[time-nuts] Voltage standards

[WarrenS]

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.


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

***********************
----- Original Message ----- 
From: "Bill Hawkins" <bill at iaxs.net>
To: "'Discussion of precise time and frequency measurement'" <time-nuts at febo.com>
Sent: Sunday, November 30, 2008 1:16 PM
Subject: Re: [time-nuts] Voltage standards


> Group,
> 
> The subject elicited a fine hardware discussion, as always.
> 
> There was some casual talk about measuring fractions of
> microvolts, implying 10E-7 or -8 accuracy.
> 
> There are lots of possible thermoelectric effects that may be
> in series with the source.
> 
> Without describing how it's done, what is the lower measuring
> limit in nanovolts? That's the limit where the last digit
> moves randomly by more than one increment. Does the method
> involve cryogenics?
> 
> I'd look it up, but I expect some of you are more current
> with the science than anything I could find with Google.
> 
> Given that extreme accuracy and stability are expensive, what
> can an amateur do for less than, say, $500 or 400 Euros?
> 
> Thanks for any enlightenment.
> Bill Hawkins
> 
> 
> 
> 
> 
>