[sdiy] Measuring Audio Sine Purity

[David G Dixon]

> How would a DIY person measure the purity of a sine wave 
> beyond using a spectrum analyzer?  To give a figure of sine 
> purity, of distortion (THD?) ?
> 
> Say you have an oscillator you built, it's supposed to be a 
> sine oscillator and you'd like to know just how pure it is.  
> What would you use?
> 
> I've used RMAA to analyze a wav file of a sine signal, but 
> that doesn't give purity numbers, it just gives the spectrum.  
> 
> Is there software that might be used with a good sound card? 
> (understanding, of course, that soundcard artifacts will 
> present themselves in the results)
> 
> I imagine that there is expensive hardware, but that's out of 
> the question for me.

I use the picoScope USB oscilloscope probe and software.  It has a host of
analysis tools and options, including spectral analysis and %THD.  I used it
to calibrate the sine shaper trimpots on the Intellijel Dixie.  I could
watch the second and third harmonics go up and down as I turned the screws.