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A question of PLL

Discussion in 'CAD' started by cici, Nov 13, 2003.

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  1. cici

    cici Guest

    The output of my lowpass filter is a dc voltage,but the value is in
    range of the control voltage of my VCO which is between 0 and 1v. How
    pull down the dc voltage? I tried to use diode, not work.
    Another thing is that when I connect the output of my VCO as the
    feedback signal, the dc votage of output of LPF is changed(when I used
    two independent
    input at the PD, it is normal). what is the reason for it?
  2. Could you provide less information, please?
  3. Normally you will not have a DC output from a filter, unless you are making
    a power supply!
    You should use AC coupling- a small capacitor inline will work usually.


    Chip Shults
    My robotics, space and CGI web page -
  4. Fred Bloggs

    Fred Bloggs Guest

    PLL usually means Phase-Lock Loop and the low-pass filter exists to
    extract the DC component of phase-detector output- and ONLY the DC
  5. This depends on your definition of DC. If, by DC, you mean a signal
    that varies (with falling energy content as frequency increases) but
    is unidirectional, then I agree. But if by DC you mean and an
    unvarying, unidirectional signal, then I don't.
  6. Fred Bloggs

    Fred Bloggs Guest

    It will be unvarying and unidirectional in lock:)
  7. red rover

    red rover Guest

    Assuming what you are locking to is unvarying.

  8. And that the whole system is without drift. but then you don't need a
  9. Only "instantaneously"
    A PLL can be used to demodulate FM. FM is continuously varying and so
    therefore will the "DC" control voltage, i.e. the control voltage is
    indeed AC.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    Understanding, is itself an emotion, i.e. a feeling.
    Emotions or feelings can only be "understood" by
    consciousness. "Understanding" consciousness can
    therefore only be understood by consciousness itself,
    therefore the "hard problem" of consciousness, is
    intrinsically unsolvable.

    Physics is proven incomplete, that is, no
    understanding of the parts of a system can
    explain all aspects of the whole of such system.
  10. I read in that red rover
    [Top-posted response re-positioned.]

    More significantly, assuming that what you are *locking* is unvarying,
    which is normally not the case.
  11. Fred Bloggs

    Fred Bloggs Guest

    Okay- put it this way, the LPF is attempting to output a constant DC:)
  12. Fred Bloggs

    Fred Bloggs Guest

    Actually, these days, the majority of applications for PLL have to be
    clock recovery, for digital and/or serial comm apps, and communications
    frequency synthesis, by sheer bulk- and it is locking to a constant
    reference- the LPF is usually the PI(proportional-integral) active
    filter with infinite gain at DC.
  13. What about clock multipliers (frequency synthesizers)? There are
    a few billion of these made each year. Ok, many of these are
    modulated for spread-spectrum, but...
  14. Well, the the control voltage serves to shift the oscillation
    frequency. If the reference has a fractional frequency offset from the
    nominal VCO frequency, the filter corrects this with a (somewhat) DC
    value. When you connect two independant references to the phase
    detector, there is no feedback in the system. If there is a frequency
    offset between the two reference the phase between them will be
    constantly drifting and either saturate the PD or lead to some sort of
    sawtooth in the phase detector output.
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