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:

Hi

What is the goal?

Temperature stability will always be a limiting factor in any of this.

The control signal is only supplying part of the C field current. The “rest of it” is coming from devices internal to the package.

Bob

Magnus Danielson via time-nuts writes:

Even in the HP5065A the C-field current is a significant part of
the stability.

I worked a lot on that back when I had time for time-nuttery and
wrote a sort of 'work-log' about it:

http://phk.freebsd.dk./hacks/HP5065A/

My conclusion was that it is not trivial to take the C-field current
definitively out of the equation for a HP5065.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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@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:

time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com

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

I think rather than power supply upgrades, another approach would be to add a clean-up oscillator. Whether GNSS, rubidium, or cesium, the short-term performance such as phase noise is based on the internal quartz oscillator specs. Basically, it is the flywheel of most frequency standards and a quality clean-up oscillator will improve more aspects of noise.
Tom Knox
SR Test and Measurement Engineer
Phoenix Research
4870 Meredith Way Apt 102
Boulder, Co 80303
Formerly of:
357 Fox Lane
Superior Co 80027
303-554-0307
actast@hotmail.com

https://www.thedenverchannel.com/news/marshall-fire/superior-man-moving-forward-after-losing-dream-research-lab-during-marshall-fire

"Peace is not the absence of violence, but the presence of Justice" Both MLK and Albert Einstein

From: Bob Camp via time-nuts time-nuts@lists.febo.com
Sent: Tuesday, August 22, 2023 3:57 PM
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@lists.febo.com
Cc: Bob Camp kb8tq@n1k.org
Subject: [time-nuts] Re: Rb Standards C-fields

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

time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com