Why discipline Rubidium oscillator?
On Mon, Nov 20, 2017 at 7:14 PM, Jerry Hancock jerry@hanler.com wrote:
Have to do a cost/benefit analysis for the wife...
I hope she is not the type of person who sets her watch 5 minutes ahead so
she will arrive on time!
--Jim Harman
Le 20 nov. 2017 à 20:53, Dana Whitlow k8yumdoober@gmail.com a écrit :
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 frequencysuddenly
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 inthis group
can explain* that* behavior (that is, held for a week before resuming old
habits), I’d love to learn about it.
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.
Dana
On Mon, Nov 20, 2017 at 1:40 PM, Bob kb8tq kb8tq@n1k.org wrote:
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
On Nov 20, 2017, at 1:34 PM, Jerry Hancock jerry@hanler.com wrote:
I know this is going to sound dumb as I know many GPSDOs had rubidium
oscillators in them. I can see why, in that during holdover, they would
tend to be more stable vs others, but given that there is a direct
mathematical relationship between the rubidium frequency and potentially
the 10Mhz desired output frequency, why do they have to be disciplined or
better yet, what advantage does it bring? Also, I can see how you
discipline a DOCXO with the external voltage, how do you discipline a
rubidium? Pulse stretching?
I guess I don’t understand how the technology works, but it seems like
an RF signal is swept that would be used to detect a dip at a pretty well
defined frequency. This dip can be used to discipline the oscillator to
something like 9Ghz or a factor of what, 900+ times better than 10Mhz. So
wouldn’t that be able to get your desired 10Mhz to 10,000,000.001 or pretty
much my level of measurement? Or does is the dip not quite that precise?
If you can point me to a write-up on this I’ll go away.
Thanks to Gilbert for providing me with at least one rubidium oscillator
that is working out of 5 though 2 others seems to stay locked for a few
hours during my testing.
Jerry
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"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:
Le 20 nov. 2017 à 20:53, Dana Whitlow k8yumdoober@gmail.com a écrit :
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 frequencysuddenly
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
inthis group
can explain* that* behavior (that is, held for a week before resuming old
habits), I’d love to learn about it.
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.
Dana
On Mon, Nov 20, 2017 at 1:40 PM, Bob kb8tq kb8tq@n1k.org wrote:
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
On Nov 20, 2017, at 1:34 PM, Jerry Hancock jerry@hanler.com wrote:
I know this is going to sound dumb as I know many GPSDOs had rubidium
oscillators in them. I can see why, in that during holdover, they would
tend to be more stable vs others, but given that there is a direct
mathematical relationship between the rubidium frequency and potentially
the 10Mhz desired output frequency, why do they have to be disciplined
or
better yet, what advantage does it bring? Also, I can see how you
discipline a DOCXO with the external voltage, how do you discipline a
rubidium? Pulse stretching?
I guess I don’t understand how the technology works, but it seems like
an RF signal is swept that would be used to detect a dip at a pretty
well
defined frequency. This dip can be used to discipline the oscillator to
something like 9Ghz or a factor of what, 900+ times better than 10Mhz.
So
wouldn’t that be able to get your desired 10Mhz to 10,000,000.001 or
pretty
much my level of measurement? Or does is the dip not quite that
precise?
If you can point me to a write-up on this I’ll go away.
Thanks to Gilbert for providing me with at least one rubidium
oscillator
that is working out of 5 though 2 others seems to stay locked for a few
hours during my testing.
Jerry
time-nuts mailing list -- time-nuts@febo.com
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mailman/listinfo/time-nuts
and follow the instructions there.
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To unsubscribe, go to https://www.febo.com/cgi-bin/
mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/
mailman/listinfo/time-nuts
and follow the instructions there.
"The power of accurate observation is commonly called cynicism by those
who have not got it. »
George Bernard Shaw
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On Mon, 20 Nov 2017 20:18:18 -0500
Bob kb8tq kb8tq@n1k.org wrote:
You can (or course) do a GPS disciplined Cs standard. That’s not easy to do,
but some are attempting it ….
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:
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,
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.
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.
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.
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.
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.
I've long wondered what causes the slow frequency drift, typically amounting
to about 3E-14 over a time span of several months.
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:
[...] The advantage of the platinum valve
system is that it "generates" single atom Hydrogen, as required
by the maser.
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?
Within the cavity there is a small glass bulb that keeps the atoms
in the right position of the cavity field.
4.5 liters in EFOS type masers - so not that small. I believe other
masers are the same order of magnitude.
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.
Lifetime ~1 second I think
I've long wondered what causes the slow frequency drift, typically
amounting
to about 3E-14 over a time span of several months.
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.
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:
[...] The advantage of the platinum valve
system is that it "generates" single atom Hydrogen, as required
by the maser.
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?
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.
Within the cavity there is a small glass bulb that keeps the atoms
in the right position of the cavity field.
4.5 liters in EFOS type masers - so not that small. I believe other
masers are the same order of magnitude.
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
4.5 liters in EFOS type masers - so not that small. I believe other
masers are the same order of magnitude.
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.
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
wrote:
On Tue, Nov 21, 2017 at 2:50 PM, Attila Kinali attila@kinali.ch wrote:
[...] The advantage of the platinum valve
system is that it "generates" single atom Hydrogen, as required
by the maser.
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?
Within the cavity there is a small glass bulb that keeps the atoms
in the right position of the cavity field.
4.5 liters in EFOS type masers - so not that small. I believe other
masers are the same order of magnitude.
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.
Lifetime ~1 second I think
I've long wondered what causes the slow frequency drift, typically
amounting
to about 3E-14 over a time span of several months.
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.
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
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On Tue, 21 Nov 2017 15:31:25 +0100
Ole Petter Ronningen opronningen@gmail.com wrote:
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.
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?
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