Time Nuts (2005)

(leapsecond.com)

22 points by jacquesm2 hours ago

5 comments

jacquesm1 hour ago
So, I originally posted this a while back when I was hip-deep in nano-second level time measurement and learning every day how little I knew about this field. We all take for granted that there are a billion nano seconds in a second and that in theory you can count those one-by-one if you have a precise enough oscillator but to actually have stability good enough to track the effects of sub-cycle changes to radio signals (phase changes) is not all that simple. Especially when the measurements themselves run over a day or more.I eventually got it all to work 'in the lab' but nowhere near stable or accurate enough for a real life application. This was all done at very low radio frequencies with super low radiated power (under the license limits) and that of course complicated matters greatly.The big game changer was to get hold of an atomic clock that I could run locally. That takes the sting out of synchronizing with external sources (such synchronization usually involves a PLL and while the long term stability is amazing over the short term it usually is quite terrible [1]) and using a relatively affordable SDR I was finally able to do this with enough precision to be able to differentiate between the ground conducted signal and the 'airwave' for one of the EU atomic clocks (DCF-77). It was really neat to see the effect of the day/night cycle on the propagation of the signal (exactly as predicted) as validation.[1] <a href="https://en.wikipedia.org/wiki/Allan_variance" rel="nofollow">https://en.wikipedia.org/wiki/Allan_variance</a>
- geerlingguy7 minutes ago
 Once you do get measurement down to nanoseconds, you quickly realize how 'bad' so many clocks you thought were good actually are. Lots of GPS modules with massive sawtooth behavior. Over a day they may be within tens of ns. But over a minute, a lot worse!
 - jacquesm3 minutes ago
 Yes, but over a decade they are ridiculously precise.And that's precisely the problem: when you need very short term stability that nicely couples into longer term stability you have your work cut out for you.Another downside of this kind of project is that to collect the data you are always looking at a minimum of 24 hours or so of sample time and at high enough rates that really adds up. Suddenly you need media that are never going to hiccup during long sequential writes and it turns out that's not a simple problem either. Before long you'll be messing around with the buffering deep inside of GNU Radio (which is amazing piece of software by the way).But: when it finally works it is extremely satisfying. To see a clock driving a frequency counter and a non-derived clock that lists 9 zeros with a 100 second sampling window is pretty cool.
aftbit1 hour ago
There's a lot of interesting stuff in the time-nut world. There's a fun mailing list at <a href="https://time-nuts.febo.narkive.com/" rel="nofollow">https://time-nuts.febo.narkive.com/</a> and Jeff Geerling has been doing a lot of stuff with clocks lately on YouTube. An interesting intro project is to set up a GPSDO and track the long-term aging of an OXCO or Rubidium standard (many of which are surplused out of telecom applications for somewhat reasonable prices on eBay).Also vaguely related, the precision clock kit by Mitxela - <a href="https://mitxela.com/shop/clock4" rel="nofollow">https://mitxela.com/shop/clock4</a>Then there's the EFOS3 Hydrogen Maser by <a href="https://www.efos3.com/" rel="nofollow">https://www.efos3.com/</a>There is a very deep rabbithole in here.
helterskelter21 hour ago
Thank you! I've had the idea of aquiring an atomic clock in the back of my head for a while now and I'm glad to see there's a like-minded community out there. I'm not even sure what I'd do with it except measure time dilation effects.
- jacquesm44 minutes ago
  I can recommend Rubidium clocks from decommissioned cell towers. $150 for a refurbished one. Beware seller rep as usual.
VladVladikoff2 hours ago
SSL error when loading this site?
- jacquesm2 hours ago
  They got stuck in time ;) Port 80 exclusively.
hp1972 hours ago
this already is a 20yo site or so. why the hype today?
- dang2 hours ago
 Historical material is always welcome on HN! We try to optiize for intellectual curiosity, not novelty.<a href="https://hn.algolia.com/?dateRange=all&page=0&prefix=true&query=by%3Adang%20welcome%20historical%20material&sort=byDate&type=comment" rel="nofollow">https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...</a><a href="https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sort=byDate&type=comment&query=curiosity%20optimiz%20by:dang" rel="nofollow">https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...</a>
 - hp19756 minutes ago
 maybe I don't understand hackernews well enough than?... I thought it was about news
 - happytoexplain31 minutes ago
 It's not a formal "news outlet". You can see what's on-topic at the top of the guidelines: <a href="https://news.ycombinator.com/newsguidelines.html">https://news.ycombinator.com/newsguidelines.html</a>
 - johnea44 minutes ago
 Unless you were already well acquainted with the time-nuts universe, this is news...
- happytoexplain2 hours ago
 Is this going around in other places today, or does posting something to HN qualify as "hype"?
- gpvos1 hour ago
 With the recent abolition of the leap second, this is as good a time as any.
- alnwlsn2 hours ago
 It was just the correct time.
 - TYPE_FASTER2 hours ago
 I mean, time really is a tough nut to crack.
- johnea2 hours ago
 When is it time to run a world class time reference in your basement?For time nuts, it's always time...If you don't get it, then, you just don't get it...
 - jacquesm9 minutes ago
 There was this hilarious picture of a guy with a full sized atomic clock strapped to his arm. Ultra precise wristwatch. Just in case.