[sdiy] Expo VCOs: some thoughts

[Tom Wiltshire]

Absolutely agree. AS3340 is an obvious choice. Be aware it likes 51K rather than 10K on its pulse output, otherwise it’s as per the CEM datasheet.

When you say “cram four on a board”, the obvious question is “how big is the board?”… ;)

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 21 May 2018, at 10:06, paula at synth.net wrote:
> 
> 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."
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