Simple pure sine-wave test source...

[Nick]

There are several free bits of software that have very good low THD sig gen's in them like Audacity, REW, Alta and Soundcard Scope. The latter is a easy to use and is very useful as it has a scope, FFT + THD measurement, etc. Some of these are quite complex and take a bit of sussing out, but you don't need to get your soldering iron out.

Essentially, all of these are the software side of a signal generator with the sound card being the DAC, so the THD is little to do with the software and a LOT to do with the quality of your soundcard - genuine 24-bt sound cards giving you >= 100 dBc are VERY expensive. Most are now USB-based - more complexity.

Again though, these are not the point - for a clean sine wave you just don't need all the gupmp - the software may be free, but the PC, operating system & hi-res soundcard are definitely not.

PCs also generate a whole bunch of noise, none of which I want in a measuring system, though I do use PC/USB tools for some of my testing.

i.e., none of this is simple.