[sdiy] OTA VCO bad or not!

[Jay Schwichtenberg]

All,

I've built my own version of the OTA VCO starting with a circuit from the LM
13700 data sheet using LM13700s and going from there. I added an PNP expo
converter from the Electronotes VCO. There are three things that I did to
tweak the circuit. First on the schmitt trigger output put some limiting on
the output, I used 1N914 to limit it to about +/- 0.6V. The signal coming
out of there is not equal so this levels it out some. Second I put in the
trim on the integrator. At a low frequency (where the synetry error is more
noticeable) I trim the offset to get a symmetric triangle out. This clears
the symmetry up pretty good. You might get away with doing just this and not
limiting with the diodes. I found that the amplitude is slightly frequency
dependent so the diodes helped that. The last thing I did was hook up Iabc
on the integrator. My thought there is that helps the temperature stability
of the OTA and thus makes the VCO better. Haven't taken the time to test
that out. I'm working on this on and off and it's still on the bread board.

Good soldering.
Jay S.


*> -----Original Message-----
*> From: owner-synth-diy at dropmix.xs4all.nl
*> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of mark verbos
*> Sent: Monday, August 21, 2006 12:34 AM
*> To: karl dalen
*> Cc: synth-diy at dropmix.xs4all.nl
*> Subject: Re: [sdiy] OTA VCO bad or not!
*>
*>
*> The triangles sometimes come out asymmetrical. The problem I have found
*> is no way to trim them. If you build a discrete OTA, like the Buchla
*> ones, you can match the transistors and put a trim at the top. Plus, if
*> you use discrete transistors, you can use an NPN expo converter.
*>
*> Mark
*>
*>