Re: [time-nuts] Rb Oscillator - rather fundamental question
All very informative and useful information for sure and good to know,
But I'm thinking the real difference between a primary and secondary
standard,
Has More to do with if there is anything else more accurate and repeatable
available.
I'd guess a Rb would of made a great cave man Primary standard.
And sounds like it will NOT be long before the Freq and drift of a CS
Primary will be consider just another secondary standard that will have to
be calibrated.
(to get the 1e-16 + or whatever accuracy/repeatability it is they are now
working on.)
ws
[time-nuts] Rb Oscillator - rather fundamental question
Sorry - I should have written a longer response - but you've put it all
straight anyway.
I wonder how long it will be before the definition of the second is changed
to use the newer types of clocks using strontium, ytterbium, mercury, or
aluminium (which I believe is the current "front runner")?
Cheers
Dave
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]
On
Behalf Of Richard (Rick) Karlquist
Sent: 23 February 2010 17:13
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Rb Oscillator - rather fundamental question
Well, what you said is true as far as it goes, but not the whole story.
Rick Karlquist N6RK
David C. Partridge wrote:
Cough - the rubidium clock or oscillator does have an intrinsic frequency,
which is the rubidium hyperfine transition of 6 834 682 610.904 324 Hz,
it's
just that the frequency generated by the transition in question isn't used
to DEFINE the second, so by definition, it must be secondary. Only a
Caesium clock is a primary standard, as the second is DEFINED to be the
time
taken for 9,192,631,770 cycles of the radiation corresponding to the
transition between the two hyperfine levels of the ground state of the
caesium 133 atom.[1].
Unless of course they changed the rules recently ...
[1] http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html
Dave
Well, what you said is true as far as it goes, but not the whole story.
The fact that a clock is based on cesium does not necessarily mean it
is a primary standard. For example the "chip scale atomic clock" uses
cesium and is a secondary standard. OTOH, certain experimental clocks
based on atoms such as rubidium, mercury, etc could be considered
primary standards in spite of the definition of the second.
It's not the type of atom, but the type of clock that is crucial.
"Cesium" usually refers to an atomic beam clock and "Rubidium" usually
refer to a gas cell device. In an atomic beam, the atoms are, on the
average, unperturbed, and will transition at exactly the 9192...
frequency in the definition of the second. Except that they are offset
from this frequency by a known amount due to the C-field. In a gas
cell device, the atoms are perturbed by the buffer gas which results
in a unknown frequency shift from the 6834... frequency. You have
to remove this offset by comparing to a primary standard.
We used to say that in theory you could build a cesium beam standard
from a kit of parts on a desert island having no other clocks, and when
you turned it on, it would be on the correct frequency (within a
tolerance) guaranteed by design/physics. There is no way you
could do this with a rubidium or cesium gas cell standard
to any kind of accuracy associated with atomic clocks (it would only be
in the general neighborhood of 6834...)
That is the difference between primary and secondary standards.
Another difference is that secondary standard have "aging" and
primary standards don't.
Rick Karlquist N6RK
I don't think I agree with that, Warren. I'd view a primary standard as
an intrinsic one -- that is accurate by definition and doesn't need
calibration against another, higher level, source.
A cesium beam standard is based on the same physical phenomenon that
defines the second, so if it's working, it's "right" within some degree
of tolerance.
As others have pointed out, a gas cell standard is subject to pulling
and needs to be set to a correct value (and also has drift over time).
And standards based on other ions may actually be more accurate and
stable than cesium, but they still need to be related to the official
definition of the second through a measurement.
In either case, they need to be referred to the cesium transition, so
they are not primary standards.
Make sense?
John
WarrenS wrote:
All very informative and useful information for sure and good to know,
But I'm thinking the real difference between a primary and secondary
standard,
Has More to do with if there is anything else more accurate and
repeatable available.
I'd guess a Rb would of made a great cave man Primary standard.
And sounds like it will NOT be long before the Freq and drift of a CS
Primary will be consider just another secondary standard that will have
to be calibrated.
(to get the 1e-16 + or whatever accuracy/repeatability it is they are
now working on.)
ws
[time-nuts] Rb Oscillator - rather fundamental question
Sorry - I should have written a longer response - but you've put it all
straight anyway.
I wonder how long it will be before the definition of the second is changed
to use the newer types of clocks using strontium, ytterbium, mercury, or
aluminium (which I believe is the current "front runner")?
Cheers
Dave
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at
febo.com] On
Behalf Of Richard (Rick) Karlquist
Sent: 23 February 2010 17:13
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Rb Oscillator - rather fundamental question
Well, what you said is true as far as it goes, but not the whole story.
Rick Karlquist N6RK
David C. Partridge wrote:
Cough - the rubidium clock or oscillator does have an intrinsic
frequency,
which is the rubidium hyperfine transition of 6 834 682 610.904 324
Hz, it's
just that the frequency generated by the transition in question isn't
used
to DEFINE the second, so by definition, it must be secondary. Only a
Caesium clock is a primary standard, as the second is DEFINED to be
the time
taken for 9,192,631,770 cycles of the radiation corresponding to the
transition between the two hyperfine levels of the ground state of the
caesium 133 atom.[1].
Unless of course they changed the rules recently ...
[1] http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html
Dave
Well, what you said is true as far as it goes, but not the whole story.
The fact that a clock is based on cesium does not necessarily mean it
is a primary standard. For example the "chip scale atomic clock" uses
cesium and is a secondary standard. OTOH, certain experimental clocks
based on atoms such as rubidium, mercury, etc could be considered
primary standards in spite of the definition of the second.
It's not the type of atom, but the type of clock that is crucial.
"Cesium" usually refers to an atomic beam clock and "Rubidium" usually
refer to a gas cell device. In an atomic beam, the atoms are, on the
average, unperturbed, and will transition at exactly the 9192...
frequency in the definition of the second. Except that they are offset
from this frequency by a known amount due to the C-field. In a gas
cell device, the atoms are perturbed by the buffer gas which results
in a unknown frequency shift from the 6834... frequency. You have
to remove this offset by comparing to a primary standard.
We used to say that in theory you could build a cesium beam standard
from a kit of parts on a desert island having no other clocks, and when
you turned it on, it would be on the correct frequency (within a
tolerance) guaranteed by design/physics. There is no way you
could do this with a rubidium or cesium gas cell standard
to any kind of accuracy associated with atomic clocks (it would only be
in the general neighborhood of 6834...)
That is the difference between primary and secondary standards.
Another difference is that secondary standard have "aging" and
primary standards don't.
Rick Karlquist N6RK
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.
WarrenS wrote:
All very informative and useful information for sure and good to know,
But I'm thinking the real difference between a primary and secondary
standard,
Has More to do with if there is anything else more accurate and repeatable
available.
I'd guess a Rb would of made a great cave man Primary standard.
And sounds like it will NOT be long before the Freq and drift of a CS
Primary will be consider just another secondary standard that will have to
be calibrated.
(to get the 1e-16 + or whatever accuracy/repeatability it is they are now
working on.)
Sorry but you have completely misunderstood the concept.
It is admittedly a difficult concept to grasp; I know it took
me a long time.
A hydrogen maser with the wall shift servo'ed out will run rings
around a compact Cs beam clock like the HP5062, used on submarines.
(An interesting trivia item is that I don't believe the 5061 can
fit through a submarine hatch). The 5062 is still a primary
frequency standard and the hydrogen maser is still a secondary
frequency standard.
Regarding "drift" of primary cesium beam standards: the 5071A has
unmeasurable drift, aging and tempco, down to a measurement limit
of at least 1E-15. It has a typical random error of a few parts
in 1E-13. The systematic error (average error of all 5071A's built)
has been established to be below 1E-14. It will always be a
primary standard even in the presence of longer reversible optically pumped
laboratory Cs beam standards of higher accuracy and better short term
stability, or cesium fountains, etc. Even the 5061A/B is considered
a primary standard, albeit with reduced accuracy, even though it
has a measurable tempco. We were very proud of the E1938A crystal
oscillator when it was able to meet the 5061 tempco spec. It
is in no way a primary frequency standard regardless of that
or any other accomplishment.
Primary means that the clock will meet its spec without being
"calibrated" against a better clock. Secondary means that
calibration against a primary standard is necessary.
Rick Karlquist N6RK
My two cents:
A primary standard is one that is directly compared with the accepted or
defined standard. In years past, an oscillator could only be called a
primary time or frequency standard if it had a clock in order to make
comparisons with the Earth's rotation. Note, there are no limits on the
accuracy or stability. A standard that is compared to a primary standard is
a secondary standard even if it is more stable.
Since 1967, the second has been defined to be the duration of 9,192,631,770
periods of the radiation corresponding to the transition between the two
hyperfine levels of the ground state of the caesium 133 atom.
Therefore, any standard which is compared directly with the transition is a
primary standard. Any standard compared to a primary standard is a
secondary standard. A standard more stable than a caesium standard is still
a secondary standard until the process the new standard is directly compared
to is designated the accepted or defined standard. A rubidium standard
compared to a caesium standard is still a secondary standard because it is
not directly compared the the caesium transition.
To summarize from the 1933 Bulletin 10 from General Radio on Frequency
Measurements at Radio Frequencies, "all frequency standards are compared
either directly or indirectly with a standard time interval derived from the
earth's rotation...It should be noted, however, that the accuracy depends on
how direct a comparison is made with the standard time interval."
Replace "derived from the earth's rotation" with "accepted or defined."
Direct comparison's yield primary standards and indirect yield secondary
standards.
John WA4WDL
From: "Rick Karlquist" richard@karlquist.com
Sent: Tuesday, February 23, 2010 3:00 PM
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Subject: Re: [time-nuts] Rb Oscillator - rather fundamental question
WarrenS wrote:
All very informative and useful information for sure and good to know,
But I'm thinking the real difference between a primary and secondary
standard,
Has More to do with if there is anything else more accurate and
repeatable
available.
I'd guess a Rb would of made a great cave man Primary standard.
And sounds like it will NOT be long before the Freq and drift of a CS
Primary will be consider just another secondary standard that will have
to
be calibrated.
(to get the 1e-16 + or whatever accuracy/repeatability it is they are
now
working on.)
Sorry but you have completely misunderstood the concept.
It is admittedly a difficult concept to grasp; I know it took
me a long time.
A hydrogen maser with the wall shift servo'ed out will run rings
around a compact Cs beam clock like the HP5062, used on submarines.
(An interesting trivia item is that I don't believe the 5061 can
fit through a submarine hatch). The 5062 is still a primary
frequency standard and the hydrogen maser is still a secondary
frequency standard.
Regarding "drift" of primary cesium beam standards: the 5071A has
unmeasurable drift, aging and tempco, down to a measurement limit
of at least 1E-15. It has a typical random error of a few parts
in 1E-13. The systematic error (average error of all 5071A's built)
has been established to be below 1E-14. It will always be a
primary standard even in the presence of longer reversible optically
pumped
laboratory Cs beam standards of higher accuracy and better short term
stability, or cesium fountains, etc. Even the 5061A/B is considered
a primary standard, albeit with reduced accuracy, even though it
has a measurable tempco. We were very proud of the E1938A crystal
oscillator when it was able to meet the 5061 tempco spec. It
is in no way a primary frequency standard regardless of that
or any other accomplishment.
Primary means that the clock will meet its spec without being
"calibrated" against a better clock. Secondary means that
calibration against a primary standard is necessary.
Rick Karlquist N6RK
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.
renamed, since the discussion has shifted.
NIST gives this definition:
"Primary Standard
"A standard that is designated or widely acknowledged as having the
highest metrological qualities and whose value is accepted without
reference to other standards of the same quantity. For example, NIST-F1
is recognized as a primary standard for time and frequency. A true
primary standard like NIST-F1 establishes maximum levels for the
frequency shifts caused by environmental factors. By summing or
combining the effects of these frequency shifts, it is possible to
estimate the uncertainty of a primary standard without comparing it to
other standards.
"In the time and frequency field, the term primary standard is
sometimes used to refer to any cesium oscillator, since the SI
definition of the second is based on the physical properties of the
cesium atom. The term primary standard is also commonly used, at least
in a local sense, to refer to the best standard available at a given
laboratory or facility."
jmfranke wrote:
Therefore, any standard which is compared directly with the transition is
a
primary standard. Any standard compared to a primary standard is a
I don't agree. By your definition the "chip scale atomic clock",
which uses cesium, would be a primary standard. I don't know
of anyone who believes that.
Rick Karlquist N6RK
In message 20100223214204.EAE711174BE@hamburg.alientech.net, Mike S writes:
renamed, since the discussion has shifted.
"In the time and frequency field, the term primary standard is
sometimes used to refer to any cesium oscillator, [...]
That rhymes with and Karls and my perception of the term:
A Cs clock is primary because when you turn it on, it latches onto
the physical phenomenon of a known and invariant frequency subject
to no systematic errors.
The reason the small Rb's do not qualify as primary is that each
unit has a slightly different frequency, due to vapour pressure,
isotopemix and other physical details, and thus you cannot know the
frequency of a particular unit, until you have measured it relative
a primary clock.
In other words, Primary and Secondary has nothing to do with which
atoms, but depends a lot on the interogations mechanism used.
So the tiny 1cm^3 Cs standards are secondary, because they are also
subject to all sorts of pulls and offsets.
The "experimental" clocks based on lonely ions and quantum embraces
are very likely primary, once somebody has measured their intrinsic
frequency relative to Cs once.
The way to find out if your new invention has a chance to become a
primary clock, is to build N of them, turn them on, and see if they
all find the same frequency once they are locked, if they do,
you're on your way to become famous.
--
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.
At 05:48 PM 2/23/2010, Poul-Henning Kamp wrote...
A Cs clock is primary because when you turn it on, it latches onto
the physical phenomenon of a known and invariant frequency subject
to no systematic errors.
The reason the small Rb's do not qualify as primary is that each
unit has a slightly different frequency, due to vapour pressure,
isotopemix and other physical details, and thus you cannot know the
frequency of a particular unit, until you have measured it relative
a primary clock.
There is no difference between Cs and Rb in that regard, except perhaps
scale.
The frequency of a Cs is subject to gravitational and electromagnetic
effects. Which is why the definition of the second was clarified in
1997 by the CIPM to refer "to a caesium atom at rest at a temperature
of 0 K."
Since absolute rest and absolute zero are impossible conditions for a
real world clock, Cs clocks do not have a "known and invariant
frequency." If they did, why would they have a C-Field knob to twiddle,
and why would TAI be a weighted average of multiple Cs clocks? I'd
guess that Cs clocks can also be thrown off by trace gasses in the
tubes and numerous other effects, impossible to completely remove.
Even if one could obtain absolute zero acceleration and temperature,
Heisenberg would still be heard.
The "experimental" clocks based on lonely ions and quantum embraces are
very likely primary, once somebody has measured their intrinsic frequency
relative to Cs once.
No they cannot be - yet. At the point where (e.g.) the second is re-defined
in terms of the aluminium quantum clock, then the aluminium quantum clocks
are then by definition the primary standards of time, and all the Cs clocks
are now secondary standards as the second is no longer defined in terms of
the Cs beam clock.
Dave
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Poul-Henning Kamp
Sent: 23 February 2010 22:49
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Primary Standards...
In message 20100223214204.EAE711174BE@hamburg.alientech.net, Mike S
writes:
renamed, since the discussion has shifted.
"In the time and frequency field, the term primary standard is
sometimes used to refer to any cesium oscillator, [...]
That rhymes with and Karls and my perception of the term:
A Cs clock is primary because when you turn it on, it latches onto the
physical phenomenon of a known and invariant frequency subject to no
systematic errors.
The reason the small Rb's do not qualify as primary is that each unit has a
slightly different frequency, due to vapour pressure, isotopemix and other
physical details, and thus you cannot know the frequency of a particular
unit, until you have measured it relative a primary clock.
In other words, Primary and Secondary has nothing to do with which atoms,
but depends a lot on the interogations mechanism used.
So the tiny 1cm^3 Cs standards are secondary, because they are also subject
to all sorts of pulls and offsets.
The "experimental" clocks based on lonely ions and quantum embraces are very
likely primary, once somebody has measured their intrinsic frequency
relative to Cs once.
The way to find out if your new invention has a chance to become a primary
clock, is to build N of them, turn them on, and see if they all find the
same frequency once they are locked, if they do, you're on your way to
become famous.
--
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.
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.
At 06:25 PM 2/23/2010, David C. Partridge wrote...
No they cannot be - yet. At the point where (e.g.) the second is
re-defined
in terms of the aluminium quantum clock, then the aluminium quantum
clocks
are then by definition the primary standards of time, and all the Cs
clocks
are now secondary standards as the second is no longer defined in
terms of
the Cs beam clock.
I disagree. A Cs clock is not a primary because it uses Cs, but because
it's the pragmatic state-of-the-art. A Cs clock which determines time
based on the definition simply doesn't exist, it meets only 1 of 3
criteria. If a demonstrably more stable clock, using any technology,
can be shown to exist, it too qualifies as a primary standard. Better
is better.