa01 (12 points)

Are you able to post an example model?

Thank you,


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: 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, 2019 by Oliver Schwartz (355 points)