VCO hacks, tweaks

[Ian Fritz]

Hi Terry --

> It looks like you did a very complete evaluation of my original VCO
> circuit, and all of your upgrades make perfect sense.  The final result is
> impressive.

Thanks. I'm quite happy with how it has turned out so far. 
 
> I wonder if anyone has previously discussed upgrading the op-amp that sums
> the control voltages going into the expo convertor.  This is an area that
> can be improved by using a good op-amp.  Although the 1 volt/octave control
> voltages are divided by a factor of 55 to generate an 18 mv/octave drive
> for the expo transistor, op-amp offset voltage drift is not divided by 55,
> and goes in to the transistor almost unattenuated, so a small offset drift
> can cause large pitch changes,   I replaced the 741s in my Minimoog with
> OP07A's for this reason.  This improvement can be applied to many other VCO
> designs.

Well, I just stumbled on that one. I started with a garden-variety FET
op amp there, but discovered by putting a finger on it that there was
quite a bit of temperature sensitivity. The better op amp helped quite a
bit. Maybe an opamp optimized for offset drift would be even better. I
like to keep the slew rate up here, though, in case I want to do some
high-frequency FM.  

> You didn't mention the type of timing cap you are using.  Is it a
> polystyrene?  These have a small temperature coeficient, and might be the
> source of the 200ppm/K drift you saw.

Ah. Yes, it's polystyrene. I noticed a bit of drift when I heated it up,
but not as much as I expected. I suspect there are several small sources
of drift left in the oscillator proper. The FET is still somewhat
sensitive, for example.

> I'm glad you caught the 3300ppm/K vs. 3500ppm/K problem, I missed that one
> completely.

I'm surprised none of us ever looked at that before! It's like magic,
correcting that.

> My original design had an error that produces a high frequency, low level
> oscillation in the expo convertor.  I assume you took care of this.  I
> don't know if this has any effect on VCO drift, but it probably won't help
> it, so it's a good idea to fix it.  If there is anyone on the list who
> needs info on this, send me an e-mail.

One feature of the OPA132 is that it's carefully designed for stability
which makes it great in servo loops. The 10pF took care of a very small
oscillation. In fact, half that value works. My old notes do indicate
that I had to tame a bit of oscillation in your original design. I
didn't think much of it -- just increased the two caps a bit.

BTW, did you notice D2? It helps with the lag between the integrator
output and the comparitor input during the ramp (due to charging C2).

Best regards,

  Ian