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GPS timebase signal

  1. Aug 4, 2007 #1
    GPS modules produce a very accurate 1 second timebase signal, rms differency of about 20nS. How do you suggest of using that to split it into a 1000 pieces = 1000Hz = 1000ms ?

    My idea is to make a generator with a quartz chrystal and drive it in to a counter module, carefully selected divide count would produce 1000kHz signal. And after a 1000 pulses(1000ms), the timebase signal will reset the counter into zero and the counting starts over. On counter there should not be a drift no more than 1 mS in a second, otherways we will miss one millisecond. And drifts less than 1ms/s is not inportant as its being calibrated away on every second. This 1000Hz signal is led to a microcontroller.

    Second idea is to use a microcontroller to make the counting. uP's are very precise on shorter timings, but they tend to have drift on longer periods. GPS timebase signal could be used to calibrate the software to run precisely on time by giving a correction pulse on every second. So it would go like this:
    uP will start counting only when it gets the timebase signal edge from the GPS. When uP reaches the count 1000, it will start to wait another pulse from the GPS and so on.

    However, if the GPS is blocked someway what should i do then?, counter should not stop in any circumstances and it should not miss any time.
    Should i run two timing clocks, other is driven by GPS and other one is driven by internal timings of uP. If GPS fails, the internal timing is valid, and the GPS clock is rejected. If GPS comes online again in some point, uP clock is copied to GPS clock to have the same time and we start of using GPS time again. Even that the uP timing is less accurate on longer periods than GPS, it still is better to have less accurate time than no time at all.

    TechSpec
     
    Last edited: Aug 4, 2007
  2. jcsd
  3. Aug 5, 2007 #2

    AlephZero

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    Rather than decoding GPS, a simpler solution could be to use a radio controlled clock. See http://tf.nist.gov/stations/radioclocks.htm (and similar transmissions exist in the rest of the world).

    AFAIK the radio transmitters for these clocks are also designed to be very stable frequency sources. Generating a 1 kHz signal from the transmitted 60kHz carrier using a frequency divider will be a lot easier than trying to stablize a 1 kHz oscillator with a 1 Hz frequency standard.

    You can get a receiver modules to build into your own equipment, if you don't want a consumer-style complete clock.

    These systems do have downtime for scheduled maintenence (those in Europe do, anyway) so if you need a continuous timing signal you will need a backup clock as well. Remember that even a $5 digital watch is an amazingly accurate and stable device considering how little it costs - especially if you run it from a stabilized power supply and keep it at near-constant temperature.
     
  4. Aug 10, 2007 #3
    Well, i consider GPS to be a harder task, but if we think that equipments should work anywhere in the world, i prefer GPS instead of radioclocks. Radioclocks seem to have different frequencies and different range, and even different protocols. And what i have found in my work and in generally when using these equipments, i would say that these radioclock-devices dont work as good as they should. Idea is great to have well penetrating signal that actually works inside buildings (limited), but these radioclock modules are usually cheap consumer products (poor quality), and very hard to find.

    If i weight their plusses and minuses, i would say that GPS has
    + global range
    + equal protocol, 1s timebase
    + easy of integration
    + relatively cheap modules, if also GPS functionality is used
    + good overall quality

    - range inside buildings
    - prize, if clock alone is used
    - power consumption

    radioclocks

    + range inside buildings
    + cheap
    + power consumption

    - frequency variations on different continents
    - cheap and poor quality, no industrial grade products (if found, not cheap)
    - protocol varies
    - no global signal
    - range is relatively poor, or focused on certain areas

    If we consider good/bad balance between these two, i see more plusses on gps. Offcourse the usage of these both techniques depend of what is the product. Both techniques has their own benefits/losses and their own applications.
    But gps is a product that fill my needs and its weaknesses are acceptable.

    TechSpec
     
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