[sdiy] VCO & EXPO experiments

[patchell]

    I was doing a bit of experimenting with VCO's and exponential
converters today...as always, I am in pursuit of a stable exponential
converter.

    What got me started on this round was a post a month or so ago when
there was a discussion about low noise, low distortion VCA's.  Somebody
posted a link to a VCA that was made by using both OTA's in an
LM13600/700 type dual OTA.  One OTA was used to linearize the other.
(Sorry, I don't remember the link, I never book marked it).

    When I saw the circuit, I was not thinking about controlling audio
levels, but rather, using it as the control element in an active temp
compensation for an expo converter.  I was already do this using a
different circuit, but, it had a higher parts count (CA3046, 1/2 of an
LM13700, LM336) vs just the LM13700 for the VCA circuit.

    Here are some preliminary results.  The VCO core is the "ASM-1" VCO
(the T.M. VCO).  Measurments were made with a Conn Strobe Tuner and a
Fluke 8050A DVM.  The scale was set for 1V/oct.

    From C1->C8, the error was +/- 2 cents, C9 was -10cents (flat) and
C10 was -50cents.  Oh yeah, the expo converter itself was made from a
CA3046.

    I so far have only taken one run to measure scale drift.  It came
out to be -700ppM/C, not too impresive.  I am going to try to tweak in
the temp compensation to see if I can improve it.

    As soon as I get the schematic drawn up, I will post it.

--
 -Jim
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