Calculate Total harmonic distortion with FFT

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The scope can't calculate the THD.
You can check the information online.
The information is as follows:
“THD of operational amplifier is the ratio of RMS value of each harmonic (2nd, 3rd... Nth) in the input signal of operational amplifier to RMS value of output signal fundamental when the input signal of operational amplifier is pure sine wave (i.e. sine wave without harmonic). In the actual test, n is generally taken as 6, because the amplitude of harmonic decreases rapidly with the increase of harmonic order. The sixth harmonic only accounts for a very small part of the total harmonic.”