[sdiy] Jürgen Haible living VCO core

[Neil Johnson]

Hi Liam,

> OK, great, I think I get this. Also "Franco Compensation" yields some
> google results for further reading.

Somewhere out there is his PhD thesis where he introduces the idea.
Worth hunting for it (try the web archive).

> > Think about what could happen when power is first applied to the op-amp.
>
> I guess it could spike negative momentarily, so this limits it to
> around -0.7V, and is protection for.. Q3?

Yes indeed.  While the power rails are coming up the internals of an
opamp are not under control, so it is possible for the outputs to
briefly go anywhere.  I suspect this is to stop the oscillator core
locking up under certain conditions.  A common place for such a diode
is in the widely used two-transistor expo converter, but many folks
forget to put the diode in, and then over time the brief reverse-bias
conditions degrade the exponential conformity of the transistors.  Not
an issue if you're just tinkering on the bench, but for a commercial
product it's just lazy/bad design.

Cheers,
Neil