[time-nuts] Re: Rb Standards C-fields

[Magnus Danielson]

Hi Bob,

For quite many rubidiums the C-field current very often derive fairly 
crudely from one of the power supply lines. As you already concluded, 
this is not the most stable source and sure you can do better. However, 
you need to back out and look on the big picture. There is a number of 
other frequency offset contributions in a rubidium, such as the wall 
shift, the gas micture, the cavity tuning and these all show dependence 
on temperature and other environments too. Some of these even compensate 
each other, so the gas mixture you attempt to make to compensate the 
wall shift. So fine, a particular design may have reasonable balance of 
these to meet it's targets.

What you can now consider is wither you aim to more precisely remove 
frequency shifts or if your aim is to improve on environmental 
stability. You can choose to do the later without necessarilly care 
overly much about the first, but you often want to do the later as you 
aim for the first. As hobbyist the second goal is a fun challenge. 
However, what you can do is to start test what voltage and temperature 
variations you can trace to this or that mechanism. There is an 
interesting interaction between these, as temperature also affects the 
oven loops which pulls less or more current which ends up varying 
voltage. Expect that it takes time to figure out which are the major 
effects and then aim to stabilize them. I am not sure that the C-field 
coil is the worst offender.

For some older rubidiums, the op-amps may not be exactly known as 
stability masters by todays measures, or to put it more pluntly, pretty 
good temperature sensors.

I can recommend you to dip your nose into "Rubidium Frequency Standard 
Primer" by W.J.Riley. It has a fair amount of practical details 
illustrated and good reasoning on them. There is more, but that's a 
relatively easy to access book.

I'm very tempted to do all this myself too, never got around to it. It 
is an interesting challenge.

Cheers,
Magnus

On 2023-08-22 07:50, Bob Stewart via time-nuts wrote:
> Is there someplace I could get some practical information about C-fields in Rb standards?  I'm not looking for the physics of how they work.  What I need is on the practical side.  This relates to the AT&T RFG-RB unit I bought.  Looking at the components, I don't see any obvious voltage references.  It has a couple of power supply chips, but nothing better than an LM-340T-5 or whatever it was.  And the C-field adjustment is a 10-turn 5K pot.
> My bottom line question is this:  Would it would make any real difference to the stability of the 10MHz output if I were to add an external voltage reference chip, such as an ADR4550A (or B or C) and a 25-turn pot, or even a DAC1220-E and a dsPIC33 to run it?  I'm assuming the center C-field voltage is 2.5V.  Given all the electronics in the thing, I'm tempted to just pull both boards out and make a new board to support the Rb oscillator with just the features I need.  But if the thing is already near the limits of the Rb oscillator there's no point.  It contains an Efratom 102100-003, for which I can't find any information.
>
> Bob
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