thru zero VCOs (was: Re: [sdiy] Temperature Compensated VCO attempt - help?

[jhaible at debitel.net]

>    > Don:
>    > > I'm really curious how the thru-zero VCO works?  (Perhaps I may have
>    > > to break down and finally buy a copy of Electronotes.)
>    > 
>    > Basiclly, start of with a triangle oscillator core and when you
>    > go though zero you reverse direction by chaning load current
>    > direction. This can be done in several different ways, but for a
>    > triangle oscillator core you allready have an OTA to do the
>    > current direction anyway, so that part comes for free.
> 
> Hey Magnus,
> 
> Sure, but I'm always interested in innovative ways to actually
> implement this.  I have yet to see one that goes through DC real
> gracefully.

Speaking of the VCO in my FS-1, this is *not* optimized for fast
transitions thru DC. It's optimized for precise, but slow transitions;
using a slow, low-offset opamp.
(Magnus: even when the actual current switching n the VCO core 
comes for free, and works fast, you also need a precise and ultra-fast
circuit that gests the switching signal and a full wave rectified
signal from your audio rate modulation signal, which isn't trivial.)

And while I don't like a BFO concept for a frequency shifter oscillator
(locking and/or drift issues), it would be an excellent choice for
audio rate thru-zero FM, wouldn't it?
(Don: Have you tried the Moog/Bode FS configuration - just the BFO part,
and simplified, as you don't even need quadrature signals - for a
thru-zero VCO?)

JH.

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