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Yet another PLL question

Discussion in 'Electronic Design' started by martin griffith, May 5, 2006.

  1. Ok, found this,
    http://www.rt66.com/~shera/
    and this is the info
    http://www.rt66.com/~shera/QST_GPS.pdf
    Its seems to be a highly recommended diy thingummy.

    It's just out of interest, I'm not trying to build a MP3 player locked
    to GPS.

    My PLL knowledge is not very good, just sort of 4046 simple stuff.

    I can't figure out why he uses a separate xtal (via U7) for doing the
    counting. Dither comes to mind

    In one para first column bottom of P2
    "If this count is constant, we know the phase difference is constant,
    that our LO is synchronized to the GPS "

    OK if the count is constant, its locked, but can it be locked to a
    close frequency. i.e if the count is always say 999, its locked, but
    if the count is always 1000, its also locked, but at a different
    frequency. So it can be locked , but at not any specific frequency.

    Or is it just me being a potential gmail account holder?

    Any comment on the article in general, apart from using a PIC?


    martin
     
  2. What I got from a cursory reading was that the count represents a phase
    difference, not a frequency difference. The local oscillator is phase
    locked to the satellite. If the count is steadily increasing or
    decreasing, there is a very small average frequency difference, which,
    if the circuit maintains lock, will be corrected.
     
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