[volt-nuts] Small capacitance

J. Forster jfor at quikus.com
Fri Jan 13 17:12:23 UTC 2012


IMO, the best caopacitance bridge is the GenRad (GR) 1615, along with it's
generator and detector. It easily can measure to 10E-15.

-John

===============


> 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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