[volt-nuts] Small capacitance

[Fred]

Nice Maual.
All you folks, thanks for the info. I have some inspiration. Think I
first try the coaxial approach. But the biggest problem will be to know
the value but a coaxial one is the most easy to measure using a VNA and
that will get me pretty close.
http://www.pa4tim.nl/?p=2929  some pictures and results op to now.

Fred


Brooke Clarke schreef op vr 13-01-2012 om 09:46 [-0800]:
> 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.
> >>
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