Guidance and advice on the design and layout of radio frequency (RF) printed circuit boards (PCBs)
This application note offers practical guidance and recommendations for designing and laying out radio frequency (RF) printed circuit boards (PCBs), with a focus on mixed-signal applications that integrate digital, analog, and RF components on the same board. The content is organized by topic to provide a "best practice" approach that should be used alongside other design and manufacturing guidelines relevant to specific components, PCB manufacturers, and materials.
RF transmission lines are essential in many Maxim RF components, as they carry RF power between IC pins and the PCB. These lines can be placed on outer layers (top or bottom) or buried within inner layers. This section covers microstrip lines, striplines, coplanar waveguides (with ground), and characteristic impedance. It also discusses corner compensation for transmission lines and transitions between different layers.
Microstrip lines consist of fixed-width conductors over a grounded plane, typically on adjacent layers. For example, a trace on the top layer requires a solid ground plane directly beneath it. The trace width, dielectric thickness, and material determine the characteristic impedance, usually 50Ω or 75Ω.
Striplines are similar but use a grounded area above and below an inner conductor, making them ideal for internal RF traces. They can be centered or offset depending on the board's layer configuration.
Coplanar waveguides offer better isolation between RF lines and other signals, with grounded areas on both sides and below the conductor. Installing via fences around these structures improves performance by shorting loop currents to the ground plane.
Characteristic impedance calculations are crucial for achieving desired signal integrity. Dielectric constants vary between outer and inner layers, so careful attention must be paid when selecting values. For instance, FR4 has εR = 4.2, while pre-preg layers may have εR = 3.8.
When routing transmission lines, bends should have a radius of at least three times the line width to minimize impedance changes. If a right-angle bend is unavoidable, angle miters can help reduce impedance jumps. Electromagnetic simulators are recommended for high-performance designs.
Changing layers for transmission lines should use multiple vias to reduce inductance. Two or more vias per line are advised, with the diameter matching the line width if possible.
Signal isolation is critical to prevent coupling. RF lines should be spaced apart, with grounded areas between different layers. High-power lines should be kept away from others, and coplanar waveguides provide excellent isolation.
Digital and RF signals should be separated on different layers to avoid noise interference. Power and ground planes need careful assignment to avoid signal loops. A "star" configuration for power distribution helps minimize ground loops and parasitic inductance.
Decoupling capacitors are essential for filtering noise. Choosing capacitors with appropriate self-resonant frequencies ensures effective bypassing across the desired frequency range. Multiple capacitors in parallel can extend the usable frequency band.
Proper placement of bypass capacitors minimizes AC ground inductance. Using two or more ground vias per component reduces parasitic effects. Thermal vias under ICs improve heat dissipation, enhancing reliability and performance.
In summary, this guide provides a comprehensive approach to RF PCB design, emphasizing layout techniques, signal integrity, and best practices for ensuring reliable and high-performance RF systems.
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