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Frequency Standard - Rubidium or GPS?

Discussion in 'Electronic Design' started by bart, Sep 2, 2009.

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  1. bart

    bart Guest

    Hi all,

    I'm looking for some sort of frequency standard (10 MHz ref.?).

    Ebay has rubdium frequency standards for under $100.00
    and there are also GPS disciplined OXCO frequency standards for
    ~$150.00 .

    My question :
    Is a 10-20 year old rubidium standard more accurate (even with aging
    drift) that a newer GPS disciplined OXCO?

    I can't afford a GPS corrected rubidium standard ( ~$700.00+).

    I just want to recalibrate my so-so frequency counters .. to hopefully
    within 10 Hz..?

    Opinions?

    Thanks for reading! :)

    Cheers!
    Bart
     
  2. Jim Yanik

    Jim Yanik Guest

    I'm surprised no one has cooked up an inexpensive WWVB receiver/freq
    reference,for hobbyist uses.I used a Spectracom WWVB unit while at TEK.
    Of course,that had a chart recorder to track the ref.osc. drift.
     
  3. GregS

    GregS Guest

    I used to look at the time changing from the Colorado station
    compairing it to the stations Cesium standard at Goldstone Apollo.
    That was for getting the right second. Ground Loran D was the prime
    day to day reference. That was tricky and not very good anyway.
    The station had Rubidium and Crystal backups, with the last being wristwatch.
    When i first started working there there was a morse code chart on the
    receiver console ?? I love backups. I really hated hearing the Sonalert
    go off from the Collins time machine and figure out whats wrong.
    That was when sonalerts were not common, but when I went
    to McDonalds and that thing went off, I would jump.
    I still have that frequency counter I calibrated from the Collins equipment,
    and I added a crystal heater before that. Cheap counter at best. I'm sure the crystal aged.
    The hams used to use the color subcarrier back then for calibrations broadcasted live.


    greg
     
  4. Rich Grise

    Rich Grise Guest

    Well, for what it's worth, my $10.99 plus tax wall clock is sync'ed to
    WWVB; I'm sure there's a 1 Hz pulse in there somewhere. :)

    I'd imagine it would make a pretty good reference if you wanted to wait,
    say, 60 seconds or so to get a good reading.

    Cheers!
    Rich
     
  5. spamme0

    spamme0 Guest

    I'd like to see the math on how much precision you're gonna get out
    of sixty samples of a 1hz clock from a $11 wall clock.
     
  6. The rubidium oscillator will likely give you the best short term stability,
    which is really what you are after for your purpose.
    If you get a rubidium oscillator (or any other reference source), you'd
    simply leave it permanately connected to the external 10MHz reference input
    on your counter. No need "recalibrate" and use your internal crystal any
    more.
    Rubidium aging would fairly negligable for your purposes (in the order of
    10^-10/year IIRC)

    Dave.
     
  7. Martin Brown

    Martin Brown Guest

    There are several off air frequency standard designs that fairly
    regularly get recycled in the various magazines. A bit like egg timers.
    In the UK at least it is trivial to take a long wave 200kHz coil and
    slug it with a bit more capacitance to tune for either 60kHz Rugby or
    77kHz DCF77. Although monolithic chips are cheaply available a discrete
    receiver is easy enough - even the original transistor radio would do.

    A reasonably helpful generic page is online at:
    http://www.timetools.co.uk/support/dcf-signal-coverage.htm

    Using the Rugby off air signal to synchronise local Rb oscillators was
    used in the late 1970's by Duffet-Smith et al to provide local
    oscillators for remote VLBI stations at low frequencies.

    ISTR they could easily detect the presence or abscence of dew at the
    Rugby transmitter from the diurnal variation of phase errors.

    Regards,
    Martin Brown
     
  8. Rich Grise

    Rich Grise Guest

    Just because it's cheap doesn't automatically make it crap. It keeps time
    as well as any clock I've ever seen, and presumably synchronizes itself
    to the WWVB signal; I could put it on my scope/freq. counter, but I'm not
    that ambitious. It was just an idea.

    Thanks,
    Rich
     
  9. GregS

    GregS Guest

    Something about 100 ms accuracy.

    Back again to the Goldstone Apollo station. In back of the Collins timing system
    was something that was not used anymore when I started working there.
    A Marantz Model 9 mono tube amplifier to drive wall clocks ! You still had
    to set the clock manually.

    greg
     
  10. spamme0

    spamme0 Guest

    I never implied that the clock was crap.
    I implied that your concept of using it to calibrate a counter in a
    minute was crap.

    Sixty seconds of a one hz signal is...wait...let me do the math...
    oh, yes, it's 60 counts.

    You are NOT gonna get 10^-6 accuracy out of counting sixty counts...ever.

    But wait...my counter has a time interval measurement.
    Ok, what's the risetime of the signal, jitter, accuracy of
    the time-interval function as specified by the counter manufacturer.
    There's a reason you don't try to calibrate a counter's reference
    in time-interval mode. Averaging over 60 clocks helps by a factor of 8
    or so on a good day for random errors. Doesn't help much on
    systematic errors.

    Assuming the average accuracy is good, there are ways to
    generate a stable reference, but not in 60 counts.

    I just wanted to see some math supporting your assertion.

    It keeps time
     
  11. nospam

    nospam Guest

    Risetime? jitter? wtf. If you get a stable reading it is a stable reading.
    Crude count the 10MHz reference clock period measurement gives you 1 part
    in 10 million resolution for a 1Hz signal.

    I had no problem calibrating my counter/timer from a GPS pps output. Get as
    close as you can with period measurement then scope the counter 10MHz
    reference triggered by the pps. Trim for zero drift. The limiting factor
    was resolution of the reference trimmer not jitter or observation of drift.
    --
     
  12. spamme0

    spamme0 Guest

    Well...it's been 30 years since I was the hardware design manager for a
    counter company....and I like to learn new things.
    While what you say is spot-on theoretically, there are practical issues
    to deal with.

    My input was related to:
    _______________________
    I just want to recalibrate my so-so frequency counters
    and
    Well, for what it's worth, my $10.99 plus tax wall clock is sync'ed to
    WWVB; I'm sure there's a 1 Hz pulse in there somewhere. :)

    I'd imagine it would make a pretty good reference if you wanted to wait,
    say, 60 seconds or so to get a good reading.
    _______________________

    I asked about the math that allowed the specified source to calibrate
    the specified counters....vague as those specs are.


    So, the first question to you is,
    "with your method, how many reference frequencies exist that produce
    a zero drift display on your scope?"
    How do you tell which is the right one?
     
  13. nospam

    nospam Guest

    As many as there are Hz in the counter reference trimming range.
    You measure the pps period to get 'on' the right Hz first, you can measure
    period after to confirm.

    --
     
  14. spamme0

    spamme0 Guest

    Got it...
    Assume the counter can measure period with more resolution
    than is implied by the reference clock rate.

    I use the period function to measure the 1Hz. and adjust the 10MHz
    oscillator
    to better than a part in 10^-7.
    Then I set my oscilloscope sweep speed to 1uS/division...oughta be
    able to
    easily resolve 10-cycles/division...and trigger the scope on the 1Hz.
    I get a display that sweeps once/second and I dial it right in.
    I just tried it...
    I don't think the trigger jitter spec on my scope is anywhere near
    a part in 10^7, but it don't matter, cause I couldn't see anything.
    Your oscilloscope has a LOT better writing rate than mine!!

    While your approach is theoretically sound, I maintain that the
    average person with average equipment can't take a so-so counter,
    a $11 wwvb-disciplined wall clock and (assuming they even have) an
    average oscilloscope
    to implement it in SIXTY seconds or so of measurement time as
    suggested by the original suggestion.

    The devil is in the details.
     
  15. nospam

    nospam Guest

    Digital scope, you are only looking at 10MHz doesn't have to be a fast one.
    Jitter from the trigger and pps wasn't enough to cause any problem
    observing drift. The pps jitters was maybe 30 or 40ns, and it was 'zero' or
    a whole 30 or 40ns, obviously synced to some about 30MHz clock in the GPS
    module.
    Pulling a pps signal from an old hand held GPS and using test equipment I
    already had let me calibrate my counter/timer with no cost.

    I doubt a wwvb wall clock would contain any useful signal. I doubt they
    lock on to anything, just occasionally correct their time from the time
    code signal.
    --
     

  16. Here's a nifty method using a VNA for phase comparison of oscillators:

    http://www.kolumbus.fi/michael.fletcher/frequency_error_using_vna.htm
     
  17. Rich Grise

    Rich Grise Guest

    OK, let it run all day or all week - eventually you'll get a usable
    reading.

    My point was merely that it's doable, if you want to wait long enough. :)

    Thanks,
    Rich
     
  18. JosephKK

    JosephKK Guest

    The last i heard the GPS sats had cesium and rubidium clocks, not
    hydrogen. Where do you get hydrogen frequency references?
     
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