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

[WarrenS]

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