[sdiy] Temperature Compensated VCO attempt - help?

[Don Tillman]

   > Date: Sun, 02 Feb 2003 18:44:30 +0100 (CET)
   > Cc: sbernardi at attbi.com, synth-diy at dropmix.xs4all.nl
   > From: Magnus Danielson <cfmd at swipnet.se>
   > 
   > Don:
   > > I'm really curious how the thru-zero VCO works?  (Perhaps I may have
   > > to break down and finally buy a copy of Electronotes.)
   > 
   > Basiclly, start of with a triangle oscillator core and when you
   > go though zero you reverse direction by chaning load current
   > direction. This can be done in several different ways, but for a
   > triangle oscillator core you allready have an OTA to do the
   > current direction anyway, so that part comes for free.

Hey Magnus,

Sure, but I'm always interested in innovative ways to actually
implement this.  I have yet to see one that goes through DC real
gracefully.

   > > I would think that the linearity of the OTA would be a major issue.
   > > OTA's aren't especially linear... in the sense that they make great
   > > triangle to sine converters.
   > 
   > This is why Scott and Jim used two different forms of
   > linarizations. Scott used the builtin linearization diodes where
   > as Jim used the other half of a LM13600 to do the linearization.

Yes, but what's the linearity error for these in practice? 

While temperature compensation is a good thing, to have temperature
compensation at the expense of any less VCO linearity would be a bad
thing (assuming you're using the VCO in the normal way).  Temperature
compensation is just a convenience and without it I can always tweak
the scale pot and be fine, but a nonlinear VCO means I can't play in
tune. 

Is a linearized OTA accurate enough?  I don't know.

I would think that this is an area where some alternate VCA topologies
would win.  A multiplying DAC f'rinstnace, though the digital part is
not really appropriate here.  Or that OVCE arraangement that the SSM
VCAs use; the feedback might help in this case.  Or one of those hifi
VCA topologies that are connected opposite from the typical, so the
control voltage accurately runs a current source for a diff pair, and
the gain is set by the base voltage on the diff pair.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com