Considerations for Designing Clock PCBs-FS Technology

タグ : pcb,pcba,fs pcb
Considerations for Designing Clock PCBs-FS Technology

The clock crystal and associated circuits should be placed in the center of the PCB and should have a good ground plane, not close to the I/O interface. Clock generation circuits are not allowed in the form of daughter cards or daughter boards, and must be fabricated on separate clock boards or load-bearing boards. Only the equipment related to the clock circuit is installed in the PCB clock circuit area of ​​FS TECH, avoid installing other circuits. Other signal lines should not be routed near or below the crystal: Clock circuits and ground planes are used under the crystal if other signals pass through the plane violating the image plane function. If the signal passes through the ground plane, there will be a small ground loop and affect the continuity of the ground plane.


Ground loops will have problems at high frequencies
Shielding measures can be used to shield the clock crystal and clock circuit;

If the clock case is metal metal, the PCB design must have copper under the crystal, and this part has a good electrical connection to the complete ground (through a porous ground). Advantages of laying ground under the FS Tech clock: The circuits inside the crystal oscillator generate RF currents. If the crystal is encased in a metal case, the DC power source discharges the transient current generated by the case through RF radiation through the ground plane, referenced to the DC voltage and the RF current circuit within the crystal.


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In short, the metal casing is a single-ended antenna, and the nearest image layer, the ground plane layer, sometimes has two or more radiation coupling effects, as the RF current to the ground.

The bottom of the PCB crystal is also conducive to heat dissipation
The clock circuit and crystal ground will provide an image plane that reduces the common mode currents of the associated crystal and clock circuit, thereby reducing RF emissions. Ground can also absorb differential mode RF currents. This plane must be connected to the complete ground plane through multiple points and through multiple holes, which provides low impedance. To improve the effect of the ground plane, the clock circuit of FS Technology should be close to the ground plane. Since most of the surface mount crystals are packaged in plastic, the SMT packaged crystal has more RF energy radiation than the metal shell crystal, and the RF current in the crystal will radiate into the space and be coupled with other devices.

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