My Stratum-0 Atomic Clock

https://coverclock.blogspot.com/2017/05/my-stratum-0-atomic-clock_9.html

by g0xA52A2A • 3 months ago

88 30 3 months ago

coverclock.blogspot.com

mzs• 3 months ago

I wonder how, if at all, you can improve precision with 4 stratum-1 clocks like he author has.

jacquesm6• 3 months ago

Ensembling clocks allows you to spot 'the odd one out' in case any one of them is having trouble so in that sense it would allow you to improve precision overall (or rather: availability of precision).

JKCalhoun2• 3 months ago

No mention of the cost of the CSAC GPSDO (only that it's "not cheap").

Too bad you couldn't hack the Americium module from a smoke detector and create a DIY atomic oscillator. Cesium seems to be preferred. (And I know nothing about this sort of thing.)

(EDIT: chatting with an LLM… I realize I had assumed that "atomic clocks" meant radioactive and so suggested Americium because it is easy to obtain. LLM schooled me and suggested "Rubidium oscillator modules" instead since they come up for a few hundred dollars or so on eBay. Still not the DIY approach I had hoped for—I think I am still channelling the old "Amateur Scientist" column from Scientific American from the day.)

geerlingguy3• 3 months ago

Typically high hundreds / low thousands for a used GPSDO with a CSAC.

And Americium is not as useful for a timing reference, as it's not as stable as Rubidium and a lot less safe to handle. Otherwise time nuts would hoard cheap smoke detectors :)

JKCalhoun2• 3 months ago

Words of wisdom (from a time nut).

ButlerianJihad• 3 months ago

Spinthariscope https://xkcd.com/2568/

tverbeure• 3 months ago

I have 2 Rb GPSDOs and a few more OCXO ones, but had never heard of CSAC modules and thought that the inevitable next step would be an HP 5061A Cs clock or later model.

So now you have me going to eBay in search one but all it turns up are BM25CSAC carburetors! What are the magic keywords to use in my search?

s0rce• 3 months ago

Americium is also chemically different and may not be as practical as cesium and rubidium to ionize and trap.

crote2• 3 months ago

He seems to be using a GPS-2700[0], which has a price tag of about $5500 / €4700. I reckon you can find a better price if you get very lucky on the second-hand market, though.

[0] https://www.microchip.com/en-us/product/GPS-2700

opello2• 3 months ago

It strikes me as strange that the article links to [1] which appears to be the same board, absent the "Viavi" logo on the main RF can, as the Microchip product you linked. I couldn't tell with a brief look if the Viavi product is offering something like software, configuration, tuning, etc. on top of GPS-2700 product.

The photo of the device on the article says "Jackson Labs" which seems to have been the previous name of "Viavi Solutions" and a review video [2] mentioned using Symmetricom atomic clock modules, which was acquired first by Microsemi (2013) and subsequently Microchip (2018)[3].

[1] https://www.viavisolutions.com/en-us/products/chip-scale-ato...

[2] https://www.youtube.com/watch?v=CogN630jUSs

[3] https://en.wikipedia.org/wiki/Symmetricom

h3lp• 3 months ago

The clocks need precise oscillations to measure time: the mechanical clocks used pendulums and springs; the maser clocks used precise microwave cavities; the atomic clocks that are dominating today use oscillations of electrons in selected atoms.

Americium is not a good atomic clock material---it doesn't have superior electronic transitions, and the nuclear transitions causing its radioactivity would get in the way.

Nuclear oscillations could also be used: there is a proposal to use a low-energy nuclear oscillation in Thorium; it would be more stable than electronic oscillations: https://en.wikipedia.org/wiki/Nuclear_clock

The distinction of what can or cannot be called 'radioactive' is somehow artificial: masers, atoms and nuclei all emit radiation, so they all are technically 'radioactive'. Conventionally, 'radioactive' radiation requires energies that cause ionization of common materials, usually quoted as above 10eV. The Thorium nuclear transition is actually below that, so technically it is not radioactive---but I'd still not want to sit next to such clock without some shielding, because even UV radiation with energies above 3eV is known to damage living tissue.

geerlingguy3• 3 months ago

I don't believe it's necessary to have multiple GPS antennas (one per device), unless signal path redundancy is required. A good GPS distribution box like from Time Machines or GPS Source can split the antenna signal to many devices without an issue.

A signal distribution box used from eBay is a lot cheaper than a good outdoor GPS antenna!

Though if you have enough cable and enough antennas already, no harm in having a little array like in OP.

jiveturkey• 3 months ago

GPS antennas typically need power. GPS receivers expect to be the sole power supplier. Simple, cheap signal distribution won't handle this. There are splitters that can allow one downstream port to supply power, but that fails when that single receiver fails. There are other splitters that are active-active. I guess you're claiming that the boxes from Time Machines are such active-active devices.

Those are not cheaper than an antenna! Although from the photo I'm not sure he bought the cheap antennas.

Speaking from experience deploying stratum-1. Not stratum-0, but the same GPS concerns.

Johnny555• 3 months ago

I don't believe it's necessary to have multiple GPS antennas (one per device), unless signal path redundancy is required

Don't those splitters typically have an amplifier, so become a single point of failure?

If you're going through the trouble to have multiple time servers on your network, you probably want to make sure that an amplifier failure doesn't take them all down at the same time.

nullc• 3 months ago

The antenna has an amplifier, usually with more than enough gain to support downstream splitting. But then sure the antenna is a SPOF.

Also the splitter will pass through the dc-bias on the first port to power the antenna, -- so whatever is hooked there is another spof.

jacquesm6• 3 months ago

Depending on your application a GPSDO may well be the entirely wrong solution depending on lots of implementation details.

driverdan• 3 months ago

(2017)

I've looked at doing this a few times. I don't have references handy but there are cheaper atomic oscillators available now, under $1000. Still to expensive for me to justify it but one of these years I'll find one cheap.

jacquesm6• 3 months ago

Go hunt up a Rubidinium standard on Ebay, $150 or thereabouts and you'll have a pretty good standard.

gh02t2• 3 months ago

I have an NTP clock that uses GPS PPS and has a local TXDO. For any reasonable amount of time that you'd be out of GPS (barring nuclear war) a TXDO should be plenty sufficient for any sane time-related needs at a tiny fraction of the cost. Serving as a frequency reference for radio or precision counters or other semi-exotic (for at home) is the only reason I can see to actually have CSAC other than cool factor. Which, fair.

jacquesm6• 3 months ago

That 'local TXDO' is most likely being governed by some kind of phase lock on the PPS or some other divisor and may well lead to terrible clock jitter in the short term while giving you insane precision over the longer term. Allan deviation plots made by a device clocked by a standard that is better than the one that you are checking is the only way to be sure how good (or bad) things really are.

I have a couple of GPSDOs here and it is fun to play them against each other, the differences in the short term can be substantial, but over anything more than a few days they are extremely accurate.

pseudohadamard• 3 months ago

You can, or used to be able to, buy 5680 rubidium references recycled from cellular base stations for around USD 100, and then get a third-party interface board to deal with the weird voltage and comms requirements. Not sure if they're still available at that price.

jcalvinowens• 3 months ago

If you use the GPIO-based PPS on RT Linux, the kernel will soon get a nice improvement in jitter: https://lore.kernel.org/all/A0383FF7-267A-4D4C-9B74-F80B9908...

I've been testing it and it is noticeably better, but I haven't yet had time to set up an A/B test to get real numbers.

My Stratum-0 Atomic Clock

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