[time-nuts] Re: Rb Standards C-fields
Bob Stewart
bob at evoria.net
Tue Aug 22 18:01:58 UTC 2023
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.Â
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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