How to Select External Capacitors for Passive Crystal Oscillators？
Firstly, it should be noted that any crystal oscillator that requires an external capacitor is a passive crystal oscillator. Additionally, note that distinguishing between external capacitors C1 and C2 (ground capacitance, ground capacitance) is not equivalent to crystal oscillator load capacitance (CL). C1 and C2 are the total capacitance values of the circuit board and components including IC and crystal oscillator.
In general, increasing the external capacitance of a passive crystal oscillator will cause a decrease in the oscillation frequency of the crystal oscillator, which is biased toward the negative direction.
For example, if the external capacitance of a passive crystal oscillator at 12MHz is 18PF, the actual output frequency is 12.002876MHz. After changing the external capacitance to 27PF, the actual output frequency of the crystal oscillator will decrease, such as 11.9998923MHz.
The larger the external capacitance of the crystal oscillator, the more stable the oscillation of the crystal oscillator, but it will increase the oscillation time, which means the crystal oscillator starts slowly. The principle is that the external capacitance will store more charges, which reduces the current intensity, thereby reducing the excitation power provided by the circuit to the crystal oscillator for oscillation.
When the output waveform of a passive crystal oscillator experiences peak shaving or distortion, this is usually caused by current overdrive.
It can be solved by connecting a resistor in series, with a resistance value typically ranging from tens of kΩ to hundreds of kΩ. If you want to stabilize the waveform, you can try paralleling a 1M feedback resistor.
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