Crystal oscillator impedance

Discussion in 'The Projects Forum' started by simo_x, May 12, 2011.

  1. simo_x

    Thread Starter Member

    Dec 23, 2010
    200
    6
    Hi all, I have this crystal oscillator
    [​IMG]
    which works fine.
    The circuit is taken from the TX-2B datasheet at page 9 first circuit.
    The oscillator works fine, but I am interested in calculate his total impedance.
    If I am not wrong, in a first moment the crystal is pushed by the current coming from R1 and then is pulled back by the L1C2 network.

    As I can see from the crystal's datasheet used, between 27 & 30Mhz in 3rd overtone the ESR of the crystal is about 150 Ohm.

    If the 27MHz frequency is applied to L1 & C1, I obtain the following impedance,

    XL = 2π · 27MHz · 2,2uH = 373,22 Ohm
    XC = 1 / [ 2π · 27MHz · 80pF ] = 73,68 Ohm

    and adding the Crystal ESR, I obtain the following resistive network

    [​IMG]

    but I am not sure it is correct.. Maybe (or off course) the situation is more complex and, as I expect, I am out of way.
    Can you help me to determine the total circuit impedance please?

    The real circuit, with power supply of approximately 9.19V consumes about 29.2 mA.

    Regards,
    Simon
     
  2. simo_x

    Thread Starter Member

    Dec 23, 2010
    200
    6
    Hi, someone can help me here, please?
    Thank You.
    Simo
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Truth, I've built my share of crystal oscillators, and have never worried about it. If you want isolation put a CMOS gate between the oscillator and the load. The imput of the gate is extremely high impedance, the ouput medium to low impedance.

    Why is this an issue?
     
  4. simo_x

    Thread Starter Member

    Dec 23, 2010
    200
    6
    Hi Bill, it is my interest determine the impedance of the oscillator to understand better how it works..
    Regards,
    Simo
     
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