[time-nuts] Voltage standards

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Dec 1 03:02:36 UTC 2008


WarrenS wrote:
> Bill Hawkins
>
> >From a theoretical standpoint, Zero Volts is the lower noise limit 
>
> If you want to reduce the measurement noise of a system you need to do one or more of the following:
>
> Lower the source impedance, by reducing the resistance of the thing you are measuring
> Lower the Bandwidth, by filtering over a longer time period
> Lower the temperature, by making it colder.
>
> Doing even any one of these things enough will in theory let the noise approach zero, When you do two (or all three) at once the noise will approach zero sooner.
> see "Johnson-Nyquist noise" for the details
>
> >From a practical standpoint, 1 nV of resolution is doable by comparing the difference between two voltage sources if one uses a lot of care and applies some form of extra filtering. 
> A  1nV  (1e-9) is way below the noise level of any voltage standard that puts out volts.
> This means reference measurements are not limited by the noise level when using a good but simple setup until the references gets to be in the 3e-10 precision range.
>
>   
Modern voltage references are somewhat noisier than standard cells and
batteries (at zero load current).
For the adventurous, recipes for constructing stable Weston standard
cells are readily available.
> The answer to "What can an amateur do to get a good low noise reference for less than, say, $500"
> IS shop at the US eBay site.
>
> WarrenS
>
>   
Bruce



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