[sdiy] Measuring THD of a sine wave

[Neil Johnson]

Ben Bradley wrote:
> As you may have discovered from reading some of the material offered,
> there are many ways to measure distortion. The AP equipment is surely
> great stuff (though I've never used or even seen it in real life), but
> it's expensive and likely overkill for what most of us need for
> synths.

That depends on what "expensive" and "overkill" mean to the reader.
There are cheaper alternatives, such as the QuantAsylum kit (if they
have stock) for much less than even a second-hand AP.

> Surely there's some freeware audio software that can record (like
> Audacity) a signal and/or do the filtering and calculations to get
> THD.

There surely is:
https://trueaudio.com/rta_abt1.htm
https://www.sillanumsoft.org/
https://www.roomeqwizard.com/

> What exactly are you wanting to do? Make a sine output on a VCO?
> There are triangle-to-sine converter schematics that work "moderately"
> well. If it's for a LFO, a twin-T or phase-shift sine oscillator with
> a variable resistor may be good enough.

If the OP just wants a variable frequency the Wien is pretty much the
best bang-for-buck you'll get.  For a VCLFO then I'd look into making
a sin/cos quadrature oscillator.  Both need careful attention to
amplitude control if you want a low distortion sine.

Or maybe the OP is just curious about how one goes about measuring low
levels of distortion and noise.

> Here's a long (9,900 posts) discussion of designing a low distortion
> oscillator (though fixed frequency and not voltage controlled), and
> many measurement techniques are discussed.
> https://www.diyaudio.com/community/threads/low-distortion-audio-range-oscillator.205304/

It shows how passionate folks get about something seemingly so simple!
 The popular Victor kit discussed in that thread is a Wien-based
oscillator - sometimes you can find them on eBay in various
configurations.

Neil