[sdiy] Sine VCO - tri to sine converter

[Czech Martin]

After some experience even 0.1% THD is audible.

You can use a VCF to scan the harmonics.

Indeed you could use a inexpensive Wien bridge 
with lamp regulator to get a decent sine wave.
If both osc. have the same frequency, you could compare
the audio. 

OTOH: if you have a good  soundcard (I mean: a real good one),
why don't you digitize and run a FFT?
Sadly enough, most sound cards will produce enough artefacts
to make the measurement impossible.

In the old days a twin T notch was used to eliminate
the fundamental. You can look at the residual wave
on the scope. A multimode VCF could do that, too.


m.c.

-----Original Message-----
From: Scott Gravenhorst [mailto:music.maker at gte.net]
Sent: Freitag, 28. März 2003 15:21
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Sine VCO - tri to sine converter


In the absence of expensive equipment, how can one determine how much
distortion (deviation from a true sine wave) a circuit produces?

I have used a simple tri to sine(like) circuit based on a 4069 inverter
running in linear mode to produce what looks like a sine wave on an Oscope. 
And to my ears, it sounds like a sine wave.  However, I know that neither
eyeballing nor earballing is terribly accurate.  I suppose that if I had a
really good tri to sine converter, and if they would stay in phase, I could
run both from the same tri source and use my oscope to subtract, then measure
what's left to get a rough idea.  But alas, I have no known "good tri to
sine" circuit either.  It's no big deal, I'm just curious as to how bad my
converter is.

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