1. Layout

1) The clock crystal and related circuits should be placed in the center of the PCB and have a good ground plane, not near the I/O interface. The clock generation circuit cannot be made in the form of a daughter card or a daughter board, and must be made on a separate clock board or carrier board. As shown in the figure below, it is best not to route wires on the next layer in the green box.

2) Only lay out the devices related to the clock circuit in the PCB clock circuit area, avoid laying other circuits, and do not lay other signal lines near or under the crystal: use the ground plane under the clock generation circuit and crystal, if other signals pass through this plane, Violating the image plane function, if you let the signal traverse this ground plane, there will be small ground loops and affect the continuity of the ground plane. These ground loops will cause problems at high frequencies.

3) For clock crystals and clock circuits, shielding measures can be used for shielding.

4) If the clock casing is metal, copper must be laid under the crystal during PCB design, and ensure that this part has a good electrical connection with the complete ground plane (through porous grounding).

5) The benefits of laying the ground under the clock crystal: The circuit inside the crystal oscillator will generate radio frequency current. If the crystal is encapsulated in a metal case, the DC power pin is the basis for the DC voltage reference and the RF current loop reference inside the crystal, which is released through the ground plane. Transient currents generated by the enclosure by radio frequency radiation. In short, the metal casing is a single-ended antenna, and the nearest image layer, the ground plane layer, and sometimes two or more layers are sufficient for the radiation coupling of the RF current to the ground. The floor under the crystal is also good for heat dissipation.

6) The clock circuit and the bottom of the crystal will provide an image plane, which can reduce the generation of common mode current to the related crystal and clock circuit, thereby reducing radio frequency radiation. The ground plane also absorbs differential mode radio frequency current. This plane must pass through multiple channels. point to a full ground plane and to 

Seek to pass through multiple vias, which can provide low impedance. In order to enhance the effect of this ground plane, the clock generation circuit should be close to this ground plane.

7) SMT packaged crystals will radiate more RF energy than metal case crystals: Because surface mount crystals are mostly plastic packages, the RF current inside the crystal will radiate into space and couple to other devices.