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General crystal/PLL question

Discussion in 'Electronic Design' started by [email protected], Apr 18, 2006.

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  1. Guest

    In the days of the first color NTSC TVs, a common practice was to drive
    the color crystal directly with the incoming colorburst and then let
    the crystal coast for the rest of the line.
    My questions are...
    How exactly did they drive the crystal? Was there something special
    about those crystals that let them be driven? (Those cans were HUGE
    back then, was it a different material, cut, etc?)
    What if I took a modern hc-49 crystal and set it up in the classic
    logic inverter oscillator circuit, but instead of an inverter I took a
    NOR gate and periodically pulsed the other input of the NOR gate with a
    narrow sync signal?
    Would the crystal eventually get 'locked' to the phase of the incoming
  2. Mark

    Mark Guest

    The color burst signal(burst of 3.58 MHz was applied to the crystal.
    No nothing special.
    Not sure what you mean by "sync signal". Yes it will work if you
    pulse it with a 3.58 MHz bust., The crystal will "ring" in phase with
    the applied burst.

  3. Guest

    Hi Mark,
    Thanks for the reply. I was typing out loud that day. I remembered
    later that I actually have the books from the 50s about color TV. Turns
    out the crystals *were* specially designed to have a lower Q than
    typical crystals, and that they were used in a ringing circuit, not a
    true oscillator AFAIK. They just whapped it with the colorburst, and
    then sent the decaying signal through a limiter, and this was enough to
    generate the color reference.

    As for the sync, I was referring to a horizontal sync pulse from a
    computer. Let's say the computer uses a 16MHz clock and I want to
    re-create this pixel clock only having access to the HSYNC. Typically a
    PLL system will try to lock a VCO with feedback from a divider,
    suitably programmed with the division ratio of 1016 in this case.

    I just wanted to avoid using the counter altogether if I could somehow
    just use the rising edge of the sync and see if I have a zero-crossing
    on my LO. Turns out this is a lot more work that just using a regular
    video genlock PLL.
  4. joseph2k

    joseph2k Guest

    So sorry sirs, the horizontal sync pulse and "back porch", where the color
    burst is, was used a "gated" phase comparison in an otherwise standard
    phase locked loop, which used a sample and hold and a reactance tube to
    tune the "VCXO". (1960's) An early sampling control system if you will.
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