[sdiy] dual vco

[David G. Dixon]

Ian,

I was going to do this when I built my first Thomas Henry VCOs, as it would
have made a very symmetric and convenient PCB layout, and I did do a double
VCO on breadboard at the time.  However, I was warned off in no uncertain
terms by Harry Bissell, Jurgen Haible, and perhaps even yourself, as I
recall, so I relented and ended up using the spare 13700 for a sine shaper.

Just say no to dual VCOs!

Having said that, it's probably at least partly due to the breadboard,
UNLESS the two circuits are on opposite sides of the "corpus callosum" down
the middle of the board (in mirror-image fashion), in which case I would
suspect that the cross-capacitance is vanishingly small, UNLESS your board
is screwed down to one of those metal plates (you know, the kind with the
three banana plugs screwed into it).  If you want to eliminate breadboard
stray capacitance issues across the divide, then arrange your two VCO
circuits in mirror-image fashion (as I presume you've already done) and
unscrew the board from any supporting plates.  If the two VCOs still sync
each other, and you've added plenty of decoupling to the rails, then it is
probably due to PS coupling within the the dual/quad chips themselves, and
that's that.

Does that make sense?

> Has anyone made a pair of VCOs using dual chips?  I have one running using
> LM13700, TL072, CD4013, etc.  The two oscillators sync up a bit, and I'm
> not sure if this intrinsic or due to still being on a whiteboard.