[volt-nuts] Zener selection: was Traveling Standards -Measuring Protocol

[Andreas Jahn]

Hello Warren,

thanks for your response.
I had a look on the manual of the Fluke 845: nice part.

But: For the first evaluations of low frequence noise I will have to use my 
24Bit ADCs.
So without larger changes on my software I will have to use the 1 minute 
averaged values.
What would be better for comparison of the low frequency noise:
Summing up the absolute values of the differences between the 1 minute 
values and
calculating some average change per minute on the values.
Or: calculating some "effective noise value" on the AC-part of the reference 
voltage.

With best regards

Andreas


----- Original Message ----- 
From: "WarrenS" <warrensjmail-one at yahoo.com>
To: <volt-nuts at febo.com>
Sent: Sunday, September 25, 2011 2:01 AM
Subject: Re: [volt-nuts] Zener selection: was Traveling Standards -Measuring 
Protocol


> ws) responses below
>
> Hello Warren,
>
> to come back to your zener diode selections:
> - do you have any source where the selection of zener diodes is described?
>
> ws) No, Just what I've posted. As far as I know, Noise, Zero TC current, 
> and Long term ageing drift, all need to be selected somewhat separately.
>
> I have read on a german page that zeners are selected by
> their "noise characteristic". And that the noise will show the long term
> behaviour.
> But its not described which kind of noise has an influence.
> Is it broad-band noise? 0.1 to 10 Hz noise? or even the lower frequency
> noise (which you have recorded on the strip chart recorder).
>
> ws) Could be, but any relation between them is unknown by me.
> I select for low freq noise over minutes and hours because that is the 
> limiting factor when making a xfer reference.
> The higher freq noise is or can be filter or averaged out.
>
>
>>From the german link it is not clear wether the selection is
> according to the value of the noise or the change before
> and after pre-ageing.
>
> ws) For ageing selection, a good indication is to measure the diode's 
> voltage each week or so as they burn in.
> Without at least some pre-ageing they drift so much you can not even 
> measure their low freq noise.
>
> And the bad news is that my LM399#1 which is drifting
> much more than my LM399#2 has only about 50% of
> 0.1 to 10 Hz noise than the LM399#2.
>
> ws) That is why you start with more like ten, not two, if you want to 
> select the best of both in a single unit.
> I'd be surprised if the "Bad"  random step type noise has much correlation 
> with your measured 0.1 to 10 Hz noise.
>
>
> By the way how do you make the plots?
> Do you always record the difference of two similar devices to
> compensate for the DC-Offset?
> Has the strip chart recorder 1uV resolution or do you have
> to add a pre-amplifier? (which one?).
>
> ws) The analog strip charts I've posted are the difference between two 
> equal voltages using a Fluke 845 null meter, generally on it's 10uv range.
> I also use a digital voltmeter and log their difference when the two 
> voltages are close but not exactly the same.
>
>
> The 10 second time constant: is it built in within the recorder
> or external (low-ohmic / high ohmic resistor + what kind of capacitor)
>
> ws) Some of both, The nice thing about the Fluke 845 besides it's up to a 
> million gain,
> is that you can hang any type of RC you want on its output, because it is 
> totally isolated from the input.
> so Any type of BIG cap is OK,  it don't really matter much, I use a couple 
> of 10kuf back to back.
> There is zero volts across them at null and accuracy and leakage is NOT a 
> issue when using isolated null meters near zero.
>
>
> with best regards
> Andreas
>
>
>
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