[sdiy] VCO - sine output - why bother?

[Ian Fritz]

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