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How To Design PCB Crystal?


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We often compare the crystal oscillator to the heart of the digital circuit. This is because all the work of the digital circuit is inseparable from the clock signal. The crystal oscillator directly controls the entire system. If the crystal oscillator does not operate, the entire system will be paralyzed, so the crystal oscillator is the prerequisites for the digital circuit to start working.

The crystal oscillator we often say is two kinds of quartz crystal oscillator and quartz crystal resonator, they are made by the piezoelectric effect of quartz crystal. Applying an electric field to the two electrodes of a quartz crystal will cause the crystal to mechanically deform. Conversely, if mechanical pressure is applied to both sides of the crystal, an electric field will be generated on the crystal. Moreover, these two phenomena are reversible. Taking advantage of this characteristic, an alternating voltage is applied to both sides of the crystal, and the wafer will produce mechanical vibration and an alternating electric field. This kind of vibration and electric field are generally very small, but at a certain frequency, the amplitude will be significantly increased, this is piezoelectric resonance, similar to the LC resonance we commonly see.

As the heart of digital circuits, how does crystal play a role in smart products? For smart home products such as air conditioners, curtains, security, monitoring and other products, all need wireless transmission modules, which use Bluetooth, WIFI or ZIGBEE and other protocols to send the module from one end to the other, or directly through the mobile phone, and the crystal oscillator is the core component in the wireless module, which affects the stability of the entire system. Therefore, the choice of the crystal oscillator used in the system determines the success or failure of the digital circuit.

the crystal on the PCB

Due to the importance of crystal in digital circuits, we need to pay attention to the following when using and designing:

1. The coupling capacitor should be as close as possible to the power pins of the crystal, and the order of placement: according to the direction of power supply, according to the capacitance value from large to small, the capacitor with the smallest capacitance is closest to the power pin.

2. The shell of the crystal oscillator must be grounded, which can radiate the crystal oscillator outwards and also shield the interference of external signals to the crystal oscillator.

3. Do not wiring under the crystal oscillator, ensure that the ground is completely laid, and at the same time, do not wire within 300mil of the crystal oscillator, which can prevent the crystal oscillator from interfering with the performance of other wiring, devices and layers.

4. The traces of the clock signal should be as short as possible, and the line width should be larger. Find a balance between the wiring length and the heat source.

5. There are quartz crystals inside the crystal oscillator, which can easily cause the quartz crystal to break and break when exposed to external shocks or falls, which can cause the crystal oscillator to fail to vibrate. Therefore, the reliable installation of the crystal oscillator should be considered when designing the circuit. The location should not be as close as possible to the edge of the board, device casing, etc.

6. When soldering by hand or by machine, pay attention to the soldering temperature. The crystal oscillator is sensitive to temperature, the temperature should not be too high during soldering, and the heating time should be as short as possible.

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