[sdiy] Measuring THD of a sine wave

[Neil Johnson]

Hi,

> 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.

Well, it's not just sine generation but anything really to do with
audio will have some THD and noise measurements to show how wonderful
it is.
In general what is usually measured, certainly in the analogue domain,
is THD+N due to the difficulty of separating noise from signal.
A good reference for all things audio-measurement is this tome from
Audio Precision:

https://www.ap.com/download/the-audio-measurement-handbook-2/

> 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?

I would rephrase Don's steps:
1. Measure total signal with precision true RMS AC voltmeter (= Vtotal)
2. Using an excellent notch filter remove the fundamental
3. Measure the residue with same RMS AC voltmeter (= Vresidue)
4. Calculate THD+N = Vresidue/Vtotal (either as percentage or dB)

With careful analogue design you can measure down to very low levels
of THD+N.  My AP System One's self-test level is around 0.0003% THD+N.
Note that that is of both the generator and the analyser.

In the AP System One the sine wave is generated by a state variable
oscillator with very careful control of amplitude, and a patented
transformer-isolated output driver (getting that level of distortion
from a transformer requires very careful design and rather neat trick
to bring the transformer into the feedback loop).

> 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?

Sign up to AP's website and you can download the System One service
manual, which has full schematics.

In more general terms, google for Bruce Hofer from Audio Precision -
he wrote up a few presentations and journal articles on low noise and
distortion audio design.

Neil