[sdiy] non-mechanical switching solutions

[Pete Hartman]

I have a problem I'm trying to solve....

...and let's just assume the problem has to be solved, I can't just
redesign around it.  In a perfect world ... but that's not where this sits.

I'm working with a ladder filter, switching 3 different capacitor ranges
(Moog 904A, yes?).  But I don't have room for a multipole rotary switch of
the necessary size.  What electronic solutions might I use for this?   (and
note that while step one is to solve four poles for the 904A, step 2 will
be to solve for the 904C which has a *9 pole* switch, which hopefully can
use the same solution)

If I use an on-off-on switch, pull ups and ground connected to the common,
I have three positions giving me 01, 11, and 10 which can be decoded to 3
out of 4 positions on a mux or a decoder.  A Mux acting as a switch to put
in or out the various capacitor values....

First thought: CD4052, but that doesn't work well in practice at all.

Second thought: DG409, better characteristics, but that still doesn't work
sufficiently well.

Dig around a while for why.... aha, the on resistance for both is
significant, especially for the capacitive stages of a ladder filter.  The
DG409 has on the order of 100R - 120R.  Too much, and confirmed to be the
issue by comparing physical connections with wire to physical connections
with 100R resistors.

Third thought: MOSFETs have low on resistance....  And an initial test of
VN0104's (on resistance on the order of 3R - 5R, much better) shows
somewhat more promise.  A little concerned about on capacitance and also
whether I can get an appropriate Vgs at the top stage of the ladder (where
the source voltage will be on the order of 10V), but so far so good; may
need a MOSFET with a lower range for Vgs-on, but the VN0104 is pretty darn
good with a max of 2.4V.... that's just at the edge of the spec though, I
think, something better would be nice.  Don't want to be in the position of
having to "select" working transistors for the top stage.

So the questions for the assembled brains bigger than mine:

1) can anyone suggest a better (yet still reasonably common) mosfet with
low Vgs and low rds-on?

2) some other (electronic, not mechanical) solution that would work in this
specific instance?  Note that while the above description assumes an SPDT
switch, I can go up to 2 poles, but not beyond.  An on-on-on would also be
an option.  Toggle switch is a requirement though.

Just to repeat; hopefully this isn't necessary in this audience, but some
other fora where I occasionally ask questions, I often get offered answers
that ignore the constraints I'm trying to fit within (in this case, no
mechanical solution, redesigning to use a physical rotary switch is not an
option).  "Your constraints are stupid" is not a helpful suggestion, yet
you might be surprised how often it comes up :).

Thanks...

Pete
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