"Weeelll, with 0.82 pF * (cm^2/mm) in air you
    better come up with a"

... bridge or other means to 'self calibrate'.

This is using a PIC - correct?

I can envision several worakable scenarios - including
the use of multiple fingers cut or 'sandwiched' together
(fabricated) from dielectric material and in a crude
'bridge' fashion as well.

Do not try to be so literal sometimes Peter - let
your imagination roam!

Jim


----- Original Message -----
From: "Peter L. Peres" <plp@ACTCOM.CO.IL>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, June 12, 2002 4:00 PM
Subject: Re: [PIC]: Variable cap A/K/A "PCB cannot carry double?"


> On Wed, 12 Jun 2002, Jim wrote:
>
> >   "Using two variable capacitors"
> >
> >Dust and variable capacitors mentioned in the same
> >sentance - is this a recipe for future trouble?
>
> No, cos' he didn't mention metal (silver) oxide on the wiper. Of course he
> could be clever and use a piece of RG174 coax to connect the moving part.
> But he did not say so ...
>
> >   - dust between plates
> >   - dust in the bearings of the movable portion
> >   - wear in the metal 'bearing'
> >   - poor contact in the bearing for RF currents
> >
> >How about two fixed plates (of a cap) into which is
> >moved a dielectric - to change the capacitance.
>
> Weeelll, with 0.82 pF * (cm^2/mm) in air you better come up with a good
> and I mean *good* readout scheme to achieve 5% accuracy over room
> temperature and 10% supply voltage span imho. even with *water* (Epsilon-R
> ~= 80) as moving dielectric. Scott Dattalo could surely chip in here if he
> would not be bound by NDAs and such,
>
> >At least this way the dependence on a metallic
> >bearing also carrying RF from one cap plate in a
> >dusty environ is eliminated ...
>
> Because the oxide would prevent any contact ? ;-)
>
> Peter
>

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