[sdiy] Expo VCOs: some thoughts

[David G Dixon]

My "Dixie" design will evidently track over 10 octaves with patient
calibration.  It uses a self-temperature-compensated 2164 VCA as the expo
converter, and also includes a 2164-based high-frequency-compensation
circuit which almost perfectly compensates for the tracking error in the
core.  You could cram four on one board, but that wouldn't include any of
the shapers -- it would give you triangle and square only, without linear
FM. 

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Tim Ressel
> Sent: Sunday, May 20, 2018 11:59 AM
> To: SYNTH DIY
> Subject: [sdiy] Expo VCOs: some thoughts
> 
> Happy Sunday! And by Sunday I mean the sun is out today...
> 
> I've been thinking about volt/octave VCO performance. My 
> customer wants a triangle core VCO with excellent tracking. 
> And of course he wants it for $0.25, but that's another 
> story. I started looking at expo VCO design from the 
> standpoint of tracking. Typically there are two pieces to an 
> expo VCO: the expo converter and the VCO core, and they can 
> be looked at separately. I'm going to tackle the core side 
> because that is where the problems are with triangle cores.
> 
> I looked at two designs: LM13700 and 4046. The focus is on 
> linearity between the control current and the resulting 
> frequency. There are other issues, but I feel linearity is 
> the biggest. In an ideal core the linearity would be perfect, 
> but as I looked at the datasheets it became clear these were 
> not ideal. Both the 4046 and the 13700 are not so great for 
> linearity. Don't get me wrong; they are okay devices. It just 
> that our performance needs are beyond their capabilities. 
> It's not their fault.
> 
> So now I am looking at a switched current mirror type design 
> similar to that found in the venerable 566. I believe the 
> non-linearities will be reduced to opamp, capacitor, and 
> comparator issues. That's all, just that ;-)
> 
> My customer also needs a small footprint as we are trying to 
> cram 4 of these on one board. As a great boss once told me: 
> "Size, price,
> performance: Pick any two."
> 
> 
> --
> --Tim Ressel
> Circuit Abbey
> timr at circuitabbey.com
> 
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