Hello....Tim, that has got to be the most elegant explanation of the CEM3340 that I
have ever read. Thank you for taking the time to type that.

Kind Regards

Timothy Adams





--- Tim Stinchcombe < timothy@... > 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
>
>

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