Crystal Oscillator Reference Circuit

  
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This is an illustrate a reference design for a crystal oscillator using the LMK04000 family. This oscillator circuit is implemented on the LMK04000 family evaluation board. This is the figure of the circuit; The tuning diode is a Skyworks model SMV1249-074LF. It is a dual diode package in a common cathode arrangement. This diode configuration has
Crystal Oscillator Reference Circuit - schematic

the advantage of blocking reverse leakage current. The loop filter components are chosen to achieve a narrow loop bandwidth, in the range of 10 Hz to 20 Hz. The actual component values will depend on the parameters chosen for PLL1 (R divider and N divider values) and the slope of the crystal tuning curve. An initial estimate of the slope of the tuning curve can be developed using Equation (3) and the appropriate parameters from the crystal data sheet. Also note that Equation (4) should be substituted for CL in Equation (3). A nominal value of 6 pF can be used for CIN. A reasonable range of values that can be used for CSTRAY is 2 pF to 4 pF. After the actual oscillator circuit has been implemented on a printed circuit board, the tuning curve should be measured to verify that the original estimate of the tuning slope was correct. Any significant difference may require recalculation of the loop filter components to maintain the desired loop bandwidth.



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