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Date：Dec 02,2022
In the usual oscillating circuit, since the crystal oscillator is used within the range of inductive reactance, the oscillating circuit operates as capacitive reactance. That is, when looking at the oscillating circuit from both sides of the crystal oscillator, the oscillating circuit is represented by a series circuit of negative resistance Ri and load capacitance CL. This capacitance value is the total capacitance on the circuit including the circuit wiring capacitance and the unit capacitance, and the total capacitance at this time is called the load capacitance CL, which is an important parameter that determines the oscillation frequency.
Frequency change amount;Frequency variable sensitivity
C0: Parallel capacitance of the crystal oscillator; C1: Series capacitance of the crystal oscillator; CL: Load capacitance
r: Capacitance ratio of the crystal oscillator; ΔfL = fL-fr; FL: Oscillation frequency under load; fr: Series resonance frequency
DL = ΔFL / fr = (FL-fr) / fr; DL is called load frequency compensation, so by differentiating formula (1) with C2, it becomes formula (2).
As shown in the diagram, the oscillation frequency varies with the load capacitance, so when the load capacitance decreases, the frequency change is greater; when the load capacitance increases, the frequency change becomes smaller. In order to expand the range of frequency variation, it is easy to be affected by small changes in the circuit (such as fluctuations in component electrical characteristics, temperature, aging, etc.) if the load capacitance of the oscillation circuit is reduced. This is the main reason for the deterioration of frequency stability. Therefore, it is important to set appropriate conditions for the final application when high frequency stability or frequency modulation is required.
In addition, the recommended standard load capacitance value by the manufacturer's instructions should be used for the load capacitance. To obtain the desired frequency and stability in the installed state of the oscillator, it is necessary to flexibly utilize the characteristics of the crystal oscillator during the design process.
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