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crystal load capacitor sizing

Discussion in 'Electronic Design' started by Jamie Morken, May 2, 2006.

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  1. Jamie Morken

    Jamie Morken Guest

    For this crystal:

    The crystal is on the right side of the board vertically
    between the two load capacitors, the bottom load capacitors
    trace is highlighted red. The board thickness is 1/16" and
    the trace width is 10mils.


    (digikey part#: SE2413CT-ND)
    (manufacturer part#: FC-135 32.7680KA-A3)

    frequency: 32.768kHz
    load capacitance: 12.5pF

    For selecting the load capacitors to use for the crystal oscillator
    circuit I am using the formula:
    CL = [(CL1 * CL2) / (CL1 + CL2)] + Cstray

    I am wondering what the stray capacitance Cstray is. If I estimate
    2pF for the above circuit then I get:

    [(21*21) / (21+21)] + 2 = 12.5pF

    Also for this crystal:

    C12 and C13 are the load capacitors and the trace width is 10mil
    and the board thickness is 1/16"


    (digikey part#: 300-8206-1-ND)
    (manufacturer part#: ABM3B-16.000MHZ-10-1-U-T)

    frequency: 16MHz
    load capacitance: 10pF

    If Cstray is 2pF, then:
    [(16*16) / (16+16)] + 2 = 10pF

    Would the two above crystal oscillator circuits have about 2pF Cstray
    given the board layout?

    Also if the actual load capacitance is high, the frequency will be
    lower, and if the actual load capacitance is low, the frequency will be
    higher. Is there a formula to give the percent change in frequency for
    a given load capacitance mismatch?

    What kind of error in frequency can I expect if I have an actual load
    capacitance that is 15pF instead of the specified 10pF for the 16MHz

  2. "Jamie Morken"
    None that I know of. It's not that clear cut.
    Probably insignificant. What is the crystal's purpose? If it's
    clocking a microcontroller, I wouldn't worry too much about it since you
    are probably only talking about a few Hz difference. If it's in a model
    rocket, you may want to consider a ceramic resonator instead of a
    crystal for durability reasons. One thing you don't want is too little
    capacitance or you'll have start-up/stability problems. Waking up from
    SLEEP mode can be a killer too. It's easy to start an oscillator with
    power application, but when a micro is sleeping it can be difficult to
    cause enough imbalance to start the oscillator going. In some cases you
    might need to have one cap at a different value to help.

    As far as the clock crystal goes, what kind of long term accuracy are
    you needing? If you are worried about individual pF of load
    capacitance, you are only talking about perhaps a 1 or 2 seconds/week
    change in accuracy. Temperature changes will probably have a larger
    effect. If you need extreme accuracy, then use a trimmer cap.
  3. Tim Shoppa

    Tim Shoppa Guest

    For an AT cut it'll probably be in the very low tens of ppm. In the
    total error budget (initial frequency offset, temp variations, etc.)
    this isn't entirely in the noise. But that's a good thing: it's why a
    trimmer cap gives you enough control to set the frequency exactly.

    All of what you said about parallel load caps and strays is pretty much
    on target, BUT in the real world the capacitor values are usually
    subject to the constraint that they ensure rapid startup of the
    oscillator and that the oscillator run stably and with a "good"
    amplitude (not too high to avoid damage to the crystal or accidentally
    run it in an overtone mode, but high enough that the microprocessor
    actually clocks). If it so happens that this is somewhere near the
    crystal's load capacitance, that's great.

    Actually ensuring reliable crystal oscillator startup and operation
    over a wide temp/Vcc/process variation range is not generally an easy

  4. Wes Stewart

    Wes Stewart Guest

    If you characterize the crystal(s) to determine the motional
    parameters then you can calculate the effects.
  5. Tim Wescott

    Tim Wescott Guest

    If you had to get reliable startup characteristics as well as accuracy
    you could probably change the distribution of the two capacitances --
    but it'll be work.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See
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