[volt-nuts] Small capacitance

Robert Atkinson robert8rpi at yahoo.co.uk
Fri Jan 13 21:05:07 UTC 2012


Hi Fred,
I'd go for a coaxial (or Triaxial with a outer screen) air dielectric design. Use largish diameter alloy tubes with a big gap. Adjust the length to suit the tube diameters. The large gap will make the effect of the oxide layer and supports (a thin plastic disc at each end). Connection strays are another issue.


Robert G8RPI.


________________________________
 From: Fred Schneider <pa4tim at gmail.com>
To: Discussion of precise voltage measurement <volt-nuts at febo.com> 
Sent: Friday, 13 January 2012, 16:23
Subject: [volt-nuts] Small capacitance
 
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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