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Frequency synthesizer-VCO

Discussion in 'Electronic Basics' started by thejim, Feb 1, 2006.

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  1. thejim

    thejim Guest

    I am reading this time about a frequency synthesizer with a VCO.
    What i am trying to understand is the use of a programmable counter.
    In the book it says that it divides the VCO frequency down to the
    reference frequency.
    Which is the ref. frequency?
  2. Hi,
    The reference frequency is the one used to compare the output
    from the divided or mixed-down VCO against, so as to control a
    synthesizer's output frequency.

    At the heart of many a synthesiser is the phase-locked loop
    (PLL) which employs a 'phase comparator'. This is a device that
    acts to make its two inputs, the reference and the comparator, be
    in phase with each other. Remember, if two signals are in phase at
    all times, it must be that they have the same frequency which is
    the whole point of the circuit.

    If one of those signals is obtained from a source called the
    'reference' and the other from a VCO, then the output of the phase
    comparator (a DC voltage when locked) may be used to drive the VCO
    frequency so that the system locks making F(ref) and F(comp) the

    The reference can be from a crystal oscillator source in the
    case of a straight synthesizer or, when the PLL is being used as a
    demodulator or filter, another signal of varying frequency which
    may contain some modulation. But that is another story.

    Cheers - Joe
  3. Hi,
    That should read -

    ........................... Remember, if the two signals differ by
    the same phase angle at all times,...............

    Cheers - Joe
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