[sdiy] Linear VCO FM Input Idea/Question

[Scott Gravenhorst]

Bear with me, I have only linear VCOs to work with...

I have an idea/question for an FM input to a FatMan (or
actually one of many linear) VCO:

The VCO control voltage enters the VCO at a summing node
using a negative feedback OPamp circuit (volt/current
converter).  I wondered how effective it might be to insert
a MOSFET as a voltage controlled resistor into the CV path
such that the source is connected to the virtual ground, the
drain is connected to the supplied CV so that the control
voltage signal must pass through the MOSFET D-S.  Since the
source is at ground (virtual), the MOSFET should act as a
nice voltage controlled resistor a decent reference.

Applying a modulating signal to the gate (biased some simple
way) would have a different effect on the VCO than would
just mixing the same modulating signal with the normal CV,
right?  I'm thinking it would be more useful, i.e., a
*mixed* signal would have a larger effect on the bass end
(linear VCO) than on the treble end.  But wouldn't using the
VCR method fix this so the bass and treble ends respond
similarly?  Ie, if I used this setup for pitch modulation
using an LFO, wouldn't I get a similar amount of wiggle at
the low end as the high?  And if this is to work, are there
any comments on what would be a good bias point for the
gate?  Should gate bias be a panel pot?  The gate will be AC
coupled to it's modulating source.

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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