Thinking outside the box a super reference
On 11/4/16 5:27 PM, Richard (Rick) Karlquist wrote:
On 11/4/2016 4:04 PM, Poul-Henning Kamp wrote:
Historically resonance cavities were used so that step/avalance
diode multipliers had enough power to excite them. Today we have
semiconductors which work at those frequencies.
A great deal of complexity in the 5061 went into
exciting an SRD at 90 MHz and getting a sufficient
line at 9180 MHz to put on a sideband at 9192.
I spent a lot of time trying to do this during the
5071 project and was never able to get anywhere
near the efficiency that the 5061 waveguide structure
was able to do. It was designed by a visiting Korean
professor, so he wasn't around to mentor me.
The 10816 also used an SRD, and it was also a
struggle, although I was able to make it work.
Fortunately, we were able to replace all this with a
DRO and PLL, and that was 25 years ago. At this time,
it is even more of a no brainer that you don't want
to knock yourself out trying to make an SRD multiplier
work. Also, these days, it is harder than ever to
purchase good SRD's.
You can get a nice GaAs VCO that will tune 9GHz with no sweat, and the
PLL parts to go around it. I'm not sure what the DRO market is these
days - You can probably build a synthesizer with lower DC power with a
DRO than using GaAs (although CMOS is getting ever better and getting up
to that kind of range). There are probably low cost low precision
(unlocked) designs like LNBs that use a DRO.
But things like 10 GHz microwave motion detectors these days use VCOs,
not DROs.
DROs are subject to microphonics and such - yes, the dominant resonance
is the puck, but the cavity it's in affects it too, so vibration and
temperature have effects.
In message 768ee5a7-1c53-06cf-cf36-ec75e290177e@karlquist.com, "Richard (Rick) Karlquist" w
rites:
Reminds me of an interesting Jack Kusters story.
There was some customer who was having problems with
his atomic clocks being noisy (I don't remember exactly
the story) but the bottom line was that they determined
it was because of helium contamination.
How would helium make his clocks noisy ?
Isn't it more likely that it was the alphas from the
radon decay that did it by their charge ?
--
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.
On Fri, 04 Nov 2016 23:04:22 +0000
"Poul-Henning Kamp" phk@phk.freebsd.dk wrote:
First: Yes, but if you pick a sensible vibration mode for your
microwave resonance, that can be done with an screw-in endcap.
Second: No, I would actually not need to tune it.
Historically resonance cavities were used so that step/avalance
diode multipliers had enough power to excite them. Today we have
semiconductors which work at those frequencies.
Yes, it is easier to create the required frequencies with high power
(getting 7GHz with 10dBm is trivial and 30dBm can be readily achieved
with single chip amplifiers) and the signal will be cleaner than what
an SRD setup can achieve (no spurs, comparable close-in phase noise
characteristic). Thus we can use cavities with lower Q which are easier
to build and tune. But we still need to ensure that the field is properly
oriented and homogenous over the whole vapor cell. For this you need
a cavity that is properly designed and most likely will be resonant at 6.9GHz.
(I don't know whether it is possible to design a non-resonant cavity with
the above properties)
But the resonanance leads to all sorts of trouble, including frequency
pulling, temperature sensitivities etc.
Frequency pulling is not so much of an issue for a vapor cell standard
as it is for hydrogen masers or the primary standards. The shifts due
to buffer gas and wall collisions are so large that a calibration is
needed anyways. Frequency pulling just adds another term. Temperature
sensitivity is IIRC lower for frequency pulling than for buffer gas shift,
but I could be mistaken. For high stability applications, temperature
stabilization of the cavity and the cell are advised anyways.
Third: A lot of the "everybody knows" about which atoms can be
used for active vs. passive atomic standards comes from the
state of the art electronics about 30 years ago.
A lot of the "everybody knows" has been challenged and rewritten
in the past 10-20 years. Although, it looks like the vapor cell
standards have not changed in 50 years, there has been a lot of
research going on and people optimized old ideas and came up with
new ones. Yes, a lot of the electronics design is rather crude,
but most of the people working in the field a physicists or electrical
engineers that just graduated, one cannot expect the level of skill
and expertise of someone who has been doing RF designs for 30 years.
Beside, these people are there to do research, not to design electronics.
The circuits are just a tool for the research, not their main topic.
Hence they rather spend 100k€ on a synthesiser from keysight instead
of spending 6 months for designing their own to save money.
Using laser-pumping and modern semiconductors, it might actually
be possible to detect the 6.8GHz photons from the Rb.
You mean an optically pumped active maser? (our vapor cell standards
are passive masers allready) This has been already done in the 60's[1]
and studied later again (eg [2]). I guess this isn't popular because
the short term stability of Rb vapor cell standards is already quite
good and the long term stability does not get improved. So it does not
justify the additional complexity.
They won't be coherent photons, like in a Hydrogen maser, but we
don't need them to be, in fact that just causes the same exact
problems as the tuned cavity anyway, as long as we can measure
the frequency well enough.
If you mean to detect the decay from the 5S hyperfine splitting,
then i have to disapoint you. There will not be much radiation to detect.
On one hand, the lifetime of the hyperfine splitting is quite long, thus
the electrons will just get stuck on the upper level after they fell
down from the 5P state (it's a forbidden transition after all).
A photon to stimulate the fall to the lower state is required, either
provided by spontaneous emission from other Rb atoms or from an external
source. On the other hand, if the photons are not coherent, they will
not build up a signal that one could detect. They will just be spikes
that get burried in noise. Yes, one could build a spectrometer which would
average over a lot of these photons, but that would require a stable
frequency source to be able to avearge for a long time. And I don't think
a simple OCXO is up to this task.
Attila Kinali
[1] "The Optically Pumped Rubidium Maser", by Davidovits and Novik, 1966
http://dx.doi.org/10.1109/PROC.1966.4628
[2] "Experimental Study of the Laser Diode Pumped Rubidium Maser",
by Michaud Tremblay and Tetu, 1991
http://dx.doi.org/10.1109/TIM.1990.1032908
https://arxiv.org/abs/physics/0508227
--
Malek's Law:
Any simple idea will be worded in the most complicated way.
Am 05.11.2016 um 00:45 schrieb David:
On Fri, 04 Nov 2016 14:39:54 -0700, you wrote:
87-Rb has a half life of something like 4.9e10 years you'll be waiting
a while for that strontium. /gp
Various online sources say that natural rubidium is radioactive enough
to fog photographic film in 1 to 2 months but that is also the case
with unprocessed uranium ore so I would not worry about it at all.
That's next to nothing. A friend of mine with interest in minerals found a
piece of Pechblende (Uranit) simply laying on the street near
St.Joachimsthal
where they used to dig for silver over the centuries and after 1945 for
uranium.
We put it on a sheet of Polaroid film for the Tektronix scope cameras and
sure enough, next morning we could see the silhouette of the stone,
completely white, probably way overexposed.
regards, Gerhard
In message 0e976194-3cc1-2bc7-1289-0d9433132d31@rubidium.dyndns.org, Magnus D
anielson writes:
They won't be coherent photons, like in a Hydrogen maser, but we
don't need them to be, in fact that just causes the same exact
problems as the tuned cavity anyway, as long as we can measure
the frequency well enough.
Active maser like the hydrogen would be possible naturally, but would
require the resonator.
I don't think they are.
With hydrogen, hitting the excited electron with the right frequency
increases the probability of coherent emission of the photon enough
that you get to the "SE" we know from LASER and MASER.
As I understand it not all excited modes of all atoms and molecules
have the not-quite-pinned-down quantum-thaumagic property to do that.
And I remember reading somewhere that the alkali atoms have been
poked and prodded to no end about this, in the hope of creating
active Cs, Rb or Sr frequency standards, but the very reluctant
(and expensive) conclusion was that hydrogen is the only one in the
family which knows the trick.
--
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.
On 11/5/16 2:59 AM, Attila Kinali wrote:
But we still need to ensure that the field is properly
oriented and homogenous over the whole vapor cell. For this you need
a cavity that is properly designed and most likely will be resonant at 6.9GHz.
(I don't know whether it is possible to design a non-resonant cavity with
the above properties)
isn't a TEM cell, as used in EMI/EMC testing, something like this? It's
a tapered transmission line and produces a uniform field within an area
inside the line.
On Sat, 05 Nov 2016 12:25:35 +0000
"Poul-Henning Kamp" phk@phk.freebsd.dk wrote:
Active maser like the hydrogen would be possible naturally, but would
require the resonator.
I don't think they are.
They are. It took a while, but they have been a thing since '64.
Though all of them have been using vapor cells.
As I understand it not all excited modes of all atoms and molecules
have the not-quite-pinned-down quantum-thaumagic property to do that.
And I remember reading somewhere that the alkali atoms have been
poked and prodded to no end about this, in the hope of creating
active Cs, Rb or Sr frequency standards, but the very reluctant
(and expensive) conclusion was that hydrogen is the only one in the
family which knows the trick.
Nope, the problem, as far as I understand it, is not that you cannot
get the atoms to emit, but to keep them in one place without perturbing
them. For hydrogen, a teflon coating does a very good job and the atom
can go for many wall collisions without losing its state/phase. Even the
early hydrogen maser got to >10^4 collisions and modern coatings offer
something like 10^6 IIRC, ie the life time is measured in seconds
to minutes.
Until recently, there didn't exist such a coating for Rb or Cs [1] and
relaxation time was measured in milliseconds. For comparison, have a look at
page 37 of Bandi's dissertation[2], the broadening due to wall collisions
is stated as 300Hz for the wall coated, evacuated cell, while the buffer
gas collision broadening is a mere 10Hz (yes, I am cheating here, the
correct comparison would be the total broadening which is 350Hz vs 600Hz).
Yes, the wall shift is smaller than the buffer gas shift (4000Hz vs 300Hz)
but this is in first order approximation a constant.
Additionally, all the coatings have rather low temperature limits
(the hydrocarbon coatings are very similar to parafin wax) and the
high temperatures that were needed for the Rb lamps didn't work togheter.
And even for modern laser pumped systems, you still need a rather high
temperature (40-90°C are usual) to get a high enough amount or Rb in the cell.
I guess you can understand that people have not been using vapor cell
active masers because they do not offer the long term stability one
seeks (due to buffer gas changes) and their short term stability is
not any better than the passive masers (limited by buffer gas relaxation).
Probably, with the new coatings and laser pumping one could build a usable
Rb active maser with evacuated cells instead of buffer gas. But sofar
nobody has tried that yet (as far as I am aware of). I guess it's more
sexy to investigate optical standards than something as old as Rb.
Especially as there are already Rb fountains[3] which offer quite a
high stability already.
Attila Kinali
[1] "Polarized Alkali-Metal Vapor with Minute-Long Transverse
Spin-Relaxation Time", by Balabas, Kraulanov, Ledbetter, Budker, 2010
http://dx.doi.org/10.1103/PhysRevLett.105.070801
[2] "Double-Resonance Studies on Compact, High-performance Rubidium Cell
Frequency Standards", by Thejesh Bandi, 2013
https://doc.rero.ch/record/32317/files/00002318.pdf
[3] "The USNO rubidium fountains", by Peil et al. 2015
http://dx.doi.org/10.1088/1742-6596/723/1/012004
Malek's Law:
Any simple idea will be worded in the most complicated way.
I think this is all described in the 1992 FCS papers,
but the executive summary is that a direct synthesizer
on 9192.63177 is to be avoided at all costs because
of the danger of it leaking into the CBT cavity.
This is also the reason why you don't multiply up
a subharmonic of this frequency.
It would also have made the synthesizer a lot more
complicated. We multiplied 10 to 320 and then used
an SRD to get to 9280 and applied a sideband at 87.36
MHz to it. The 87.36 MHz synthesizer was a phase
locked VCXO using a 5th OT crystal.
It took me a long time for me to sell this to Len Cutler.
It was pretty advanced for 1989.
Rick
On 11/4/2016 7:12 PM, paul swed wrote:
Rick on the pll DRO I agree with you for today.
So is it built for 9180 and then the 12.63 is mixed with it? Or is it
actually a direct PLL precisely at the frequency so not even the
synthesizer is used?
Regards
Paul
WB8TSL
On Sat, 5 Nov 2016 06:53:45 -0700
jimlux jimlux@earthlink.net wrote:
On 11/5/16 2:59 AM, Attila Kinali wrote:
But we still need to ensure that the field is properly
oriented and homogenous over the whole vapor cell. For this you need
a cavity that is properly designed and most likely will be resonant at 6.9GHz.
(I don't know whether it is possible to design a non-resonant cavity with
the above properties)
isn't a TEM cell, as used in EMI/EMC testing, something like this? It's
a tapered transmission line and produces a uniform field within an area
inside the line.
Yes, they fullfill the "constant field" condition, but not the "properly
oriented" condition. The field in these cells has still a quite considerable
curl. One can of course minimize that by building a larger one, but
then there are smaller structures with similar properties, when one does
not need the the wideband property of those TEM cells.
Attila Kinali
--
Malek's Law:
Any simple idea will be worded in the most complicated way.
On 11/5/2016 12:18 AM, Poul-Henning Kamp wrote:
In message 768ee5a7-1c53-06cf-cf36-ec75e290177e@karlquist.com, "Richard (Rick) Karlquist" w
rites:
Reminds me of an interesting Jack Kusters story.
There was some customer who was having problems with
his atomic clocks being noisy (I don't remember exactly
the story) but the bottom line was that they determined
it was because of helium contamination.
How would helium make his clocks noisy ?
Isn't it more likely that it was the alphas from the
radon decay that did it by their charge ?
I probably am not remembering the story exactly, but
I do definitely remember it had to do with unexplained
helium, which they tried to blame Kusters company for.
He said there was a simple explanation, namely radon
gas emitting alpha particles that turn into helium
and helium can diffuse into "sealed" containers.
He basically told them that if they dropped the issue,
it wouldn't be necessary for him to publicize the fact
that their plant was full of radon. It was really
funny when Jack himself told it.
Rick