The difference and relationship between the load capacitance of the crystal oscillator and the external capacitance

“It is often encountered that some people confuse the load capacitance of the crystal oscillator with the external capacitance, and some people even mistakenly think that it refers to the same parameter. It is important to point out that if you think this way, you are very wrong.

“

It is often encountered that some people confuse the load capacitance of the crystal oscillator with the external capacitance, and some people even mistakenly think that it refers to the same parameter. It is important to point out that if you think this way, you are very wrong.

Principle of passive crystal oscillator

The following will analyze and distinguish for you:

Load capacitance refers to an important internal electrical parameter of the crystal oscillator.

In general, on the PCBA of Electronic equipment that is not very sensitive to power consumption, the common crystal load capacitances are 15PF, 18PF, and 20PF.

Then, for electronic products such as watches, mobile phones, Bluetooth headsets, etc., which obviously have a high demand for low power consumption, crystal oscillators with small load capacitances are often used on PCBA, such as 6PF, 7PF, 9PF, 10PF, 12PF.

The load capacitance of the crystal oscillator has been locked by the processing technology according to the requirements in the production process, and cannot be changed in the application.

The external capacitor of the crystal oscillator refers to the electronic components connected in series with the crystal oscillator frequency input pin and output pin respectively on the PCBA board. The value of the external capacitance is determined by the crystal oscillator load capacitance and the circuit board stray capacitance (including IC capacitance), which is usually the sum of the two.

In the application of oscillation circuit, the relationship between the load capacitance, stray capacitance and external capacitance of the crystal oscillator is as follows:

Note:
1. CL: The load capacitance of the quartz crystal resonator.
2. CS: refers to the stray capacitance, including the stray capacitance value inside the IC, the capacitance between the circuit board wiring, the parasitic capacitance between the layers of the PCB board, etc.
3. C1 and C2: refer to the two external capacitors of the quartz crystal resonator in circuit applications respectively.

There are two application purposes of external capacitors:

Fine-tune the crystal frequency to be as close to the target frequency as possible. Rule: The larger the external capacitor, the more negative the crystal oscillator output frequency. On the contrary, the smaller the external capacitance is, the more positive the output frequency of the crystal oscillator is.

The external capacitor can stabilize the oscillation circuit. This is why it is recommended to add a capacitor of the same value to the crystal frequency input pin and output pin.

※Finally, two points need to be reminded:

1. The function of the external capacitor is to fine-tune the frequency of the crystal oscillator. If the frequency deviation of the crystal oscillator is too large, it is necessary to consider the reasons for the accuracy of the crystal oscillator itself, such as whether the crystal oscillator accuracy cannot meet the requirements of the chip and is changed to a crystal oscillator with higher precision.

2. External capacitors are only used for passive crystal oscillator applications. In the circuit application of active crystal oscillator, no external capacitor is required.