[sdiy] VCO Linear Voltage

[David G. Dixon]

> >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.

My apologies, Ian, but I've got the 2164 Expo VCO on the brain (as you are
only too painfully aware).  I wasn't thinking about those "other" VCOs with
conventional expo converters based on matched pairs.  The 2164 can process
currents of either polarity.