more VCO cores

[Ian Fritz]

Juergen --

I'm sorry to say the answer is no, I just hacked at the circuit until I got
it to track right. (So much for "celebrity"!) I don't even remember how I
got the idea of varying the OTA drive level. I just wrote the effect off as
some non-ideal behavior that I would never understand (especially since I
could barely understand the ideal behavior).

On a related subject, at one point in time I discovered that all of the expo
converters I had built had small high frequency oscillations. (See EN #98,
p. 15.) Terry recently found a similar effect when he revisited his VCO
design. This effect causes the tracking to go sharp at the high end also. I
strongly suggesting checking carefully for this problem anytime tracking is
a problem. My present solution is to use a BB OPA132 for the op-amp in the
converter. This amp is optimized for feedback-loop applications, and greatly
reduces the amout of external compensation needed to kill the HF
oscillations.

BTW, I also invented a VCO core that I believe may be unique, although I
never took the time to develop it. A very brief description is in EN# 111,
p.19.

  Ian



----- Original Message -----
From: "Haible Juergen" <Juergen.Haible at nbgm.siemens.de>
To: <ijfritz at earthlink.net>; "DIY" <synth-diy at mailhost.bpa.nl>
Sent: Wednesday, February 16, 2000 7:51 AM
Subject: more VCO cores


> Speaking of EN celebrities and VCO cores:
> Yesterday I ran across a 3080-based triangle HF-VCO
> from Ian Fritz again (Used to clock some ROM tables etc.).
>
> There was this note about using exactly the right
> input voltage step for the 3080, although the OTA is just
> used as a current *switch* rather than a linear multiplier.
> Too high or too low voltage would cause tracking nonlinearities.
>
> I'm asking because I've run into the very same trap myself
> (sort of reinventing a "broken wheel"): I thought the best
> would be to overdrive the OTA input in order to avoid temperature
> dependence of the output current. But I've encountered the
> same tracking problems. I did not find a solution.
> Ian *has* found a solution, but the article says he didn't exactly
> know *why* a certain level works and others do not.
>
> Is the mystery solved meanwhile ?
>
> JH.
>