Just as a matter of interest, the CEM3340 can be adapted to through
zero linear FM operation, see this link to Jurgen Haible's dual VCO
design. http://www.oldcrows.net/~jhaible/tonline_stuff/hj2vco.gif

Cheers,
Allan



--- In Doepfer_a100@yahoogroups.com , "Tim Stinchcombe"
<timothy@t...> wrote:

> Hi Florian, Matthew,
>
> > > > Here is a little something on the topic of thru-zero FM.
> > > > http://www.uni-bonn.de/~uzs159/vco6.html
> >
> > > This looks like the Curtis CEM 3340 is a thru-zero VCO too. Am

I

> > > wrong or right
>
> Unfortunately it is not. Oscillators like the CEM3340 and the A-

110

> require a 'standing current', positive for the 3340, negative for

the

> A-110. For normal 1V/octave (exponential) use this is constant,

and

> passes through an exponential converter (controlled by the freq

CV)

> and thence charges a capacitor. The capacitor is discharged

suddenly

> in the A-110, hence it's a sawtooth core; in the CEM3340, current
> mirrors reverse the current and discharge the cap at the same rate

as

> it was charged, hence a triangle core. For linear FM control of

both

> of these, the linear FM input simply adds or subtracts current

from

> the standing current, by an amount linearly proportional to the
> applied voltage: if this voltage becomes of such a size so as to
> reduce the (magnitude of the) standing current to zero, the
> oscillator simply stops, as there is nothing to charge the cap! To
> prevent this happening generally requires quite a lot of trickery

to

> reverse everything at the appropriate moment, hence all the

switches

> and comparator in the Schmitz design.
>
> Yet another thing on my list to look at once I finish my studies

and

> have the time!
>
> Tim