PCB design of crystal oscillator circuit

we often liken the crystal to the heart of the digital circuit, this is because all the work of the digital circuit can not be separated from the clock signal, the crystal directly control the entire system, if the crystal oscillator does not work then the whole system is also paralyzed, so the crystal oscillator is to determine the digital circuit to begin work prerequisites.

we often say that the crystal oscillator, quartz crystal oscillator and quartz resonator two, they are using the piezoelectric effect of quartz crystals made. Applying an electric field to the two electrodes of a quartz crystal can cause the crystal to deform mechanically, whereas if a mechanical pressure is exerted on both sides of the crystal the electric field is generated on the crystal. And, both of these phenomena are reversible. With this characteristic, alternating voltages are applied on both sides of the crystal, and the wafer generates mechanical vibrations and produces alternating electric field. This vibration and electric field are generally very small, but at a certain frequency, the amplitude will be significantly increased, this is the piezoelectric resonance, similar to what we often see LC loop resonance.

due to the importance of crystal oscillator in digital circuits, we need to handle it carefully when using and designing:

1. Crystal Internal quartz crystal, by external impact or fall prone to damage to quartz crystals, resulting in crystal vibration does not start, so in the design of the circuit to consider the reliable installation of crystal oscillator, its position near the CPU chip priority placement, away from the board edge.

2. In manual welding or machine welding, pay attention to the welding temperature. The crystal is sensitive to temperature, the temperature can not be too high when soldering, and the heating time is as short as possible ｡

3. The coupling capacitor should be as close as possible to the crystal power supply pin, position placing order: according to the power supply flow direction, according to the capacitance value from large to small in turn, the capacitance of the smallest capacitor closest to the power pin.

4. The shell of the crystal oscillator must be grounded, the outward radiation of the crystal oscillator, and the interference of the external signal to the crystal oscillator can also be shielded.

5. Do not wire under the crystal oscillator, to ensure full paving, while in the Crystal Oscillator 300mil range do not wiring, this can prevent crystal interference other wiring, devices and layer performance.

6. Clock signal traces should be as short as possible, the line is wider, in the wiring length and away from the source of heat to find balance.

7. Handling the Parcel

(cylindrical crystal oscillator) with a rectangular pad that is similar to the crystal shape when the housing is grounded, let crystal "lie flat" on this pad, in the two long sides of the pad open a hole (hole to fall in the pad, if you can use a multilayer pad to replace the hole is better, the two multilayer pads to be connected with a rectangular pad), Then the crystal "hoop" with a copper wire or other bare conductors, the ends of the copper wire are welded in the two holes or pads you open. This can avoid the high temperature welding to the crystal vibration damage, but also to ensure good grounding.

of course, some people in the crystal to add solder joints, grounding treatment (in welding must pay attention to the effect of temperature on crystal oscillator)