Thursday, March 3, 2011

Capacitor selection for a Crystal Oscillator

A typical crystal oscillator circuit and an example layout design are shown in the figures. A crystal manufacturer normally specifies the load capacitance, CL, needed for a crystal in its datasheet. This value is the load seen by the crystal which is in series with C1 and C2, including all parasites. Typically, overall stray capacitance, Cstray, for pin, board, and PCB trace is between 2 pF and 7 pF. The relation between them can be represented by the following formula.

CL=(C1 * C2)/(C1 + C2) + Cstray

C1 and C2 are normally chosen to have equal value. Therefore,

C1 = C2=2 * (CL - Cstray)

For example, assume that Cstray is 4 pF and crystal load capacitance CL is 18 pF, C1 = C2= 28 pF. The normalized value of 30 pF can be used.

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