[volt-nuts] Small capacitance

[Ivan Cousins]

  Fred:
You can measure the capacitance of a series connected capacitor with 
high resolution.
Have a look a manual for the Keithley 590 CV Analyzer.
The concept is explained in the manual. See the diagram(s) and text.
I regularly measure Schottky diode junction capacitance with sub fFarad 
resolution.
The analogy is to use a VNA to measure a capacitor in the series mode 
(S21) rather than in shunt mode (S11). By measuring a DUT in a series 
measurement mode the full dynamic range of the receiver can be used. For 
measuring impedances that are less than 50 ohms, measure the DUT in 
shunt mode. For measuring impedances that are greater than 50 ohms, 
measure the DUT in series mode.
This concept is also explained, (by looking at the equations and 
diagram(s)) in the HP4275A LCR Meter.
Some concepts can be used in many measurement instruments, i.e. a CV 
Analyzer, LCR Meters, and VNAs.

Some of the early NBS documents showed how to build a ring guarded, 
circular, parallel plate capacitor.
I do not have a reference handy at the moment.
A Google Scholar search with the right set of key words should turn up 
something.

PS The Keithley 590 CV Analyzer is set to measure at 100KHz or 1MHz.
I have used the Keithley 590 , HP4275A and VNAs for the last  decade (or 
more).
Some concepts work in all three instruments.

Ivan Cousins