Hi

There is no direct relation for an Rb to 10 MYz. Cs beam tubes are what have a direct relation.
Even then, the qualifier is “under standard conditions”. They are sensitive to magnetic field. Rb’s
also are sensitive to magnetic field. Both can be tuned by varying the field. In the case of an Rb
that also takes care of a multitude of other issues.

In the case of Rb, there is a distribution of cells coming out of the manufacturing process. Some
are pretty close to the “right” frequency. Others are way off (as in 100’s of KHz or more). All of them
are capable of meeting the required specs. DDS techniques allow those cells to be used in a
production part. That increases the yield and thus drops the production cost.

Since you now magically have a DDS in the Rb, you can do all sorts of interesting things. If you
suddenly need a 9.99900 MHz standard …. here it is … If you need to do temperature compensation
via a lookup table … it just takes a bit of testing and some code to make it happen. Indeed, the DDS
does also give you some issues. Without some sort of cleanup oscillator, you will have spurs and
phase noise on the output.

Lots of fun ….

Bob

And even without problems like external magnetic fields, Rb oscillators do
drift with
age. Over a period of several years they may drift as much as ~1E-9, which
is a huge
error for serious time nuts.

In my pre-retirement job I rode herd on an active Hydrogen maser system,
and even
that has a clear drift tendency.  Generally a couple or three times per
year I had to
make a frequency adjustment in the neighborhood of 3E-14.  And still being
privy to
its performance, I was amused to note that its drift tendency was
interrupted by the
hurricane Maria.  On the day of eye passage over the site the frequency
suddenly
decreased by a few parts in 10^14, held about constant for roughly a week,
then
resumed almost its original value and drift rate thereafter.  If anybody in
this group
can explain* that* behavior (that is, held for a week before resuming old
habits), I'd
love to learn about it.

Dana

On Mon, Nov 20, 2017 at 1:40 PM, Bob kb8tq kb8tq@n1k.org wrote:

Bob, I was referring to the rubidium standard of 6834682610.904 Hz.  For some reason I thought it was closer to 9Ghz.

I assume then rubidium standards oscillate (if that is the correct term) somewhere around that number but not exact or is it in the detection where things fall down?

On Mon, Nov 20, 2017 at 8:28 PM, Jerry Hancock jerry@hanler.com wrote:

I think you are confused by the difference between primary and
secondary standards.

A typical rb gas cell is a secondary standard.  Its exact frequency is
distorted by a number of factors like gas pressure, interaction with
the cell walls, and ambient magnetic fields which cannot be canceled
by the design of the standard.  This is why it is useful to discipline
a telecom rb against GPS, disciplining can be accomplished through
control of a biasing magnetic field.

Something like a cesium beam standard is able to internally cancel
most of these biases "under standard conditions".  A drift free
frequency source can also be constructed using rubidium, such as
rubidium fountains just as a secondary standard could be constructed
using cs-- like cs gas cell standards (such as the sa.45 CSAC).

[Hopefully I haven't mangled things].

Hi

It’s very much a “somewhere near that number” sort of thing with an Rb. The
“thing” you are looking at is quantum mechanical in nature. Unfortunately that
by its self does not make it perfect. A beam tube (as opposed to a gas cell)
isolates things better.

A 5061 is a beam tube device. A 5065 is gas cell based. It is very important to note that
accuracy and stability are two different things …. The beam tube is more accurate.
The gas cell is more stable (over some range of tau).

A normal Rb standard has a bit of this and that in the bulb. These other gasses
help in various ways. They each also add a bit of “pull” to the frequency one way
or the other. They get you away from your “magic number” but the benefits they
deliver are worth the trouble. The exact gas mix gets into the “secret sauce” of
the Rb manufacturer. They each optimize things a bit differently. The walls
of the bulb get into the act ….

Beam standards are actually a bit old these days. The more modern approach
would be a fountain (toss the ion straight up and let it fall back to you). An even
more modern approach would be a trapped ion standard. The amount of money
involved goes up dramatically with each of those steps. You get rid of this and
that subtle effect with each improvement. Accuracy gets better and better.

Lots of choices !!!

Bob

One step at a time.

2yrs ago when the time-bug hit, I had a crystal oscillator.  6 months later, DOCXO then GPSDO then Rubidium soon to be with GPSDO and there aren’t too many steps after that…

I also gave my brother the bug the other day…

Hi

Ummm ….. errrr ….. multiple GPSDO’s …. L1/L2 GPSDO(s) …. Cs standard (s) … Maser(s) …. Ensembles of all of the above ….

There’s lots of steps still to take ….

Bob

Not to junk up the mailboxes, but I have the multiple GPSDOs.  Don’t know what you mean by L1/L2 GPSDO, is that a quality statement?

Also, what would the next step cost me for a Cesium Beam?  Roughly?  And what order of magnitude improvement would that be for the cost?  Have to do a cost/benefit analysis for the wife...

Hi

Most GPSDO’s run the GPS signal only on one “band” (L1). If you want to eliminate the errors in the ionosphere
correction process you go to.a double band (L1 and L2) GPS receiver. Since there are fewer potential errors in
the GPS signal, you may have fewer net errors in your GPSDO.

For the Cs - the issue is a stand alone reference. How can you be sure that GPS is not lying to you? The only way
to check something like that is with a stand alone reference. That’s somewhere between priceless and worthless
depending on which half of the family you happen to ask.

You can (or course) do a GPS disciplined Cs standard. That’s not easy to do, but some are attempting it ….

Lots to do…

Bob

On Mon, Nov 20, 2017 at 7:14 PM, Jerry Hancock jerry@hanler.com wrote:

I hope she is not the type of person who sets her watch 5 minutes ahead so
she will arrive on time!

--Jim Harman

You don’t mention the make of the instrument, but I suspect the same basic technology is used by all.
To quote from the Oscilloquartz page on their CH1-76A product:
«  The quantum device is used as a frequency discriminator in an automatic frequency tuning system of a crystal oscillator. »
They don’t however quote stability relative to air pressure. However…..
It is known that atmospheric pressure changes can induce OCXO frequency changes due to deformation of the crystal envelope causing stray capacitance changes.
As the eye of a hurricane has greatly reduced air pressure than normal, by as much as 15%, it could be related.

"The power of accurate observation is commonly called cynicism by those who have not got it. »
George Bernard Shaw

Oops, I should have mentioned:  The unit was a Symmetricom MHM-2010 (I hope
I got that right), except that it was built before the company was bought by
Symmetricom (which was later bought by Micro-Semi in turn).

This is an active maser, meaning a self-sustaining oscillator whose gain
medium
was a volume of hydrogen atoms (at low pressure) maintained in a population-
inverted state by squirting a thin stream of state-selected H atoms into a
glass
bulb, and simultaneously pumping on the bulb to maintain a low pressure.
The
inside of the bulb was treated so that the collisions with the surface did
not
usually cause a quantum state change of the H atom involved.  I've read that
the average excited atom typically "survived" a large number of such wall
collisions before being "consumed" by contributing a quantum of energy to
the oscillating mode; this has always amazed me.

So the primary frequency-determining mechanism is the collision-broadened
line width of the gain mechanism. However, the cavity resonance exhibits a
noticeable frequency-pulling effect, and our maser has a feedback loop that
strives to keep the cavity tuned to the center of the medium's gain
profile.
But I think this loop is not a tight loop, ergo not completely successful.

Anyway, a small amount of RF power (a fraction of a pW as I understand it)
is extracted from the cavity as the useful output.  This drives a frequency
synthesizer to make a useful standard frequency output.  The "divide ratio"
of that synthesizer is adjustable in fine steps, with one step being a
fractional
frequency change of about 7E-17.

I've long wondered what causes the slow frequency drift, typically amounting
to about 3E-14 over a time span of several months.

Dana

On Tue, Nov 21, 2017 at 1:39 AM, Mike Cook michael.cook@sfr.fr wrote:

On Mon, 20 Nov 2017 20:18:18 -0500
Bob kb8tq kb8tq@n1k.org wrote:

It is actually quite easy. You measure the satellite carrier and code
phases relative to the output signal of the Cs, collect all the data,
wait a couple of weeks until the IGS final product becomes available,
then you calculate the frequency and phase difference and steer the
Cs accordingly using a control loop with a tau in the order of 3 weeks
at minimum (due to the delay in IGS data), rather 10 to 50 weeks.

Simple, right? :-)

		Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson

On Tue, 21 Nov 2017 04:24:51 -0600
Dana Whitlow k8yumdoober@gmail.com wrote:

They are not maintained in this state. Rather the Hydrogen atoms are
"used up" by emiting a photon at 1.4GHz. They are then "removed"
by leaking out of the opening of the bulb and pumped away.
The source for the Hydrogen atoms is usually a heated platinum
valve (a heated plate of platinum that is thin enough that the
Hydrogen will leak through). The advantage of the platinum valve
system is that it "generates" single atom Hydrogen, as required
by the maser. These atoms go through a specially formed magnet
that deflects the atoms that are in the wrong state (c.f. Stern-Gerlach
experiment). Those in the right state make it into the cavity.
Within the cavity there is a small glass bulb that keeps the atoms
in the right position of the cavity field. In order not to perturbe
the atoms too much, the bulb walls are coated with Teflon.

Yes, IIRC normal numbers are several 10s to 100s of wall collisions
before the atom loses its state due to wall colisions and without
contributing to the signal.

The loop is quite tight. But there are multiple effects that prevent
perfect operation. Major problems are the low signal levels and the
shifts due wall colisions and cavity pulling.

Mostly changes in the wall coating leading to a different wall collision
shift and mechanical changes of the cavity dimension (think air pressure
and creep) leading to a different cavity pulling. To a lesser extend
it's the changes in the quality of the vacuum and number of Hydrogen atoms
in the cavity.

			Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson

On Tue, Nov 21, 2017 at 2:50 PM, Attila Kinali attila@kinali.ch wrote:

Picking nits here.. It was my understanding that the splitting of molecular
hydrogen into atomic hydrogen happens using RF in the dissociator - not in
the platinum leak valve. Is my understanding incorrect?

4.5 liters in EFOS type masers - so not that small. I believe other
masers are the same order of magnitude.

Lifetime ~1 second I think

Also aging of electronic components - coarse tuning of the cavity is done
by temperature, and any drift if the temperature-sensor/amplifiers etc will
result in drift. At least for EFOS type masers.

Ole

On Tue, 21 Nov 2017 15:14:59 +0100
Ole Petter Ronningen opronningen@gmail.com wrote:

Good question. I don't know. I've only read a dozen or so papers on
maser construction. I have never owned or operated one. Much less
taken appart and studied its construction.

Hehe.. Yes. It's "small" compared to the cavity. Depending on the
exact cavity construction, the storage space can be as small as
a tenth of the total cavity volume.

		Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson

Thats interesting, I would think a small volume would result in increased
spin exchange - do you have any papers detailing the tradeoffs with
big/small storage bulbs?

Hi Ole,

What does 'EFOS' mean?  I hadn't heard the term before.

I think I've heard the one-second lifetime figure before.

I do hear mixed reports about where the conversion to atomic H
occurs, and consider the jury to still be out on that question.

I had thought that the volume of the storage bulb was much
smaller in out maser, perhaps in the pint to quart range.  For a
frequency of ~1420 MHz, I guess it would take a cavity that is
operating in a somewhat higher than fundamental mode if the
volume is in the gallon regime as you suggest.  But with the
narrow gain profile width of this transition, I supposed there'd
be no risk of the thing running in the wrong mode.

Dana

On Tue, Nov 21, 2017 at 8:14 AM, Ole Petter Ronningen <opronningen@gmail.com

On Tue, 21 Nov 2017 15:31:25 +0100
Ole Petter Ronningen opronningen@gmail.com wrote:

I would have to go through my collection to find those that mention
anything on it.

The tradeoff is bascially, that you want to have an as large volume
as possible to minimize wall and atom-atom collisions. But there is
only a certain volume within the cavity, where the field has the
right orientation. The higher the mode of the cavity, the smaller
the volume (relative to the cavity size). Higher cavity modes are
used to shrink the overall cavity size, without the need of loading,
which introduces losses.

			Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson