I've done some measurements of the THD of the SDG2042X using a QA401 audio analyzer to see how it stacks up to the datasheet:
Total Harmonic Distortion: 0.075% @ 0 dBm, 10 Hz ~ 20 kHz
These are the raw numbers from my measurements:
And in a graph to see the trend more easily:
Here's what the spectrum looks like for 20Hz @ 1V RMS:
And for 1kHz @ 0dBm/0.224V RMS:
And 1kHz @ 7V RMS:
And lastly 20kHz @ 3.6V RMS:
You can see 2 channels on these images, and the left (yellow) channel is the SDG2042X, and the right (red) channel is the signal coming from the built in generator of the QA401, for comparison. The SDG can go up to 7V RMS, while the built in generator will go almost to 2V RMS, so in some of the screenshots the voltage levels between the channels will be different.
The SDG does better than the datasheet specifies in all cases. It does quite well across the audio frequency range, with 20Hz being slightly worse than other frequencies. Above 20Hz you get around 0.004% and 0.005% THD up to about 3.5V RMS before it steeply climbs to around 0.03% and 0.04%.
I found it fun looking into this. I didn't think it would do so well, since I've been hearing how it's useless for audio. It's useless for measuring modern DACs and preamps of course, but it will be good enough for many power amplifiers. Low distortion, but not ultra low distortion.