On Wed, 12 Jun 2002, Jim wrote:

>   "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!

My imagination roams way more than I really want to let it and I can back
my sayings with tens or hundreds of hours in the lab. I have researched
variable capacitor position and angle sensing thoroughly enough to be able
to say that it works iff you find a way to reduce the sensing input
capacitance 10 to 20 times (from what is a normal IC input cap. of 5pF).
It turns out that this is VERY HARD to do lacking specialised parts. A
nuvistor in electrometer configuration will do this for example. Certain
SMD FETs are acceptable but the majority of amateur parts don't cut it,
even if the dodgy layout practices seen on the net would not play a role
...

I have also researched RF bridges with both coils and caps, also half
bridges and single ended devices. This is more promising and I've had some
good results.

For example none of the field mill schemes shown here used a driven shield
on the input to reduce shunt capacitance. Why ? I will not elaborate on
this.

Peter

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