[sdiy] non-mechanical switching solutions

[Rick Jansen]

Miniature reed switches? Sort of mechanical, but hey.

rick

> On 27 Jun 2016, at 16:15, Pete Hartman <pete.hartman at gmail.com> wrote:
> 
> 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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