6 ms·

NIST-F1 Cesium Fountain Atomic Clock

64 points by fzliu 5y ago

lend000 5y ago
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?
- jacksonkmarley 5y ago
  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.
- adrian_b 5y ago
  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.
adhesive_wombat 5y ago
Cesium fountains are such a crazy idea that sound like they're some fevered sci-fi concept.
- adrian_b 5y ago
  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 5y ago
    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 5y ago
      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 5y ago
    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.
a9h74j 5y ago
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-references/3824-chip-scale-atomic-clock-csac https://www.microsemi.com/product-directory/clocks-frequency... 10^-11 short-term stability; about 10^-9/month drift
- traceroute66 5y ago
  > 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-appliance/ https://engineering.fb.com/2021/08/11/open-source/time-appli...
- segfaultbuserr 5y ago
  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/133269-5071a-datasheet https://www.microsemi.com/document-portal/doc_download/13326...
phkahler 5y ago
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.
- mikewarot 5y ago
  The HP-5061 Cesium Beam clock (I've repaired a few) frequency modulates the sample frequency just a little bit, using a 137 hertz sine wave. It then compares the phase of the output with the frequency shift. If the output goes up in phase with the rise in frequency, the frequency is too low. If the output goes down in phase with the rise in frequency, the frequency is too high. This signal is used to very slowly pull the 5 Mhz local oscillator into lock with the bandpass of the Cesium beam. At that point, the output dips on both excursions from zero, a 274 hz "second harmonic" is detected, and this turns on the "Lock" light, letting the user know the standard is now on frequency. [Edit/More Info] Here's a link to the Operating and Repair Manual.[1] (PDF) There is a simplified block diagram on page 35 of the PDF. (Figure 4-1) 1 - http://www.leapsecond.com/museum/hp5061a/5061a.pdf http://www.leapsecond.com/museum/hp5061a/5061a.pdf
- qwezxcrty 5y ago
  In physics experiments, applying some dithering and demodulate to generate a error signal zeroing at zero derivative is a standard technique.
JumpCrisscross 5y ago
Would there be merit to putting a cesium fountain in zero g? Or would relativistic wobble neutralise the benefit of longer observation periods?
- mikewarot 5y ago
  There is no place in the universe with Zero g. Free fall (orbit) around the earth is still subject to most of the gravity you feel on the ground.
  - JumpCrisscross 5y ago
    > no place in the universe with Zero g…free fall Which is what physicists call zero g. For purposes of this application, that practical zero g (versus the red herring you constructed) is the phenomenon of value, as it would permit suspending the cesium atom in the fountain.
codezero 5y ago
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.
- stephen_g 5y ago
  Much more accurate optical lattice clocks definitely are affected by gravity - scientists have determined hight by measuring the difference between clocks at different elevations, linked by optical fibre [1], and that article also makes reference to atomic clocks that have been transported by plane having been observed to have recorded slightly different amounts of elapsed time than ones that stayed on the ground (as is expected). I assume it is affected to some extent by the gravitational force of the moon and the sun, but I expect that would be harder to measure. 1. https://www.sciencedaily.com/releases/2013/10/131002103036.htm https://www.sciencedaily.com/releases/2013/10/131002103036.h...
wrmcgehee 5y ago
For anyone with even a passing interest in frequency standards, I recommend watching this recent seminar by Bill Phillips on recent changes to the SI. Bill provides some interesting history on atomic clocks, context on why Cs was chosen, and perspective on the remaining problems with the definition of the second. https://www.youtube.com/watch?v=9okt-3xLbH8 https://www.youtube.com/watch?v=9okt-3xLbH8

64 points by fzliu 5y ago

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?

jacksonkmarley 5y ago
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.
adrian_b 5y ago
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.

adhesive_wombat 5y ago

Cesium fountains are such a crazy idea that sound like they're some fevered sci-fi concept.

adrian_b 5y ago
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 5y ago
  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 5y ago
    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 5y ago
  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.

a9h74j 5y ago

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-references/3824-chip-scale-atomic-clock-csac https://www.microsemi.com/product-directory/clocks-frequency... 10^-11 short-term stability; about 10^-9/month drift

traceroute66 5y ago
> 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-appliance/ https://engineering.fb.com/2021/08/11/open-source/time-appli...
segfaultbuserr 5y ago
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/133269-5071a-datasheet https://www.microsemi.com/document-portal/doc_download/13326...

phkahler 5y ago

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.

mikewarot 5y ago
The HP-5061 Cesium Beam clock (I've repaired a few) frequency modulates the sample frequency just a little bit, using a 137 hertz sine wave. It then compares the phase of the output with the frequency shift. If the output goes up in phase with the rise in frequency, the frequency is too low. If the output goes down in phase with the rise in frequency, the frequency is too high. This signal is used to very slowly pull the 5 Mhz local oscillator into lock with the bandpass of the Cesium beam. At that point, the output dips on both excursions from zero, a 274 hz "second harmonic" is detected, and this turns on the "Lock" light, letting the user know the standard is now on frequency. [Edit/More Info] Here's a link to the Operating and Repair Manual.[1] (PDF) There is a simplified block diagram on page 35 of the PDF. (Figure 4-1) 1 - http://www.leapsecond.com/museum/hp5061a/5061a.pdf http://www.leapsecond.com/museum/hp5061a/5061a.pdf
qwezxcrty 5y ago
In physics experiments, applying some dithering and demodulate to generate a error signal zeroing at zero derivative is a standard technique.

JumpCrisscross 5y ago

Would there be merit to putting a cesium fountain in zero g? Or would relativistic wobble neutralise the benefit of longer observation periods?

mikewarot 5y ago
There is no place in the universe with Zero g. Free fall (orbit) around the earth is still subject to most of the gravity you feel on the ground.
- JumpCrisscross 5y ago
  > no place in the universe with Zero g…free fall Which is what physicists call zero g. For purposes of this application, that practical zero g (versus the red herring you constructed) is the phenomenon of value, as it would permit suspending the cesium atom in the fountain.

codezero 5y ago

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.

stephen_g 5y ago
Much more accurate optical lattice clocks definitely are affected by gravity - scientists have determined hight by measuring the difference between clocks at different elevations, linked by optical fibre [1], and that article also makes reference to atomic clocks that have been transported by plane having been observed to have recorded slightly different amounts of elapsed time than ones that stayed on the ground (as is expected). I assume it is affected to some extent by the gravitational force of the moon and the sun, but I expect that would be harder to measure. 1. https://www.sciencedaily.com/releases/2013/10/131002103036.htm https://www.sciencedaily.com/releases/2013/10/131002103036.h...

wrmcgehee 5y ago

For anyone with even a passing interest in frequency standards, I recommend watching this recent seminar by Bill Phillips on recent changes to the SI. Bill provides some interesting history on atomic clocks, context on why Cs was chosen, and perspective on the remaining problems with the definition of the second. https://www.youtube.com/watch?v=9okt-3xLbH8 https://www.youtube.com/watch?v=9okt-3xLbH8