[time-nuts] Rb lamp lifetime...

WB6BNQ wb6bnq at cox.net
Sun Nov 2 03:14:29 EST 2008


Hi Dick,

I guess I should have been more clear.  By “locking” I meant using the
1pps output of the Trimble and a
separate house standard connected to a separate controller board such as
Brooks Shera’s circuit.

The oscillator in the Trimble is ok, but it is not the same as a high
quality crystal oscillator with a larger
thermal mass and a double oven.  The Trimble specs for a 1 second
measurement period (i.e., using it as an
external reference for your counter) is only good to 1xE-9.  To get the
1.16xE-12 takes a 24 hour period as
noted in the spec sheet.

An hp 10544B is spec’d for short term stability at 1E-11 for 1 second
averaging.  It would be a better
flywheel (when GPS is screwing up) for a house reference source using
the GPS to keep the long term average in
check.

For the extreme, if you want to spend the money, use a Rubidium
oscillator.  The difference being what happens
during any period when the GPS is not in control.  Of course a Rubidium
would make a good short term portable
source without the GPS control.

If possible it is better to keep a reference standard unperturbed and
just keep track of the actual value.
>From that data you know the deviation and slope of any drift.  It is all a matter of keeping records.  In
primary labs that is what is done with primary standards as most are not
“adjustable,” they are not messed
with and only the measurement values are reported.  It is up to the
owner to know what to do with the
information.  Of course there are some items that are adjustable and may
very well be adjusted, would depend
upon the item and the customer needs.

In the case of a high quality Rubidium, it is stable enough to be
treated as a short term reference.  In such
a case it would only be compared to, not controlled by the GPS.  Careful
measurement, extremely careful
adjustment and good records would allow it being used as high quality
portable standard if it is kept hot the
whole time during its use.

Bill....WB6BNQ


"Richard W. Solomon" wrote:

> I am confused (a normal state).
> Why would locking an external oscillator to the Trimble
> be better than using the 10 MHz output of the Trimble ?
> Wouldn't the external oscillator follow the Trimble signal ?
> Since I have one and just use the 10 MHz output you got my
> attention.
> I do have a 970 MHz "brick" locked to one of my HB GPSDO's
> that I use for Service Monitor Calibration. Perhaps you were
> thinking of locking a Sig Gen using the Trimble ??
>
> 73, Dick, W1KSZ
>
> -----Original Message-----
> >From: WB6BNQ <wb6bnq at cox.net>
> >Sent: Nov 1, 2008 8:31 PM
> >To: Yuri Ostry <yuri at ostry.ru>, Discussion of precise time and frequency measurement <time-nuts at febo.com>
> >Subject: Re: [time-nuts] Rb lamp lifetime...
> >
> >Hi Yuri,
> >
> >Unless you need the longer term stability of an undisciplined Rb source, I think
> >you would be better served to get one of the “Timenuts group” Trimble Thunderbolt
> >GPS units to use as a home standard.  It is available via this page:
> >
> >http://www.tapr.org/kits_thunder.html
> >
> >and info about the unit is available at this URL:
> >
> >http://www.leapsecond.com/tbolt-faq.htm
> >
> >Add an external very high quality crystal oscillator locked to the Trimble and
> >you will have the same stability of a good Rb source.  Most would use a high
> >quality Rb source, undisciplined, as a tool for comparing or generating
> >specifications of crystal oscillators.
> >
> >It seems that the Rb standards last longer turned off.  My understanding (subject
> >to disagreement) is the Rubidium in the lamp ends up coating the walls of the
> >lamp and reducing the transmission through the glass, i.e., the signal gets more
> >and more noisy as time goes on.  Also, it seems that the Rubidium may permeate
> >through the walls of the glass housing and thus reduce the amount of Rb over
> >time.  I have not heard of anyone reversing the condition.  Of course, other
> >things can go wrong with the Rb source besides the lamp.
> >
> >A very high quality crystal oscillator would most likely last longer with fewer
> >problems.  I have several +40 year old hp counters with decent oscillators still
> >operating.
> >
> >Bill....WB6BNQ
> >
> >
> >
> >Yuri Ostry wrote:
> >
> >> Hello,
> >>
> >> Readed list archives and googled a lot, and seen two opposing points of
> >> view - one is that there is nothing to age in Rb lamp, another is that
> >> Rb lamp is degrading when in operation (without details, how and why).
> >>
> >> I'm planning to get some Efratom Rb oscillator to use it in a
> >> constantly running home lab frequency reference. Trying to understand
> >> for myself, is it worth to get spare unit (or even two) of the same
> >> model just to have replacement lamp on hand.
> >>
> >> One more question - does someone seen Rb standard that is
> >> malfunctioned due to degraded lamp, that, at same time, does have
> >> good vacuum. If so, which model, and how the failed lamp looks like?
> >> Was there any attempts to "rejuvenate" the lamp? (for example, heating
> >> to remove glass darkening, if any, or similar experiments).
> >>
> >> By the way, I'm very curious about physical process that may cause
> >> lamp degradation.
> >>
> >> Rubidium-87 that is included in the lamp (according to
> >> manuals) is a radionuclide, beta emitter. It decays very slowly (4.7
> >> billion years half life) to stable Strontium-87, emitting 282.62keV
> >> electrons and (anti?)neutrino. I don't think that 87Rb decay may
> >> interfere with unit operation after tens of years, unless 87Sr is
> >> "poisonous" so much so it can interfere with lamp operation even in
> >> tiniest amounts.
> >>
> >> The only reference I located to date, is following article, that is
> >> not available to general public. It is hard to understand for me just
> >> from this abstract, does it specifically related to space environment
> >> factors, or it is something that may cause degradation of Rb clocks
> >> that is operated in average lab on Earth. ;)
> >>
> >> > A Mechanism of Rubidium Atomic Clock Degradation: Ring-Mode to
> >> > Red-Mode Transition in rf-Discharge Lamps
> >> > Camparo, J.   Mackay, R.
> >> > Aerosp. Corp., El Segundo;
> >> >
> >> >
> >> > This paper appears in: Frequency Control Symposium, 2007 Joint with
> >> > the 21st European Frequency and Time Forum. IEEE International
> >> > Publication Date: May 29 2007-June 1 2007
> >> > On page(s): 45-48
> >> > Location: Geneva,
> >> > ISSN: 1075-6787
> >> > ISBN: 978-1-4244-0647-0
> >> > INSPEC Accession Number: 9805223
> >> > Digital Object Identifier: 10.1109/FREQ.2007.4319027
> >> > Current Version Published: 2007-10-01
> >> >
> >> >
> >> >
> >> >
> >> >
> >> > Abstract
> >> > In the vapor-cell atomic clock, long-term stability can be
> >> > influenced by slow variations in the discharge lamp's output via the
> >> > light-shift effect. Additionally, over a multi-year mission lifetime
> >> > the lamp's aging can degrade its optical pumping efficiency.
> >> > Understanding the mechanism(s) that drives these changes is
> >> > particularly important for spacecraft devices, where the atomic
> >> > clocks are called upon to function continuously and reliably for
> >> > many years. Here, we consider the two well-known, but little
> >> > studied, modes associated with alkali rf-discharge lamp operation:
> >> > the ring mode and the red mode. Consistent with previous research,
> >> > we find that the ring mode is best for optical pumping, and that the
> >> > clock-signal amplitude degrades significantly when the lamp operates
> >> > in the red mode. Examining the emission spectrum as the lamp
> >> > transitions between these two modes, we show that the ring-mode to
> >> > red-mode transition is driven by radiation trapping within the lamp.
> >>
> >> --
> >> Sincerely,
> >>  Yuri UA3ATQ/KI7XJ                     mailto:yuri at ostry.ru
> >>
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> >
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