[sdiy] Expo VCOs: some thoughts

[Michael E Caloroso]

The CEM3340 is a triangle core VCO.

There were a couple of CEM newsletters that someone had scanned and
put online, one of them explained the 3340 operation pretty well.
Anyone have the URL?

MC

On 5/20/18, David G Dixon <dixon at mail.ubc.ca> wrote:
> 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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