[volt-nuts] Small capacitance

[ed breya]

How accurate does it need to be? Does it need to be "exactly" 1 pF, 
or do you want it near there but accurately known? Stable mica caps 
in the 1 pF range are fairly common. It's hard to get close initial 
tolerance in that range, but if you measure one on an accurate bridge 
or LC meter, you can then have a known reference as good as the 
measurement, as long as you can replicate the physical conditions. 
The C variation in the connections can easily exceed the device 
value, so the measurement system should be a 4-wire type. If it needs 
to be an "exact" value, an air-variable cap in series with a mica 
could be set where you want, but the physical variation issue would 
be even more critical.

Ed

At 08:23 AM 1/13/2012, Fred PA4TIM 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 got some standard caps in the range 100-1000 pF but i want to be 
>able to measure it in fF.