Thanks for the information.
According to REW's manual, "...if the 'Use AES17-2015 standard notch' option is selected the fundamental power for THD and THD+N calculations will be the power within a one octave span around the fundamental frequency. If that option is not selected it will be the power in the main lobe of the fundamental. When the fundamental level approaches the noise floor using the standard notch will produce a much higher figure than the fundamental main lobe, in those cases it is better not to use the standard notch...".
It does not seem to be good to use this option for the following reasons:
1. Octave bands do not have a fixed bandwidth. They become wider as frequency increases. In contrast, the main lobes of the fundamental and harmonics do have a fixed bandwidth for a fixed FFT size (The bandwidth is reduced as the FFT size increases). Their widths do not change with frequency. In THD calculation, it is obviously "unfair" if only the fundamental power is calculated based on one octave span around it while the harmonic powers are still calculated based on the main lobe bandwidths. On the other hand, it is also not correct if the harmonic powers are calculated based on one octave span around them because the octave span will be too wide at high frequencies (see table below).

- 1-1_OctaveBands.png (27.88 KiB) Viewed 93 times
2. It is quite understandable to use this option in THD+N calculation to mimic the hardware method, however, in THD+N vs Frequency measurement, the fundamental frequency will be stepped from 20Hz to 20kHz. Imaging how wide one octave span will be at 20kHz. Even in THD+N @1kHz measurement, one octave span would be from 707Hz to 1414Hz, which is very wide compared with the main lobe method. The noise power inside that range will all be treated falsely as the power of the fundamental.
Having said the above, in Multi-Instrument, it is possible to calculate THD+N with AES17-2015 notch filter like REW using "Energy in user defined frequency bands" function as follows:
1. Define two frequency bands, one for the total energy, the other for the fundamental energy, e.g. 20Hz~20kHz, 707Hz~1414Hz

- 1.png (25.34 KiB) Viewed 93 times
2. Define a UDDP using the following formula to calculate THD+N:
10*log10(pow(10,[fBand1RMS_A(EU)]/10)-pow(10,[fBand2RMS_A(EU)]/10))/pow(10,[fBand1RMS_A(EU)]/10)
That is: 10*log10([energy in band1] - [energy in band2])/[energy in band1])

- 2.png (16 KiB) Viewed 93 times
The following picture shows an example of THD+N measurement. The THD+N measured by the AES17-2015 notch filter method is -82.75 dB while -82.58 dB is measured by the normal method. The former is lower because the bandwidth of the notch filter at the fundamental is much wider and the resulting fundamental power contains the contribution from noises in that octave band.

- 3.png (125.39 KiB) Viewed 93 times