Computing stuff tied to the physical world

Rubidium Clock – part 2

In Hardware on Mar 11, 2012 at 00:01

After yesterday’s intro of my “get your own atomic clock”, which is really just doodling, here’s the next step:

DSC 2952

The clock, and the PCB panel it came attached to, has been placed in an all-plastic enclosure along with a little 15V @ 1.7A switching power supply. This thing needs quite a bit of power and actually gets quite hot. Nevertheless, I expect that placing it inside this relatively small plastic enclosure will not be a problem because much of the heat seems to be generated simply to keep the Rubidium “physics package” inside at a certain fixed temperature. For that same reason, I suspect that the heat sink on which this clock is mounted is not so much meant to draw heat away, but to maintain a stable temperature and improve stability.

Speaking of stability… here are the specs of this unit from eBay:

Screen Shot 2012 03 10 at 13 26 51

To get an idea: 10 to the power -11 frequency stability is less than 0.3 milliseconds per year error!

This particular unit (they are not all identical, even when called “FE-5680A”) also needs a 5V logic supply.

I haven’t yet decided how to bring out various signals, so I’ll hook up the 50Ω BNC connector on the back first and wait with the rest. Also needed: a LED power-on light, LED indicators for the “output valid” and “1 pulse-per-second” signals (via a one-shot to extend the 1 µs second pulse), and a 7805 regulator. Here’s the front – so far:

DSC 2953

I don’t intend to keep this energy-drain running at all times, but it’ll be there at the flick of a switch to generate a stable 10 MHz signal when needed. One of the things you can do with it is calibrate other clocks, and compare their accuracy + drift over time and temperature.

Geeky stuff. For a lot more info about precise time and frequency tracking, see the Time Nuts web site.

Tomorrow, I’ll describe some of the trade-offs w.r.t. time for JeeNodes and wireless sensors.

  1. Note that the output is not really stable at the moment the “lock” light comes on (although more than good enough for most purposes). The best stability comes after several hours, when everything has reached thermal equilibrium. Also, the accuracy can very likely be improved by comparing with GPS time over several days, and applying the needed frequency offset correction using the RS-232 port commands (see the online FE-5680A FAQ ). For ultimate in stability, control the case temperature to within 0.1 C (a thermostatically controlled cooling fan can do this, in a reasonably stable ambient).

    • Thanks – yes, I’ve seen some info about this (and your notes, isn’t the web a fantastic resource!). I may at some point go after that GPS sync (I think Everyday Practical Electronics once published a circuit for that – can’t seem to find it right now), but for now anything well below 1 ppm is plenty for me. The lure of infinitely accurate clocks hasn’t escaped me (and the exponential amount of effort needed to get there…) but for now I just want to have a solid baseline to compare crystals against.

  2. Yes, using it as a GPSDO gives you the best long-term accuracy, but I meant just a manual one-time frequency offset correction after watching the drift rate vs. a GPS receiver 1 PPS output for a while. This would improve your absolute accuracy, but not short- or long-term stability. These units often are not well calibrated as you receive them, although if only <1 ppm is your goal, they’re most likely OK as-is.

    • I’ll see what I can do. I do admit that I’m curious about the actual accuracy. In Europe there’s also the DCF77 radio frequency standard – an article in Elektor Jan 2012 has a PLL-synced receiver with which the author claims ±250µs accuracy per day with proper adjustment. That’s 3 ppb, if my math is correct.

  3. I’ll confess to falling for cool gadget desire and looking on ebay. All the listed Rubidium clocks are in China, which does make me a little worried. I know there are plenty of perfectly honest ebay members in China, but I’ve also got that horrible little voice in my head reminding me there are also a far few less honest who wouldn’t think twice about getting a metal box and label fabricated and sticking a 10Mhz crystal inside, especially for a £50 price tag.

    Okay, yes, I admit it, I’m a paranoid Englishman who looks at foreign places with suspicion ;-)

    I’ve seen quite a few on ebay which look very similar to the one you and our favourite excitable Australian have shown, but there are others where subtle things, like the labels, just look “different”.

    So are there any pointer to sourcing a genuine good one?

  4. The “time-nuts” mailing list http://www.mail-archive.com/time-nuts@febo.com/info.html members have probably accounted for over 100 used FE-5680A units, all from China, and no one has reported getting a fake. Some of them did not work upon arrival; a few got fixed, and some were returned & replaced. You’re probably ok if you buy from someone with 99%+ positive rating and who has already sold several of them. I purchased 3 from “2010bluebook”, 2 working OK, one would not lock, but I fixed it. By the way prices on these have gone way up in the past few months.

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