[sdiy] Measuring THD of a sine wave

[John Speth]

On 11/11/2022 4:39 PM, Tiago Ventura via Synth-diy wrote:
> The technical literature in general, *whenever* it bumps into the
> topic of sine wave generation techniques the difficulty of generating
> such a signal is always mentioned. And then, several techniques and
> circuits are presented and analyzed. The THD% of these circuits are
> mentioned for comparison purposes.
>
> How the heck are these distortion levels measured or calculated in practice?
> What are the main techniques and procedures applied in order to
> measure the total harmonic distortion (THD%) of a given sine wave?
>
> The idea of using a second sine wave as a comparative reference of
> what is "pure" is pretty obvious, but what kind of source would be
> able to present an acceptable "purity" for this purpose? (a digital
> source sounds ridiculous for this but I can't think about anything
> else!)
>
> Remembering that it is essential to also calculate the accuracy of
> this measurement. Are there other alternative techniques for making
> such a measurement? Is there anything available in the technical
> literature exploring this subject?
>
> Any thoughts/comments/suggestions would be greatly appreciated.

I don't know how THD measurements are done in lab equipment. However, as 
the measurement of total harmonic distortion specifically mentions 
harmonics, I suppose the sensible way to measure it is to measure the 
amplitudes and phases of all output harmonics and sum them using some 
unknown (by me) formula that yields THD. I believe THD assumes a pure 
sine input (typically 1 kHz; 0% THD by definition). Such a pure signal 
is never perfect in practice but considered good enough in equipment 
that generates THD input. Quantifying output harmonic characteristics 
shouldn't be too hard. Spectrum analyzers have been doing it for many years.

It doesn't matter if it's analog or digital sine generation We can 
eliminate most of the input distortion using appropriate filters and 
generation techniques and/or higher sample rates (if digital).

You might be able to find the info you seek if you comb through the 
Audio Precision web site. Here's a quote I found about THD after a 
couple of minutes of searching it: "THD refers to the sum of all the 
harmonic distortion products in a measurement."

JJS