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I have a voltage controlled amplifier that I would like to calculate the THD of using the power dissipated at the (resistive) load.

My question is: if the input voltage frequency is 100Hz, would I use 100Hz as the fundamental frequency, or would I use 200Hz, since the power waveform would be P = IV = sin(100t)*sin(100t) = sin(100t)^2, which has a frequency of 200Hz.

Thanks for any help.

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  • \$\begingroup\$ You use the fundamental frequency of the signal you want to know the THD of. If you want THD of three voltage signal, then use the fundamental frequency of the voltage signal. \$\endgroup\$ Commented Sep 2, 2020 at 23:24
  • \$\begingroup\$ What would I use to determine the THD using the power dissipated at the load then? \$\endgroup\$ Commented Sep 2, 2020 at 23:36

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*My question is: if the input voltage frequency is 100Hz, would I use 100Hz as the fundamental frequency, or would I use 200Hz, since the power waveform would be P = IV = sin(100t)sin(100t) = sin(100t)^2, which has a frequency of 200Hz.

Use 100 Hz, since the 200 Hz rectified power signal is what you get from a 100 Hz sine input (and only a 100 Hz input). Consider that the first harmonic of 100 Hz is 200 Hz, but the power waveform is at 400 Hz. Each unique input frequency has a unique power frequency, so just reference everything to the input frequencies.

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  • \$\begingroup\$ The issue I get is when I perform an fft on the power dissipation waveform the 200Hz harmonic is greater than the 100Hz, so my THD value is very large and I don't believe it's actually indicative of the actual THD. \$\endgroup\$ Commented Sep 2, 2020 at 23:49
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    \$\begingroup\$ You should do the fft on the voltage, convert to power at the output of the FFT if you need to. \$\endgroup\$ Commented Sep 2, 2020 at 23:55

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