[volt-nuts] Small capacitance

Brooke Clarke brooke at pacific.net
Fri Jan 13 17:46:51 UTC 2012


Hi Fred:

Measuring components is very different from measuring things with connectors.
In order to get good answers you need to account for stray impedance as well as compensate for things like cable length.

One way to eliminate stray capacitance is to use guarding, for example see:
http://www.prc68.com/I/HP4274_4275_LCR.shtml#G

For the frequency range where they are available the four terminal pair RLC bridges are by far the most accurate method 
of measuring impedance.
For more on that and a treasure trove of info on imedance measurements get a copy of the free HP Impedance Measurement 
Handbook and the associated Accessories Selection Guide for Impedance Measurements.  Links to those at:
http://www.prc68.com/I/Z.shtml

For really accurate impedance measurements it's best to use an "analyzer" (sweep frequency test with graphic display) as 
opposed to a "meter" that has a numeric display. The better impedance analyzers have the ability to fit a number of 
models to the measured complex impedance vs frequency plot and give you the circuit values.

For semiconductors their is the HP C-V meter that uses a current source to charge a capacitiance and measures the 
voltage.  This data plotted over time gives the semiconductor doping profile.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html


Fred Schneider wrote:
> 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.
>>
>> _______________________________________________
>> volt-nuts mailing list -- volt-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
>> and follow the instructions there.
> _______________________________________________
> volt-nuts mailing list -- volt-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.
>
>



More information about the volt-nuts mailing list