Figure 23. Microstrip Design On A 2-Layer Pcb; Figure 24. Coplanar Waveguide Design On A 2-Layer Pcb - ST STM32WL Series User Manual

A 4‑layer PCB can also be used as soon as the RF trace remains controlled to 50 Ω.
Some general guidelines when routing an RF PCB are listed below:
• RF traces must be short and straightforward. Make the transmission lines short and straightforward to
avoid reflections, save power, and reduce high‑frequency issues.
• Use an impedance simulation tool to ensure the characteristic impedance of the RF track to 50 Ω. Contact
your PCB maker to verify if the values on the stack-up selected can be guaranteed.
• Try to maintain the characteristic impedance (50 Ω) constant. Avoid discontinuities, such as different pad
sizes on transmission lines, bends, or T-junctions, or changing the RF trace width along the line.
• Minimize the distance between the STM32WL5MOC module RF output to the antenna connector.
• The reference ground of the RF signal must be complete. Adding some ground vias along the RF tracks
can improve the RF performance.
• Keep critical signals away from RF. High-frequency signals can induce some undesired effects in critical
signals such as electric and/or magnetic coupling.
UM3127 - Rev 1
Figure 23. Microstrip design on a 2‑layer PCB
Figure 24. Coplanar waveguide design on a 2‑layer PCB
UM3127
Hardware layout and configuration
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