[sdiy] Fwd: VCO - sine output - why bother?

[Ben Bradley]

If you want to go a little more 'advanced' with a low distortion sine
wave oscillator, you can do a lot better than using zener diode
limiting for the amplitude. The idea of going through eight stages of
integrators seems overcomplicated. An AGC circuit to regulate the
level/gain will give a lot less distortion, though it may not work
well over a very wide frequency range. OTOH. since we're using lots of
VCAs (and this is just my off-the-cuff idea), you could put a tracking
VCA/integrator in the AGC loop filter so a lower frequency sinewave
has a lower frequency AGC loop controlling it.

I'm on diyaudio where there's a huge thread on the topic.
http://www.diyaudio.com/forums/equipment-tools/205304-low-distortion-audio-range-oscillator.html

On Mon, Aug 29, 2016 at 2:26 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
>
>
>> -----Original Message-----
>> From: Synth-diy [mailto:synth-diy-bounces at dropmix.xs4all.nl]
>> On Behalf Of Mattias Rickardsson
>> Sent: Monday, August 29, 2016 2:26 AM
>> To: Ian Fritz
>> Cc: synthdiy diy
>> Subject: Re: [sdiy] VCO - sine output - why bother?
>>
>> On 28 August 2016 at 23:46, David G Dixon <dixon at mail.ubc.ca> wrote:
>> > I make sine wave generators out of stripped-down four-pole COTA
>> > filters with fixed feedback gain.  It's the easiest and
>> cheapest way
>> > I've found to make sine waves of uniform amplitude and low
>> distortion.
>> > With 2164, the tracking is decent, too.
>>
>> On 29 August 2016 at 00:37, Ian Fritz <ijfritz at comcast.net> wrote:
>> > Why use four integrators when you only need two?
>> >
>> > http://electro-music.com/forum/topic-19841.html&postorder=asc
>>
>> Probably because he wants 45 degrees and 135 degrees as well. :-)
>>
>> /mr
>
> Yes, this is true, but the sine waves need to be limited.  I use
> back-to-back zeners off the feedback loop for this.  I also apply 1.44 gain
> to each stage to overcome the natural 6dB attenuation.  The only problem
> with this technique is that the sines from the first couple of stages are
> not as clean as the next two.  If you want really really clean quadrature
> sines using this technique, it is best to go to 8 stages (requiring two
> 2164s) and take the 6th and 8th sines.  That gives the circuit 6
> opportunities to filter out the zener clipping.  By this method the THD can
> be well less than 0.1%.  With only 4 stages, the THD from the 4th stage sine
> is still probably around 0.2 to 0.3%.  If you want to temperature-compensate
> the CV, then you need to sacrifice two of the 2164 VCAs, and can only do 6
> stages.
>
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