[sdiy] VCO - sine output - why bother?

[David G Dixon]

I just checked out your schemo.  Very cool, Ian!  I don't know why I didn't
come across that before when I was looking for quadrature oscillators.  I'm
going to try it with 2164.  I'll let y'all know how it works.  If it's
successful, I'll lay it out on a PCB (through-hole, of course).

> -----Original Message-----
> From: Ian Fritz [mailto:ijfritz at comcast.net] 
> Sent: Monday, August 29, 2016 5:03 PM
> To: David G Dixon; 'Mattias Rickardsson'
> Cc: 'synthdiy diy'
> Subject: Re: [sdiy] VCO - sine output - why bother?
> 
> On 8/29/2016 12:26 PM, David G Dixon 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.
> >
> >
> Yes, that's why I abandoned the COTA approach in favor of the 
> limit-cycle approach. For the four-stage version, the waves 
> are all the same amplitude, and second and third harmonics 
> are more than 50dB down for all four stages. The limiting is 
> very gentle, as each stage is just slightly underdamped. The 
> balance is so fine that it takes ~200 cycles for the unit to 
> stabilize at power on. (But once is is stabilized its 
> frequency can be changed instantaneously.)
> 
> Ian
> 
> --
> ijfritz.byethost4.com