A fundamental problem of exponential VCOs?

[Joachim Verghese]

Hi again, it's been a while...

> Has anybody tried this - one exponential converter, driving two
> oscillator circuits? In other words, split (or mirror) the
> exponential current into two integrator/comparator circuits,
> and then alter some parameter to make the oscillator sections
> slightly different. Some thoughts include:  [...]

Unfortunately, altering the trip point(s) won't help, since
the control current (and thus the beat rate) will still double
for each 18 mV increase at the expo input.

I gave this problem some thought a few years back when I built
a Taurus clone. I wanted to have that characteristic constant
beat rate feature, but didn't want to use linear VCOs because
of the difficulty of generating and processing exponential CVs.
Ok, the Taurus only covers one octave at a time, but I still
wanted that feature...

One solution I thought about back then was to build a dual output
expo converter using 3/5 of a 3086, i.e. one voltage follower and
two exponentiators (instead of splitting a single control current),
and then put a 2kohm (or so) pot in place of the HF-track
compensation resistor of the second VCO to control beat rate.

Another idea was to insert an emitter resistance at one of the
exponentiators so as to alter the control scale, but the resistance
would have to be rather small.

Unfortunately, I never tried building any of the above circuits.
And I think I had some other ideas as well which I never tried.
Actually, I ended up with a totally different solution -- one
that I'm a bit embarrased to speak about around here -- ok,
<small font on> I used DCOs <small font off>. I can hear you all
boo at me now, but for this particular application it worked
out pretty well. (Timer/counter based DCOs can be made to have
quite good resolution at bass frequencies).

Anyhow, I think Juergen's idea of adding a small offset to the
control current sounds good, although we're talking about nano-
ampere currents. Another application that requires good current
mirrors, i.e. matched tranny-pairs. (Why are those MAT-0x:s so
damn expensive....).

BTW, speaking of the Taurus, for those of you who are into '70s
prog rock, the upcoming Genesis release reportedly features
some nice Taurus + 12-string work. I think I'm getting excited...
<http://www.genesis-web.com/>. Sorry about the digressions.

-joachim