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How to Accurately Measure External Oscillator

Discussion in 'Microcontrollers' started by jpanhalt, Sep 17, 2015.

  1. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    I understand Diver300 The pull range has everything to do with the Q of the Crystal and 900 ppm off is not possible with Q of 10k or 1/100ppm.

    There is a relationship between SRF and PRF in every crystal.

    We often say SRF is Self Resonance Frequency as this is most common, but it should be called Series Resonant Frequency , SRF ( for all caps , series tuned crystals, dielectrics, insulators, or any material between two conductors, even a loop antenna)

    Then we have Parallel Resonant Frequency, (PRF) ( for all inductors, par. tuned crystals, wires, antenna ) which is sometimes called-"anti-resonance" by resonator experts.
    Crystal angles are cut by seconds which is 1/3600 of a degree and this affects the 3rd order shape of all AT cut crystals You can predict the SRF from PRF and visa-versa from the values below which also happens to control the Q of the resonator. Cpar is in pf, while Cser is in fentofards, fF
    • Cser: 1~50 fF (femtofarad = 10^-15 F)
    • L: 5~50 mH typ
    • R: 10~100Ω typ
    • Cpar: <10 pF typ
    [​IMG]
    [​IMG]


    Typically SRF quality factors are specified by R values and PRF are defined by external C to get specified tolerance.


    And that's why I think Mr AI's crystal was actually a SRF or series Xtal but tested as a PRF (Colpitts mode) which tends to have the offset he noted. ( but this is just a forensic guess)
     

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