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How to calculate THD using calculator?

greatqs

I found the thd() in calculator only has  5 arguments (e.g. from, to, # samples, fundamental frequency).

For example, I'm doing a FFT with sample frequency of 8192KHz with from=1ms to=2ms and fundamental frequency is 5KHz.

I want to calculate the THD within 20 ~ 20KHz instead of 0 ~ 4096KHz which thd() does since it doesn't include Fmin/Fmax arguments.

 

Thanks,

qs

Parents
  • The THD, as a parameter, is defined as the rms sum of all the fundamentally related harmonics in a signal. Hence, the function, I believe, computes the total harmonic distortion in a manner consistent with the definition (given the FFT length).

    To compute the harmonic distortion for a lesser number of harmonics or over a particular frequency range, simply determine the power (or value of the dft function) for the relevat harmonics and perform the rms sum to compare it with the power in the fundamental.

    An alternative might be to actually include a filter in your circuit analysis simulation that limits the bandwidth to the range of interest and then use the THD function on its output.

    I hope this provides some help!

    Shawn

    http://www.aspowertechnologies.com/resources/pdf/Total%20Harmonic%20Distortion.pdf
Reply
  • The THD, as a parameter, is defined as the rms sum of all the fundamentally related harmonics in a signal. Hence, the function, I believe, computes the total harmonic distortion in a manner consistent with the definition (given the FFT length).

    To compute the harmonic distortion for a lesser number of harmonics or over a particular frequency range, simply determine the power (or value of the dft function) for the relevat harmonics and perform the rms sum to compare it with the power in the fundamental.

    An alternative might be to actually include a filter in your circuit analysis simulation that limits the bandwidth to the range of interest and then use the THD function on its output.

    I hope this provides some help!

    Shawn

    http://www.aspowertechnologies.com/resources/pdf/Total%20Harmonic%20Distortion.pdf
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