Everybody with a GPS-disciplined oscillator has access to time and frequency from the Naval Observatory at the sub-100 ns level, optionally augmented to +/- 1 ns with reasonably affordable gear like https://www.sparkfun.com/sparkpnt-gnss-disciplined-oscillato... .
A fountain clock is on a whole different level than any of these. The same researchers who build fountains also work on even better optical lattice clocks, none of which you can buy from Sparkfun. These are research tools that don't have a market, at least not yet.
The SI second definition will likely move from Cs-133 at 9 GHz to Sr-87 at 400 THz before too long ( https://www.nist.gov/si-redefinition/second-future ), but that probably won't shake up the existing market too much.
This is because the motion of celestial bodies and spacecraft is dominated by gravitational forces which depend only on G·M, and that motion can be measured extremely accurately with e.g. Doppler radar.
TIL. I guess maybe that explains why the second is used as the base of the SI (https://en.wikipedia.org/wiki/2019_revision_of_the_SI) post-2019, if my understanding of it is correct?
https://www.iflscience.com/this-incredible-islamic-fountain-...
Something like that still around, and/or available? Any updated designs?
Personally I have no need for ultra-accurate timekeeping. But hey... an atomic clock is way cooler than a Nixie clock or oven-controlled Xtal oscillator. And no... huge 2nd hand atomic clock found on eBay etc doesn't cut it. Too big /heavy / power-hungry.
https://www.nist.gov/noac/technology/time-and-frequency/chip...
https://spectrum.ieee.org/chipscale-atomic-clock
Microchip launched their latest version earlier this year:
https://www.electronicspecifier.com/products/frequency-contr...
Spectratime's version (mRO-50) went into a $2M piece of eye-candy that mechanically syncs a watch.
https://www.urwerk.com/collections/ur-chronometry/amc
https://www.urwerk.com/sites/default/files/press/docs/urwerk_amc_eng.pdf[1] https://www.microchip.com/en-us/products/clock-and-timing/co...
[2] https://www.teledyne-si.com/en-us/Products-and-Services_/Pag...
If you're interested in a small atomic clock and don't absolutely require the very low power consumption of the SA.45s and the very small package size you can get better performance, reliability, and cost in something a big larger and quite a bit higher power consumption.
The CSAC improved atomic oscillators a lot more in power than size... but its timekeeping performance is so/so as far as atomic clocks go.
In particular, the surplus market has a lot of telecom rubidium that can be had quite inexpensively.
I'm personally a fan of the PRS-10 (which also exists in a benchtop form, the SRS725). They seem to regularly sell on ebay for about $300 with a little breakout board for power. And unlike many small rubidiums they have very good phase noise. You can sync them to GPS time using a 1pps input (though I believe on the bare modules the 1pps sync is optional so if you want to use it be sure to get one that has it).
Timekeeping of atomic clocks is still waaaayy better than next-best technologies (like temp-compensated Xtal oscillators). And size/power/$ constraints matter. So CSACs for the win imho.
There is also, of course, the issue of infrastructure dependence. Particularly since wireless telephony has moved almost exclusively to GNSS time we're going to have a really bad time if kessler syndrome takes out the GNSS satellites.
Edit: Here is an example adev chart for a inexpensive atomic clock vs what appears to be a pretty good timing GPS receiver: https://www.thinksrs.com/images/instr/prs10/PRS10diag2LG.gif
So in that case the GPS accuracy only beats out the free running atomic clock at intervals greater than 200,000 seconds or so.
Here is a collection of older timing receivers: http://www.leapsecond.com/pages/3gps/gps-adev-mdev.gif
A quick look didn't turn up any cheap non-timing receivers, but my experience is they're pretty bad (I mean relative to atomic standards, of course).
There are better timing receivers than the ones charted above, of course, but they are not $5. Their cost is now in the same general ballpark as surplus atomic clocks.
Of course, if you have both you can sync one to the other with whatever time constant maximizes the composite performance and have the best of both.
(and primary atomic clocks don't have this drift issue, but sadly the days of the occasional sub $1000 5071 showing up on auction sites seem to be over. :P )
I searched on Mouser and found that part no longer has a listed price; there are fancier chip-scale atomic clocks from the same company, the only one with a listed price going for $3,528.23.
Others from a different manufacturer are just over $2,000. A kind of awesome thing is that one of those chips is marked
Frequency 10.000000 MHz
It's awesome that that's not just an estimate or some kind of exaggeration!
Not sure how good/useful/not broken they are since I've never had one, but over the last 10 years I've seen them in a good number of hobbyist projects.
Or you could just go for GPS. That's technically still atomic clocks, but in space!
https://www.nist.gov/news-events/news/1999/12/nist-f1-cesium...
Earlier this year there was a big leap in so-called "nuclear clocks" which uses the resonant frequency of energy states of a nucleus itself as opposed to electron orbitals around it. Besides the "more frequency = more better" factor that has always driven clock accuracy -- thorium-229 nuclei excites in ultraviolet wavelengths -- nuclear clocks are better isolated than electron orbital-based clocks because the frequency band where they interact is impossibly narrow. In fact, the reason why it was only recently demonstrated is due to the difficulty of producing the required frequency at a high enough precision to interact reliably. This could lead to more accurate and more compact and cheaper clocks.
Discussion 4 months ago: https://news.ycombinator.com/item?id=42362215 | Major Leap for Nuclear Clock Paves Way for Ultraprecise Timekeeping (nist.gov)
Note that this is distinct from syncing from GPS, which is a thing people obviously do too, but CVTT can achieve much higher accuracy.
Because you're synchronizing extremely stable clocks the difference between them will primary be an offset (plus/minus a slope from relativistic effects of different altitude). Because of this you can average a large number of readings, so the only major source of error will be systematic effects in propagation/orbit/etc.
Historically, sync was obtained via traveling clocks-- e.g. you sync one atomic clock up and load it, running, in a station wagon... which is the same thing that is most often done for voltage standards today (as atomic voltage standards remain rare, compared to atomic clocks-- I think the least I've paid for one is $15 excluding the ones that were free).
But vibration isn't great for anything with a crystal oscillator in it, and the most modern atomic fountain clocks don't work if they're accelerating in any direction except the designed 'up' direction (gravity), because the little cloud of cooled atoms will fall out of the measurement channel, which makes sync by station wagon not viable.
Of course, once you talk about syncing there is always a question of what you're syncing to. UTC doesn't exist until after the fact. Laboratories measure their offsets via CVTT and UTC is calculated after the fact as past offsets to each of the contributing clocks.
I've also been reading about nuclear clocks[1]... skipping over the uncertainty of the entire atom's chaotic oscillations entirely!
throw0101a•9mo ago
Presumably there's diminishing returns, but as the article says we're at one part in 2.2e-16, are there practical application of going further?
0_____0•9mo ago
maxnoe•9mo ago
https://www.nist.gov/si-redefinition/second-future
slowmovintarget•9mo ago
When Einstein made his thought experiments with light clocks and described what should be observed, he didn't think we'd actually make light clocks... but we have the technology to do that literally (really, truly, actually... so sad we no longer have a word in English that means that).
[1] https://www.youtube.com/watch?v=y5V9qAETcw8