Advantages of laying grounding under the clock crystal on a single PCB
Precautions for single clock PCB mainly includes the following aspects:
1) The clock crystal and related circuits should be placed in the center of the PCB and have a good grounding plane, not close to the I/O interface. The clock generation circuit cannot be manufactured in the form of sub card or sub board, but must be manufactured on a separate clock board or carrier board.
2) Only arrange the equipment related to the clock circuit in the clock circuit area of the PCB, avoid other circuits, and do not arrange other signal lines near or below the crystal: if other signals pass through the plane, use the clock generation circuit and the ground plane below the crystal, whICh violates the image plane function. If the signal passes through the grounding plane, there will be a SMAll grounding loop and affect the continuity of the grounding plane. These grounding loops will cause problems at high frequencies.
3) For clock crystals and clock circuits, mask measures can be used to mask;
4) If the clock housing is made of metal, the copper must be placed under the crystal when designing the PCB, and this part must have a good electrical connection with the entire grounding plane (grounded through a hole).
2. Advantages of laying the floor under the clock crystal:
The circuit inside the crystal oscillator will generate RF current. If the crystal is packaged in a metal housing, the DC power supply pin is the reference of the DC voltage reference and the RF current loop reference inside the crystal, and the transient current generated by the RF radiation of the housing is released through the grounding plane. In short, the metal shell is a single ended antenna, and the near image layer, ground plane layer and sometimes two or more layers are enough to couple RF current radiation to the ground. The floor under the crystal is also conducive to heat dissipation. The clock circuit and the bottom of the crystal will provide an image plane, which can reduce the common mode current generated by the related crystal and clock circuit, thereby reducing RF radiation. The ground plane also absorbs differential mode RF current. The plane must be connected by multiple points. For a complete ground plane, and multiple through holes are required to provide low impedance, in order to enhance the effect of the ground plane, the clock generation circuit should be close to the ground plane. SMT encapsulated crystals will radiate more RF energy than metal enclosed crystals: because surface mounted crystals are mostly plastIC encapsulated, the RF current inside the crystals will radiate into space and be coupLED to other devices.
3. Total clock track
It is better to use radial topological connection for fast rising edge signals and clock signals than to use a single common drive source network in series. Each trace shall be wired according to its characteristic impedance.
4. Clock transmission line requirements and PCB layering
Clock wiring principle: a complete image plane layer is arranged beside the clock wiring layer to reduce the wiring length and perform impedance control.
5. Incorrect cross layer trace and impedance mismatch may cause:
1) The use of vias and jumps as traces leads to the incompleteness of the image loop;
2) The surge voltage on the image plane is due to the voltage on the signal pin of the device changing with the signal;
3) If the 3W principle is not considered in the wiring, different clock signals will cause crosstalk;
6. Routing clock signal:
1) The clock wire must be located in the inner layer of the multilayer PCB board. And ensure the use of striplines; If you want to go to the outer layer, you can only take the microstrip line.
2) Walking on the inner layer can ensure a complete image plane. It can provide a low impedance RF transmission path and generate magnetic flux to offset the magnetic flux of its source transmission line. The closer the distance between the source and the return path, the better the demagnetization effect. Due to the enhanced degaussing capability, each complete planar image layer of the high-density PCB can provide 6-8dB suppression.
3) Advantages of clock cloth Multi layer boards: There are one or more layers that can be determined to the complete power supply and ground plane, and can be designed as a good decoupling system to reduce the area of the ground loop, reduce differential mode radiation, reduce EMI, and reduce the impedance level of the signal and power loop, which can maintain the consistency of the track impedance throughout the process, And the crosstalk between adjacent traces is on the PCB