RF PCBs and mixed-signal systems are challenging for many designers. Careful attention must be paid to the PCB stackup design, process for designing transmission lines, placement of components, and floorplanning the PCB layout to ensure predictable return path. Many RF PCB layout designs will include printed elements, like an RF filter, antenna, isolation structures, and unique waveguide geometries.
RF systems can make copious use of RF filters and matching network designs to ensure signal quality. If you need to design an RF PCB layout with RF filter circuits and structures, your design tools will play a major role in determining the system’s performance and they can help you stay productive. Once you’ve determined your relevant signal bandwidth and you’ve determined RF filter requirements, your design tools can help you stay productive as you create your RF PCB layout.
The RF PCB Design Process
RF PCB designs at any frequency carry a range of PCB layout requirements and must follow similar processes. Because so many RF designs will need to manipulate RF signals, the design team needs to consider signal bandwidths to determine RF filter design requirements in their PCB layout. RFdesigns usually proceed under the following process:
- Determine signal bandwidths: This will determine what type of filtering is needed in the PCB layout. It also determines the dielectric constant as dispersion must be considered when planning the design.
- PCB stackup design: Once the signal bandwidth and dielectric constant are determined, the PCB stackup can be designed to ensure required PDN impedance and impedance of transmission lines.
- Schematic capture and component placement: If schematics are ready and components have been selected, the design can be captured in a blank layout and the designer can start arranging components.
- Filter and antenna placement: Printed elements like RF antenna structures and RF filter designs need to be carefully designed and placed in the PCB layout to ensure they have the desired signal characteristics.
- Routing: Once components are placed in the PCB layout, the design needs to be routed with consistent transmission line impedance.
- Documentation and manufacturing prep: A finished design needs to be prepared for manufacturing by generating a set of standardized deliverables from design documents.
RF filters take special care when designed as printed structures on a PCB, just an antenna, via fence, or other isolation structure. When designed from discrete components, there are important aspects of signal integrity that must be considered when creating the PCB layout.
RF Filter PCB Layout Structure
Before arranging components in the PCB layout for RF filter circuits, the PCB stackup needs to be created to provide the desired system impedance within the system’s signal bandwidth. In addition, an RF filter should be simulated in a schematic sheet to examine how the filter topology will influence signal behavior.
RF Filter PCB Layout and Routing
Once your PCB stackup is designed and filtering requirements have been determined, it’s time to create an RF PCB layout for your filtering circuits. Filter circuits and impedance matching networks have some specific requirements that need to be considered during layout and routing:
- Floorplan the design: Keep the RF analog section away from digital components and ensure the return path from the digital section does not overlap into the digital section.
- Use short traces: This is less about loss and more about ensuring traces between components and printed elements do not act like transmission lines at high frequencies. When traces are long, signals can reflect between components and create strong radiation at certain frequencies.
- Account for parasitics in the layout: The presence of parasitics in a high-frequency design cause the real impedance to be different from the design impedance. This requires using techniques like network parameter analysis and field solvers to understand parasitics.
- Account for parasitics in components: Components with reactive impedance have parasitics that become evident at high frequencies. This is very important when selecting capacitors and inductors.
- Place RF components above a ground plane: The goal here is to ensure isolation and consistent input impedance seen by a signal as it enters the RF filter.
Because impedance control is so important to RF layout and routing, your design tools need to be able to evaluate design impedance and enforce the necessary impedance profile. The best design tools can do this and provide simulation capabilities to examine signal behavior as you create your RF PCB layout.
Ensure Impedance Control in RF PCB Layout and Routing
The traces between components and printed circuit elements in an RF filter circuit need to be carefully routed while ensuring controlled impedance in the PCB layout. The best routing tools can take your stackup design data and enforce a specific impedance profile as traces are routed. This is a central aspect of signal integrity in high-speed digital and high-frequency analog design, and it will ensure your RF filter circuits and structures have the right bandwidth and filtering behavior.
The Best Design Software for RF PCB Layout
In the list of RF PCB design and layout requirements shown above, we can determine several design rules that need to be carefully followed. Modern PCB design tools should operate by giving designers the ability to customize their design rules to comply with their specific application requirements. For RF PCBs, this requires considering everything from impedance to ground plane return path tracking to ensure signal integrity, RF isolation, and stable power in your circuit board’s PDN.