[sdiy] Measuring THD of a sine wave

[Donald Tillman]

It's actually pretty straightforward:

1. Put the sine wave through a notch filter tuned to the same frequency.
2. Look at resulting squiggle on a scope.
3. Assure that the notch filter is doing its job.
4. Fine tune the frequency to completely null out the fundamental.
5. Measure the amplitude of the resulting squiggle.
6. And report that as a percentage of the original sine amplitude.

By "Notch Filter" I mean a filter that can fully null out a particular frequency to zero.  And of course doesn't add any distortion on its own.

  -- Don
--
Donald Tillman, Palo Alto, California
https://www.till.com

> On Nov 11, 2022, at 4:39 PM, Tiago Ventura via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Greetings folks!
> 
> 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.
> 
> Tiago Ventura
> 
> Greetings from Porto Alegre/Brazil
> -30.03376474999071, -51.238242916392814
> https://www.instagram.com/psychoincorp/
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