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crystal oscillator frequency

Discussion in 'Electronic Design' started by Johnny Chang, Feb 11, 2008.

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  1. Johnny Chang

    Johnny Chang Guest

    can a crystal's oscillating frequency be changed by changing the
    capacitance across the crystal to something other than what the
    manufacturer specified?

    will it be stable?
     
  2. Yes if it is used in parallel resonance mode. You can then theoritically
    tune it down to the serial resonance frequency (a variation of a couple of
    KHz for a MHz range crystal) by increasing the loading capacitor. No (or
    very little) if it is used at its serial resonance. But 99.9% of the
    CMos-based crustal oscillators are parallel mode.
    No problem.

    Friendly yours,
    Robert
    www.alciom.com
    The mixed signal experts
     
  3. Fred Bloggs

    Fred Bloggs Guest

    Yes but it cannot change much, 500ppm is considered a large deviation
    for AT and SC cut crystals.
     
  4. Leon

    Leon Guest

    You can also use a series inductor to tune it above the marked
    frequency. 'Pulling' crystals like this is a technique often used by
    radio amateurs to cover part of a band with a single crystal.

    Leon
    --
    Leon Heller
    Amateur radio call-sign G1HSM
    Yaesu FT-817ND transceiver
    Suzuki SV1000S motorcycle

    http://www.geocities.com/leon_heller
     
  5. MooseFET

    MooseFET Guest

    You can also shift it by putting a capacitor in series with the
    crystal. This leads to a better oscillator in most cases. The
    circuit looks like this:

    Y1
    C3 !-!
    From amplifying device -------+-----!!----! !----+--- To amplifying
    device
    ! !-! !
    === C1 === C2
    ! !
    GND GND

    The ratio of C1/C2 is constant so that amplifier needs a nearly
    constant gain. Changing C3 changes the total capacitance seen by the
    crystal.
    "No problem" may be the wrong answer here. How stable do you need the
    resulting oscillator to be? When you pull a crystal, you do suffer
    some reduction in stablity because the capacitor's value matters more.
     
  6. Phil Hobbs

    Phil Hobbs Guest

    You have to be really really careful about that, though...if the
    inductor is too large, the tank will resonate with the crystal being
    capacitive. In general there are two frequencies where a given crystal
    will have a given capacitive reactance. Also the crystal doesn't have
    nearly as high a rate of change of reactance with frequency out there,
    so the resulting Q is much lower.

    Cheers,

    Phil Hobbs
     
  7. Joop

    Joop Guest

    A series inductor tunes it DOWN in frequency. A series capacitor tunes
    it up.

    One group that explored the obtainable range describe their stuff
    here:
    http://www.amqrp.org/projects/NB6M contestvxos/VXO Experiments.html
    For more examples search for "wide range VXO".

    Cheers,

    Joop
     
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