Crystal oscillator PCB design considerations

Crystal oscillators are often used on circuit boards. We often compare crystal oscillators to the heart of digital circuits. This is because all the work of digital circuits 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 affected. The system is paralyzed, so the crystal oscillator determines the prerequisite for the digital circuit to start working.

The crystal oscillators we often talk about are quartz crystal oscillators and quartz crystal resonators. They are both made by utilizing the piezoelectric effect of quartz crystals. Applying an electric field to the two electrodes of a quartz crystal causes mechanical deformation of the crystal, whereas applying mechanical pressure to both sides of the crystal produces an electric field on the crystal. Moreover, these two phenomena are reversible. Taking advantage of this characteristic, applying alternating voltage on both sides of the crystal will cause the wafer to vibrate mechanically and generate an alternating electric field at the same time. This vibration and electric field are generally very small, but at a certain frequency, the amplitude will increase significantly. This is piezoelectric resonance, similar to the LC loop resonance we commonly see.

Due to the importance of crystal oscillators in digital circuits, we need to be careful when using and designing them:

1. There is a quartz crystal inside the crystal oscillator. When it is hit or dropped from the outside, it is easy to cause the quartz crystal to break and damage, which will cause the crystal oscillator to not vibrate. Therefore, when designing the circuit, the reliable installation of the crystal oscillator must be considered. Its position should be placed close to the CPU chip first and away from the board. side.

2. When manual welding or machine welding, pay attention to the welding temperature. The crystal oscillator is relatively sensitive to temperature. The temperature should not be too high during welding, and the heating time should be as short as possible?

3. The coupling capacitor should be placed as close as possible to the power pin of the crystal oscillator. The order of placement is: according to the direction of power inflow, and in order from large to small in capacitance. The capacitor with the smallest capacitance is closest to the power pin.

4. The shell of the crystal oscillator must be grounded to prevent the crystal oscillator from radiating outward and to shield the crystal oscillator from interference from external signals.

5. Do not lay any wiring under the crystal oscillator. Ensure that the ground is completely covered. At the same time, do not lay any wiring within 300mil of the crystal oscillator. This can prevent the crystal oscillator from interfering with the performance of other wiring, devices and layers.

6. The wiring of the clock signal should be as short as possible and the line width should be larger. Find a balance between wiring length and distance from heat sources.

7. Carry out land parcel processing

(Cylindrical crystal oscillator) When the shell is grounded, add a rectangular pad with a similar shape to the crystal oscillator, let the crystal oscillator "lay flat" on this pad, and open a hole near the two long sides of the pad (the holes should fall In the pad, it would be better if a multi-layer pad can be used instead of the hole. These two multi-layer pads should be connected to the rectangular pads), and then use a copper wire or other bare wire to "hoop" the crystal oscillator. , the two ends of the copper wire are welded in the two holes or pads you opened. This can avoid damage to the crystal oscillator by high-temperature welding, and ensure good grounding. Of course, some people add solder joints to the crystal oscillator for grounding (be sure to pay attention to the effect of temperature on the crystal oscillator when welding). The above are the precautions for the crystal oscillator on the circuit board. I hope it can help you.