[sdiy] Expo VCOs: some thoughts

[Tim Ressel]

100 x 109mm


On 5/21/2018 7:21 AM, Tom Wiltshire wrote:
> 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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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com