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Phase Margin Question for Phase Locked Loops

Discussion in 'Electronic Design' started by [email protected], Jan 1, 2005.

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  1. Tim Wescott

    Tim Wescott Guest

    Then why do you need any help?

    If the circuit provides negative feedback at DC and has insufficient
    phase margin at some frequency, then slapping in an inverter will give
    you positive feedback, with some _very_ peculiar results. If you're
    using an XOR or other mixer for the phase detector then it'll invert
    automatically, and you won't be changing the gain/phase relationship of
    the loop.
    No, a few more than that: Passive, single ended filters. Active filters
    using op-amps, inverting, non-inverting, and double to single-ended.
    Closed-loop control systems using several different types of
    microprocessors, DSPs and DSPs roped to FPGAs.

    I haven't used pneumatics, mechanical amplifiers, magnetic amplifiers or
    dynamotors yet, but you never know.
    From what population of PLLs are you drawing this "most"? If you're
    working with the current crop of PLL-based synthesizers-on-a-chip then
    using an inverting filter means you have to reverse the sense of the VCO
    command/frequency relationship, or you need to flip a bit in the phase
    detector someplace. If you're using an XOR phase detector then the
    point is moot, of course.
  2. Guest

    You mean the magnitude of [GH] through unity.

  3. Guest

    I most certainly do. See Barkhausen in my
    original post.

    An inverter between the
    Well, we have two integrators in this case, the
    op-amp with capacitive feedback and the VCO.

  4. Guest

    We would like to improve the
    current design, if we could. But
    it certainly already works. We
    would like to omit some fast acquisition
    1N4148 back-to-back diodes that we have
    in parallel to the output resistor, as
    they supposedly distort our audio modulation
    at low freq. However, the lock-up time
    is slower without them, so we'd like to
    adjust the poles and zeros for faster
    lock without them, without losing too much
    phase margin/stability.

    Well, that was my orginal question,
    do all phase detectors, whether XOR or
    phase/freq, have the 180 degree phase shift
    that you need. Apparently, yes.

  5. Andrew Holme

    Andrew Holme Guest

    Yes. I forgot the N. I was thinking of a loop I did with N=1. I actually
    prefer to use G(s) for open-loop gain and H(s) for closed loop gain; so,
    personally, I would write it as:

    G(s) = kPD * A*(s+B)/s * kVCO/s * 1/N

    You say in another post that you want to improve the lock time. I recently
    designed a loop that had to lock very quickly. I found it useful to do
    Bode plots and step response using SCILAB.
  6. Guest

    Most of the literature uses G(s) for the forward gain
    and H(s) for the feedback path.

    But, to each their own.

    What sort of phase margin did you shoot for?
    And what type of filter did you use?

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