[HN Gopher] NIST-F1 Cesium Fountain Atomic Clock
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NIST-F1 Cesium Fountain Atomic Clock
Author : mrintellectual
Score : 43 points
Date : 2022-04-02 07:16 UTC (1 days ago)
(HTM) web link (www.nist.gov)
(TXT) w3m dump (www.nist.gov)
| adhesive_wombat wrote:
| Cesium fountains are such a crazy idea that sound like they're
| some fevered sci-fi concept.
| adrian_b wrote:
| Cesium fountains were a great advance, but some time during the
| next decade they will become completely obsolete.
|
| The optical atomic clocks have already much better
| performances, but they are not mature enough to replace cesium
| fountain clocks, because they still cannot operate continuously
| for long times and they are hard to transport for now.
|
| As soon as the optical atomic clocks will become more rugged
| and reliable, the cesium fountain atomic clocks will become
| obsolete.
|
| Only the miniature cesium atomic clocks, like those made by
| Microchip, will remain useful for a longer time, because much
| more years will be needed until the optical atomic clocks will
| become so small and cheap (i.e. a few thousands dollars like
| the miniature Cs clocks).
| nullc wrote:
| The CSAC has found a lot of use in applications where just a
| high performance OCXO would be fine-- but large OCXOs draw a
| low of power and particularly in low power mode the CSAC
| doesn't. I wouldn't be too shocked to see improved MEMS
| oscillators displacing CSACs in applications ahead of future
| miniature optical clocks.
| adrian_b wrote:
| True.
|
| If the miniature Cs clocks would not have been at least 10
| times more expensive than OCXOs (several thousands $ vs.
| several hundreds $), they could have replaced them in
| almost all their applications.
| adhesive_wombat wrote:
| You know it's the future when collecting and tossing a ball
| of supercooled gas up a tube and catching it again using
| lasers and measuring time with it to a part per ten
| quadrillion is the old and busted method.
| adrian_b wrote:
| While the fountain clocks use atoms that fall slowly in a
| chamber, the optical clocks use ions (a single ion or more
| ions) or neutral atoms (usually a relatively large number
| disposed regularly in a so-called lattice) which are
| trapped in fixed positions in vacuum, using various
| combinations of lasers and electromagnetic fields.
|
| So the control over the ions or atoms is even more advanced
| than in fountains, because they stay fixed (except for
| thermal vibrations) like in a solid, even if they are
| widely spaced in vacuum.
|
| The reason why a real solid is not used is that the atoms
| or ions are too near one from the other in a solid, and
| their interactions modify the atomic resonance frequencies.
|
| When a kind of artificial solid is made, where the atoms or
| ions stay in vacuum, in fixed positions, but which are much
| more widely separated than in a natural solid, the
| interactions between the atoms or ions can be minimized.
| Because the atoms or ions are cooled to very low
| temperatures, their movements are reduced to relatively
| slow vibrations, so there are no large frequency deviations
| caused by the Doppler effect, like when the atoms or ions
| are free to move in a gas.
| phkahler wrote:
| It seems like a challenge to tune anything to the exact resonant
| frequency of the caesium, since it's an attempt to find a maximum
| output. My first thought was to take multiple measurements at
| different frequencies and curve fit the response to get a
| maximum. But then you'd have to somehow compensate for
| differences among sources and maybe detectors.
|
| Hill climbing to the top is challenging.
| JumpCrisscross wrote:
| Would there be merit to putting a cesium fountain in zero g? Or
| would relativistic wobble neutralise the benefit of longer
| observation periods?
| a9h74j wrote:
| This might be common knowledge, but it was a recent TIL for me.
| _Chip scale atomic clock_ :
|
| https://www.microsemi.com/product-directory/clocks-frequency...
|
| 10^-11 short-term stability; about 10^-9/month drift
| segfaultbuserr wrote:
| I'm also sharing some common knowledge among electronics
| enthusiasts, but other readers may find it interesting: besides
| these custom top performance ones in physics labs or miniature
| chip-scale ones for embedded applications, it's worth pointing
| out that "regular" cesium atomic clocks are readily available
| as standard commercial products, anyone with enough cash can
| just purchase it today and mount it on a rack in your server
| room tomorrow.
|
| The workhorse in the industry is the Hewlett-Packard^W
| Agilent^W Symmetricom^W Microsemi^W... oh I meant the
| _Microchip 5071A Cesium Clock Primary Frequency Standard_. [1]
| The TAI & UTC time are literally powered by these clocks -
| more than half of the atomic clocks in national standard labs
| are Microchip 5071As.
|
| Also, if much lower performance is acceptable, a rubidium
| frequency standard is extremely affordable. You can get second-
| hand modules (usually retired from base stations) for just
| $200. All they do is outputting an extremely accurate 10 MHz
| reference frequency, you can use a frequency divider to get an
| 1 PPS signal and drive a mechanical clock, or connect it to
| your oscilloscopes, spectrum analyzers, or frequency counters
| as an external timebase, or time a digital clock using a
| microcontroller, many interesting possibilities.
|
| And of course, if you have access to an outdoor radio antenna,
| you can outsource the task of generating an atomic-accurate
| frequency to a government's shortwave radio station or GPS,
| your tax dollar does the rest.
|
| [1] https://www.microsemi.com/document-
| portal/doc_download/13326...
| nullc wrote:
| The rise of GPS clocks has really killed the commercial
| market for cesium beam clocks-- the 5071 is a design over
| twenty five years old. It's a fine piece of engineering (I
| have three, two I obtained broken and repaired) but its age
| is starting to show, including the fact that it is
| phenomenally expensive to get replacement tubes for (the
| tubes are limited life).
|
| Similar is true for rubidium standards, though there are some
| somewhat more modern models-- though since they aren't
| primary standards most places that use them will still use
| GPS to keep them on frequency. A primary standard like a 5071
| can internally compensate for every major systematic effect
| and so they can autonomously derive the second without
| external calibration. Telecom rb's can't self-calibrate for
| their gas cell pressure.
|
| All this has lead to a worrisome dependence on GPS just as a
| precise source of frequency.
|
| Hopefully in the long run we'll see single chip optical
| clocks with GPS-clock beating performance at competitive
| prices-- if they got even close to GPS they would rapidly
| displace it, as the need to put up an antenna for GPS is a
| real nuisance, and GPS jammers are a sadly too common source
| of trouble.
| wbl wrote:
| It's a pain to get the top of second calibrated on a
| ceasium beam clock.
| traceroute66 wrote:
| > This might be common knowledge, but it was a recent TIL for
| me. Chip scale atomic clock
|
| Old news indeed.
|
| Facebook have been using it in production for a while now[1]
| and released all the details under the Open Compute Time
| Appliances Project.
|
| [1] https://engineering.fb.com/2021/08/11/open-source/time-
| appli...
| lend000 wrote:
| Nice description. I didn't know atomic clocks just sampled Cesium
| atoms for a second at a time every once in a while -- I always
| thought they were somehow continuously amplifying that
| oscillation. So in reality, they are just recalibrating a more
| traditional circuit at regular intervals, which serves as the
| clock for some digital circuit?
| adrian_b wrote:
| Most atomic clocks work so, i.e. they serve only to calibrate
| periodically an oscillator, which is the only one which matters
| for measuring the time during short times, e.g. under an hour.
|
| Many atomic clocks use just quartz oscillators (high quality
| Oven Controlled Crystal Oscillators).
|
| The more expensive atomic clocks use either dielectric
| resonators (e.g. sapphire resonators), or superconducting
| cavities (e.g. from niobium or lead) or optical resonators
| (e.g. single-crystal silicon or germanium resonators for
| infrared light) which are cooled to very low temperatures
| (cryogenic resonators), to ensure much higher quality factors
| than possible with quartz resonators (the oscillators with
| cryogenic resonators can also have a better short-term
| stability than the active hydrogen masers).
|
| The atomic clocks that produce continuous oscillations, so that
| they do not need another oscillator, are active masers or
| lasers. While there are many experimental types, the only
| commercial type are the active hydrogen masers.
|
| The cheaper hydrogen masers are passive hydrogen masers, which
| are also used only to calibrate periodically another type of
| oscillator.
| jacksonkmarley wrote:
| Usually something with good short-term stability, e.g. a
| hydrogen maser, generates a steady signal that can then be
| measured against a frequency reference such as a caesium
| fountain, and the output can be adjusted to correct for
| frequency drift.
| codezero wrote:
| Does the gravity force affect measurement? I'm curious if there
| is a tidal effect that can be observed from the various locations
| of the Moon and Sun.
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(page generated 2022-04-03 23:00 UTC)