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Mixing pll singals with high freqs.

Discussion in 'Electronic Design' started by [email protected], Jun 15, 2007.

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    I have a problem with understanding some designs in phase locked
    loops. I want to stabilize signal from some kind of electooptical
    oscillator; it works like a normal VCO, with frequency about 87MHz and
    Kvco~=50Hz/V. This signal may be mixed in DBM/phase detector with
    reference signal and output after filtering in low pass filter is used
    to tune/detune VCO. This can be done also on higher harmonic (and we
    get lower phase noise).

    In some publications I saw a little bit advanced scheme: 10th
    harmonic from electooptical oscillator (870MHz) is mixed in DBM with
    high frequency signal from synthesizer (about 880MHz), difference
    signal is filtered in low pass filter and then is mixed with reference
    10MHz signal in phase detector.
    And another scenario: signal from electooptical oscillator (120 MHz)
    is mixed with 1GHz then filtered with band-pass filter for 1120MHz,
    divided by 256 in prescaler and mixed in digital phase detector with
    reference frequency. Why signal is mixed with high frequency, but
    detection in phase detector is made on low frequency? Because diode
    phase detectors don't work on high frequencies?

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