SV: Re: [sdiy] Expo converters, lin log etc.

[Magnus Danielson]

From: Karl Ekdahl <elektrodwarf at yahoo.se>
Subject: SV: Re: [sdiy] Expo converters, lin log etc.
Date: Mon, 5 Feb 2007 01:02:13 +0100 (CET)
Message-ID: <181028.12936.qm at web26203.mail.ukl.yahoo.com>

> Hi all
> 
> > If you don't want to play around with a 4046 for
> > this, it becomes a bit messy.
> > The "normal" sawtooth method (see for instance my
> > ASM-1 schematics as well as
> > say MiniMoog schematics) is simpler. You ought to
> > learn yourself that one.
> > The "normal" triangle variant (see my Buchla schemas
> > as well as the Bergfotron)
> > is a variant of the sawtooth with a slightly more
> > complex twist, but not an
> > essentially different solution. The 4046 solutions
> > is an implementation variant
> > of these.
> 
> That's pretty much what i realized, no need to do it
> overly complicated to start with.

Indeed.

> Instead the classic "cap charging through a resistor
> and threshold mechanism that suddenly discharges the
> cap". Any core like that that has a fast enough
> discharge time (so that it's neglible) should be.. 
> ..exponential or linear in respect to current and/or
> voltage? It's practically just a RC constant right?

The CCO core is always linear. It is the expo transistors which makes it
go exponential in responce. It is trivial to convert an exponential VCO into a
linear VCO. Drop the expo-transistors, choose the right resistor-value from
the CV summer into the CCO core part and you are done.

The CCO core frequency depends on charging current, comparator voltage (i.e.
the peak-to-peak voltage) and capacitor value. If you look into my ASM-1 page
you find the math there. Not *THAT* complex math actually. I'm sure you can
manage.

Just read the VCO theory section and contemplate over it for a while. It is
really just a creative use of the capacitor formula U = Q/C, knowing that the
charge Q is Q = I*t (or rather, I integrated over time, but you don't need that
for a static analysis). The op-amp buffers up the capacitor and when the
voltage is high enought the reset-circuit shorts the cap out and thus
discharge it and the whole process starts again. The resistor in series with
the cap helps to cancel the reset-time, but that is a technical refinement
you can skip for the moment.

I've had my ASM-1 VCO go to 160 kHz and ranging over 21 octaves. That _should_
do it for most usages.

Cheers,
Magnus