PLL Reference Signal

Discussion in 'Embedded Systems and Microcontrollers' started by bobo01, Apr 19, 2013.

  1. bobo01

    Thread Starter New Member

    Apr 17, 2013
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    Hello,

    I am trying to understand the concept of a PLL. I understand that it essentially locks a VCO to a reference signal. I don't understand what this reference signal is and where it comes from? Why does the reference signal maintain the needed frequency?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. bobo01

    Thread Starter New Member

    Apr 17, 2013
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    Short answer I found from there is:

    "A high stability oscillator like a crystal oscillator etc. are used."

    That is what I assumed, but then why use a PLL at all. Is it not better to use a crystal oscillator and a program counter?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    With a PLL you also have the posibility to multiply the frequency of the reference frequency.
    With an oscilator and a counter, you can only go down in frequency.

    Bertus
     
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  5. bobo01

    Thread Starter New Member

    Apr 17, 2013
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    Hello,

    So, that VCO, is esentially a frequency synthesizer? And it needs that reference crystal as the voltage may vary too much?

    Thanks.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The VCO is part of the synthesizer.
    Take a look at the attached PDF's.

    Bertus
     
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  7. MrChips

    Moderator

    Oct 2, 2009
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    My ARM mcu is clocked at a rock-stable 168MHz but it uses an 8MHz xtal.
    That is an example of what a PLL can do.
     
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