[sdiy] Expo VCOs: some thoughts

[paula at synth.net]

Why not go for the AS3340?

Paula

On 2018-05-20 19:58, Tim Ressel wrote:
> 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."