The Difference Between Load & External Capacitor of Crystal Oscillator

Load capacitance refers to an important internal electrical parameter of the crystal oscillator. In general, the load capacitance of the crystal oscillator is 15PF, 18PF, and 20PF on the PCBA of electronic devices that are not too sensitive to power consumption. Electronic products such as wristwatches, mobile phones, Bluetooth headsets, etc., obviously have a high demand for low power consumption. Therefore, PCBA often uses a crystal oscillator with small load capacitance, such as 6PF, 7PF, 9PF, 10PF, and 12PF.

In addition, the load capacitance of the crystal has been locked in the manufacturing process according to the demand through the processing process, and cannot be changed in the application.

The external capacitance of the crystal oscillator is the electronic component connected in series with the crystal frequencies input and output pins respectively on the PCBA board.

The size of the external capacitance is determined by the load capacitance of the crystal and the stray capacitance of the board (including the IC capacitance), which is usually the sum of the two.

The external capacitor is applied for two purposes. To fine-tune the crystal frequencies for the target frequency as close as possible.

Rule: The larger the external capacitor, the more negative the output frequency of the crystal. Conversely, the smaller the external capacitor, the more the output frequency of the crystal tends to change in a positive direction.

The external capacitor can play a stabilizing role in the oscillation circuit. This is the reason why it is recommended to add a capacitor of the same value to the input and output pins of the crystal frequencies.

PS:

The external capacitor only serves to fine-tune the frequency of the crystal.

If the crystal frequencies is too large when working, you need to consider the reasons for the accuracy of the crystal itself. For example, whether the crystal accuracy can not meet the requirements of the chip and change to a higher precision crystal.

The external capacitors are only used for passive crystal applications. In the circuit application of an active crystal oscillator, external capacitors are not required.

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