We often encounter people who confuse the load capacitance of a crystal with the external capacitance, and some even mistakenly believe that they refer to the same parameters. Here is the analysis and distinction for you.
Load capacitance refers to an important internal electrical parameter of the crystal.
In general, the power consumption of less sensitive electronic devices PCBA, the common crystal load capacitance of 15PF, 18PF, 20PF.
Such as watches, cell phones, Bluetooth headsets and other electronic products with significantly higher demand for low power consumption, PCBAs often use a smaller load capacitance of the crystal, such as 6PF, 7PF, 9PF, 10PF, 12PF.
The load capacitance of the crystal in the production process has been locked according to demand through the processing process, in the application can not be changed.
The external capacitor of the crystal is the electronic component that is connected in series with the crystal frequency input pin and output pin 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 IC capacitance), which is usually the sum of the two.
In oscillation circuit applications, the relationship between crystal load capacitance, stray capacitance and external capacitance is schematically shown below.
Notes:
1. CL: The load capacitance of the quartz crystal resonator.
2. CS: refers to the stray capacitance, including the internal stray capacitance of the IC, the capacitance between the circuit board wiring, and the parasitic capacitance between the layers of the PCB board.
3. C1 and C2: refers to the two external capacitors of the quartz crystal resonator in the circuit application, respectively.
External capacitors are used for two purposes.
To fine-tune the crystal frequency, so that it is as close to the target frequency as possible. Rule: The larger the external capacitor, the more negative the output frequency of the crystal oscillator. 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 for the oscillation circuit. This is why it is recommended to add a capacitor of the same value to the input and output pins of the crystal frequency respectively.
Finally, we need to remind two points.
1. the external capacitor only plays a role in fine-tuning the crystal frequency. If the crystal frequency is too large when working, you need to consider the accuracy of the crystal itself, such as whether the crystal precision can not meet the requirements of the chip and change to a higher precision crystal.
2. The external capacitor is only used for passive crystal applications. In the circuit application of the active crystal, no external capacitor.
