🚀 go-pugleaf

From article <1992Dec1.202151.23062@das.harvard.edu>, by squish@endor.uucp (Shishin Yamada):
> Hello all,
> 	I am involved with a project that has at it's centerpiece a
> very small device with a changing capacitance that we would like to
> measure. The chief problems have been signal noise and other unknown
> capacitances. I was wondering if anyone had any suggestions or further
> ideas for improvement.
> 
...............
> 
> shishin "squish" yamada
> squish@endor.harvard.edu
> division of applied sciences
> harvard university

Long ago I built a system to measure insect activity that essentially
measured capacitance variation. It was VERY sensitive, and worked OK
even with lots of parallel capacitance. I used the capacitance as
part of an oscillator (running at about 4.5 MHZ) and then used the 
output of an FM discriminator, appropriately amplified, to provide
the output. It had a long time-constant feedback to a varactor in the 
oscillator to reduce long-term drift.
We later tried it as a burglar alarm. It worked great there too.
I used a TV IC as the discriminator (that's why the freq. was 4.5 MHz).
I don't remember any more details.
This is great as long as you are looking for changes. It might
not be so good at measuring absolute values.

george
-- 
*  I don't speak for AT&T.       *    George Kinder   gwk@cblpe.ATT.COM  *
*  AT&T doesn't speak for me.    *    AT&T Bell Laboratories             *
*  We both like it this way.     *    Columbus, Ohio       614-860-2238  *