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

[Bob Camp]

Hi

If the unit has been on power for any significant portion of the ~30 years since it was built,
it likely does not have much life left in it. The telecom Rb’s do have a finite number of years
of life in them. 

Bob

> On Aug 22, 2023, at 2:01 PM, Bob Stewart via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hello Magnus,
> I haven't spoken to you for a very long time.  The date of manufacture on this is 9547, which I take to be the 47th week of 1995.  Of course I have no idea how much usage it's seen in that time.  
> 
> So, are you suggesting that I toss the boards in the unit, and even go so far as to taking the Rb unit apart and changing the opamps?  I don't mind tossing the telco support boards, since I don't have any info on what can be done through the control interface.  But I think I'd like to start with simply adding a small board with a voltage reference and a better pot.  It might be useful to just bring the wires out and use an external pot to begin with.  But as I mentioned already, will the results likely be worth the effort?
> 
> Bob 
> 
>    On Tuesday, August 22, 2023 at 08:03:26 AM CDT, Magnus Danielson via time-nuts <time-nuts at lists.febo.com> wrote:  
> 
> 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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