[sdiy] Measuring THD of a sine wave

[Paul Perry]

THD% isn't the whole of it. For example, a very thin spike at the peaks may
not add up to much in THD terms, but be very obtrusive indeed in some
applications (eg LFO). Conversely, it's surprising how much THD there can
be sometimes with (to me) little or no audible effect.

paul "cloth ears" perry Melbourne Australia

On Sat, Nov 12, 2022 at 11:59 AM Matthew Arthur via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Very sharp q filters tuned to the same frequency, then you measure the
> amplitude of what's left and compare it to the total amplitude.
>
> I believe the manual for the hp 333a distortion analyzer has a detailed
> description of it's principles of operation
>
>
> On Fri, Nov 11, 2022, 7:44 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
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