[volt-nuts] Small capacitance

[Fred Schneider]

I am measuring small capacitances, just for sport but that is more difficult as I thought. 
I need something that gives a known capacitance around 1 pF. 
I cut several pieces FR4 in different sizes and measured them several ways, but the problem is the dielectric constant if I use K = 4.5 and distance 1.33 ( it is 1.35 thick form outside copper to outside copper so the 1.33 is also a guess) i get calculated results that are in line with my measurements. Not the same values but the same ratios.

I used a digital VNA in shunt mode, a TF1717 bridge from Marconi, the frequency shift methode as described by F.E. terman in RF measurements, a modern LCR meter ( not an expansive very good one) a function generator at 50 KHz and thn measuring the current through the capacitor, an O-opamp plugin with a setup i made to measure small currents ( delta V and delta t are constants so i measure delta i) and the last something call a Capacitance-Frequentie converter i designed, a constant DC current, a constant delta voltage integrator and comprator) and as a result a changing delta T, so frequence is related to capacitance.
All measurements are close but not enough ;-) 

so I need a sturdy standard capacitance. Any suggestions, something using air will be best I think but two metal plates should be straight and mounted solid opposite. I used aluminium but forgot the dielectric constant of the oxide so it is not just air. Only if I use K=1.41 I am close. Two seperate peases of pcb ? And then there is the edge effect ect. Most formulas I find are aproximations.

I got some standard caps in the range 100-1000 pF but i want to be able to measure it in fF.

Fred PA4TIM





Fred PA4TIM

Op 13 jan. 2012 om 15:04 heeft "Poul-Henning Kamp" <phk at phk.freebsd.dk> het volgende geschreven:

> In message <CAE6XXrhydntKXjzq6W8ZA46Fs048CD-Wvrcbotz7kciGd57j5Q at mail.gmail.com>
> , Will writes:
> 
>> A Peltier element
>> is almost as easy to drive as a heater resistor, but dissipated heat
>> probably makes the thermal design much more challenging.
> 
> Actually the major trouble with Peltier is controlling them, because
> they are asymetric with respect to transport direction.
> 
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
> 
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