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Narrow pll loop filter

Discussion in 'Electronic Design' started by [email protected], Dec 2, 2007.

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  1. Guest

    Hello,

    I'm trying to design a PLL with narrow loop filter, about 10Hz. The
    most popular (2nd order)
    designs are active PI filter and pasive RC filter, but calculations
    gives me mF capacitors and mOhm
    resistors. I see, that phase shift for DC very low freq. in by PI
    filter is
    about 90 deg. I think that Bessel or Czebyshev (with slowly changing
    phase shift, without instant "phase jump")
    filter would be great, but these filters produces different phase
    shifts for near-DC component and I don't know,
    if phase loop will work. Also filters like MAX280 are very convenient.
    Does anyone know, how to build shuch a filter?



    E.C.
     
  2. What do you mean exactly? Is 10Hz a desired close loop noise bandwidth
    or what?
    Looks like the excessive open loop gain, so the poles have to be located
    at very low frequency.
    Avoid the higher order filters in the PLL unless you really need them.
    It complicates the situation dramatically.
    Try dropping the loop gain.


    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    http://www.abvolt.com
     
  3. Guest

    I want to measure phase noise of generator using low noise VCO tuned
    by PLL (as described in HP/Agilent Product Note 11729B-1 "Phase Noise
    Characterization
    of Microwave Oscillators"). In quadrature, at phase detector output I
    will see sum of two noises: from VCO and DUT (my generator). But
    tracking phase loop
    will suppress phase noise inside loop filter bandwidth, so this
    filter should be very narrow loop (eg. 10Hz or less).
    My generator is quite stable, it drifts very slowly, and I think that
    10Hz will be enougth to hold the lock of tracking loop.

    It is too complicated for me, I'm not familiar with Laplace
    transform :-(
    It sounds very good, I'll build a simple divider on low noise opamp
    with 1/10-1/20 gain.
    At now, VCO gain is about 5e6 Hz/V, D.U.T. works at 80MHz, and Kd is
    0.46 rad/V.

    Thanks,

    E.C.
     
  4. This is a fundamental problem with this sort of PLL.
    There are two parameters you can vary: the time constant of the zero,
    and the gain (which sets the pole locations). You can often do
    something about the gain (by changing the reference and feedback
    dividers, or the phase detector current), but the zero can be a hard
    nut to crack.

    The time constant (i.e. RC product) associated with the zero will need
    to be around 5-10ms to get that closed loop bandwidth, for a
    reasonable damping ratio.

    (For at least part of the output spectrum) the output phase noise is
    determined by the resistor value (from en = sqrt(4kTR) V/sqrt(Hz)), so
    you want to keep the resistor value as low as you can.
    Work backwards from your phase noise specification to get the maximum
    allowable resistor value. If it's 1Mohm, good. If it's 1kohm, you
    might have problems as the cap gets unreasonably large.

    But you also want to avoid large capacitors, as low leakage types get
    expensive and physically large. Al or Ta caps are cheap, but have
    leakage current, which might cause a unacceptably large steady state
    phase error. Hi-K ceramic caps can have microphonics (which may cause
    hard-to-debug phase noise at the PLL output). Low-K cermic caps are
    only available up to a few uF or so, and are expensive.


    I tend to go for passive loop filters (I'm that kind of guy). In the
    past, I've used Wima metalised film caps to 1uF. The very last time I
    designed a PLL (for a SONET clock), I got lazy and just used a 10uF
    X5R cap that was already in the CAD library. It worked fine.

    Regards,
    Allan
     
  5. Howard Swain

    Howard Swain Guest

    Hi E.C.,

    Do you mean your VCO gain is 5 MHz / V ?
    That would be very sensitive.
    I'd think you'd need a crystal oscillator (or something referenced to a crystal)
    for both sources to even think about measuring that close in. But I could be wrong.
     
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