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Looping Binary Counter, Stable 32768 oscillator

Discussion in 'General Electronics' started by GMV, Jul 29, 2005.

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  1. GMV

    GMV Guest

    I am in need of a circuit that will place minute mark
    ,second 59 or second zero of UTC, into my homebuilt circuit
    for calibration purposes. I have thought of possibly building
    a 32768Hz oscillator and using a looping binary counter that can be
    calibrated using WWV or WWVB but this all sounds silly to me
    since there must be calibration clocks already out there.
    I need either one with a TTL output or even better an FET switch or something
    like that with a mark I can feed directly into the back end of my
    circuit just before the signal hits the A/D converter.
    The stability of the oscillator would be most important
    not to deviate more than +/-0.03 Hz.
    My laptop has a RTC that looses one second of time every four point five hours
    so it is completely unsuitable for anything other than getting the time
    close enough to understand what the calibration mark means.

    Could someone who is an engineer provide me with
    a schematic to do this ?
    The basic idea seems simple for those educated in the EE field.
    It is the stable oscillator that eludes me.

    Any help here is appreciated.
  2. Dan Hollands

    Dan Hollands Guest

  3. GMV

    GMV Guest

    I think you might be from another country so i just
    want to say here in the usa we have a time signal at
    60KHz that keeps cheap maybe $20.00 clocks in sync
    with the atomic one in colorado and if only some
    genius would include an output on the back of one of those
    clocks for second zero from the beginning to end of
    second zero on the minute mark UTC that is all any amateur should need.
    The GPS alternative is maybe 10 times more expensive then
    necessary if you live in the USA.
    But thanks for the input i will download and read it even
    though I am pretty sure they are not producing the device I seek.
    The accuracy of the time loop I am talking about will be set manually
    or electrically by listening to WWV or WWVB broadcasts.
    We are not talking millisends here but only +/- 0.2 seconds or
    the human reaction time.

  4. Charles Jean

    Charles Jean Guest

    Looks like you are after about 1 ppm stability. Dalis/Maxim makes the
    DS32kHz chip which is a temperature compensated crystal oscillater.
    Don't know if this will work for you, but you might want to take a
    look at the data sheet. With a couple of counter chips and this chip
    you could get a 1 pulse/min output. The specs say frequency
    stablility from 0-40 deg C is +/- 2ppm. Several years ago I built a
    clock circuit from this chip, set it to WWV time, and measured its
    time vs WWV over a period of 300 days. It gradually and linearly
    gained time over WWV, and the total time gained over WWV was 13.7
    seconds, or about 0.7 ppm. This was done inside the house, so the
    ambient temp over this period probably varied +/- 4-5 deg F. Don't
    know if this meets your requirements, since you didn't mention ambient
    temp, but thought I'd post this in case you could use it. If you like,
    I can send you the circuit and data.

    "Sic hoc legere scis nimium eruditionis habes."
    (If you can read this, you're overeducated.)
  5. You might see if you can pick up a HeathKit GC-1000 cheap off eBay. The GC-1000
    synchronizes the local oscillator with WWV signals at 5, 10 or 15 MHz and has a
    3.6 MHz TTL output. There was an option for outputting an ASCII time stream on
    an RS-232 port, too. With an external antenna in the attic, my GC-1000 stays
    "synchronized" to WWV most of the time and the few times I've checked the output
    with a frequency counter has been dead on 3.6 MHz.

    Then all you would need is a simple TTL counter chain to divide by 3,600,000 and
    you have a 1 pps signal.
  6. GMV

    GMV Guest

    Yes By all means send me your circuit.
    I am interested in a +/- 0.2 second
    error over a three day period at about
    81F to 83F range I think there is a peltier
    device that might be used to warm the crystal
    using a PID loop and keep its temp pretty
    steady at say 98.6 degrees.
    You could cool it but that means dew
    which is tripleuncool.
    You are thinking much like I do here.
    Yes I am interested in seeing your design.

    I have just purchased a 141KIT general timer with a
    12MHz clock that I hope to modify into a looping
    counter but the darn thing does not provide
    capacitors for adjusting the frequency of the clock
    so i have my doubts about my ability to calibrate
    the frequency. I intend to use my SW receiver
    to verify its operation close to the correct freq.

    I have no ham license to experiment with AC circuits
    so I must be careful not to break transmitting laws when
    dealing with these oscillators.

  7. GMV

    GMV Guest

    Sounds like a good idea but at 4 bits per counter thats a lot of counters to use.
    It would be so nice to find a single chip that would be a 20 bit counter but I know of none.
    I am wary of sending 3.6MHz any distance because
    of possible RFI generation.

  8. RFI shouldn't be a problem since the GC-1000 feeds the 3.6 MHz out via a BNC
    connector so you would be using coaxial cable to connect it to your counter

    You are correct that you would need several stages of dividers if you use
    standard TTL or CMOS logic parts. To be more precise, you would need two
    divide-by-2 stages, two divide-by-3 stages and five divide-by-ten stages. While
    you could do this with a bunch of 4-Bit binary counter stages, I would probably
    use some of the devices that provide two decade counters in one package. IIRC,
    there are also counters that provide a divide-by-12 counter in a single package.

    To me, the better way of doing this would be to use a CPLD programmed to do all
    the dividing. I would probably use something like the Xilinx XC9536-15PC44C
    ($3.30 from DigiKey) which has more than enough macrocells and I/O pins for
    something this simple. The Xilinx ISE WebPACK is free for download and can be
    used to develop the logic and program the device.
  9. GMV

    GMV Guest

    Thanks Mr. White but I just bought one of these
    for a bit over $30 and intend to calibrate the
    crystal if possible to within +/- 9 Hz to get
    the precision and accuracy I am looking for.

    That should point you to the info.
    The only bad thing about the kit is it gives you no means
    by which to adjust the clock.

    People all over the world in industry are
    terrible about calibration.

    You would think they would want to be proud of their work.

    Not much more effort and you can adjust the clock
    but no one is designing things like that anymore.

    A shortwave radio with a BFO circuit works quite well as a
    freq counter for these single tone oscillators.
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