[sdiy] VCO Linear Voltage

[Ian Fritz]

At 03:34 PM 11/17/2009, David G. Dixon wrote:
> > True, but that method limits you to 100% modulation.  You get a much more
> > useful range of sounds with a VCO that is capable of through-zero
> > modulation, which can go to 1000s of percent.
>
>Ian, for Through-Zero Linear FM, could it be as simple as applying a gain
>of, say, -1 < G < 1 to the reference voltage feeding the reference current?

I may not be understanding what you are asking.  If you reverse the 
direction of the reference current? ... well, you can't.  The currents in 
an expo converter only go in one direction.  IOW, if you tried that, the 
half of the pair in the opamp feedback path would be reverse biased.  And 
of course the expo transistor still can only put out current in one direction.

In my unit, the reference current is reversed by feeding it into an OTA 
control-current port with a bipolar voltage input for the 
modulation.  Bernie's method is to FWR the modulation input so it is always 
positive and to reverse the direction of the core whenever the the 
modulation signal is negative.  But in any event you have to have to add 
some kind of multiplier or switching circuit to implement TZFM.

I've gotten many questions similar to this one the past few months.  Can't 
you "simply" do blah blah blah with an existing VCO?   Sorry, but no.

Ian