Key takeaways:
- A radio frequency (RF) printed circuit board (PCB) is a delicate balancing act between multiple phenomena where even a small change can cause large ripple effects in other parts of the board.
- The operating frequency is a primary factor influencing material selection and many other decisions.
- Regular PCB software does not include the capabilities needed for high-quality RF designs. RF boards require more powerful tools that have specialized in-depth knowledge of RF.
Complex RF PCBs are at the heart of many modern critical devices, like radar transceivers, telecom base stations, and wireless routers. Designing RF PCBs is a complicated task requiring years of experience and proficiency because every little change can have severe ripple effects on functionality and performance.
RF PCB design software assists in engineering these sensitive boards with high confidence and productivity. Learn why this software is essential to RF design and what features to look for when evaluating them.
What is RF PCB design software?
RF PCB design software is a specialized category of electronic design automation (EDA) tools for designing, simulating, and verifying PCBs that operate at high frequencies in the following bands:
- RF band: Ranging from tens of megahertz (MHz) up to 3 gigahertz (GHz), it covers most wireless communications (like Wi-Fi, 2G/3G/4G telecom, and Bluetooth), basic satellite communications, and global positioning systems.
- Microwave band: The 3-30 GHz band is used by radars and Wi-Fi 5/6/7.
- Millimeter wave (mmWave) band: The 30-300 GHz range is used for 5G/6G telecom, 802.11ad gigabit Wi-Fi, radars, high-throughput satellites, and automotive vehicle-to-everything
At these high frequencies, several physical phenomena that are otherwise negligible show prominent effects on a PCB’s functionality and performance. They include transmission line effects and electromagnetic interference (EMI) from copper traces, propagation losses in the PCB layers, parasitic effects of inductors and capacitors, higher resistances due to skin effects, noise, phase shifts, and more.
Even minor changes to an RF circuit board design — like slightly increasing the dimensions of traces — can intensify these effects and severely impact functionality or performance. RF PCB design is a delicate balancing act that, when done manually, can severely hamper design productivity.
This is where RF PCB design software helps, ensuring high first-pass yield and faster time-to-market. It provides sophisticated circuit simulation and analysis models to see the impacts of the above phenomena after every change in the PCB layout or schematic. It automates the optimization of RF board layouts and routing using in-depth knowledge of RF phenomena.
Let’s dig deeper into why RF PCB design tools are practically a necessity nowadays.
Why is RF PCB design software essential for creating wireless electronic circuits?
Figure 1. High-frequency effects
The delicateness of RF PCBs calls for precise control over all design aspects and high-frequency phenomena. Even minor changes must be carefully evaluated for potential ripple effects on the PCB’s functionality and performance. We outline some of these phenomena, aspects, and effects in more detail below.
High-frequency (HF) phenomena
Frequency is, by far, the most important factor because of its first-order and higher-order effects. These must be simulated and mitigated using RF PCB design software.
Since the wavelengths of HF signals are comparable to the dimensions of components and circuit traces, effects like resonance and standing waves occur. They must be mitigated through precise control over trace geometries and floor layouts.
Electromagnetic effects become pronounced. Copper traces start behaving like transmissionlines instead of simple wires. Signals experience reflections, delays, and attenuation due to impedance mismatches. Engineers must account for factors like transmission line lengths relative to the typical signal wavelengths.
Another problem is increased resistance due to the skin effect — the tendency of HF alternating currents to concentrate near conductor surfaces instead of flowing evenly through the entire cross-section.
Then there are the parasitic effects. Every inductor is also a capacitor, and every capacitor is an inductor. They cause unexpected resonant frequencies and phase shifts. All analyses and simulations must use their equivalent circuits for accurate results.
Nonlinearities in active components like diodes and transistors become more pronounced. Their unexpected harmonics and intermodulationnoise can degrade desired signals.
Signal integrity issues
Signal integrity covers concerns like:
- separation of RF and digital sub-circuits in mixed-signal PCBs
- impedance matching
- reflection analysis
- crosstalk analysis
- grounding issues and ground connection integrity
- shielding to minimize EMI
Techniques like controlled impedance, differential signaling, and ground planes are necessary to maintain signal integrity. RF PCB design environments integrate all these analyses into user-friendly workflows to enable quick checks and visualizations.
Power integrity concerns
Power integrity concerns include:
- decoupling supplies to maintain stable voltages in high-speed HF PCBs
- isolating the RF and digital grounds in mixed-signal PCBs
Thermal effects
Heat dissipation and thermal management are essential to prevent effects on signal integrity. Substrate materials with lower coefficients of thermal expansion (CTE) and loss tangents are essential. Design software must be able to model and predict the thermal behaviors of these materials and their layer stack-ups. They must also predict the impacts of thermal behaviors on electrical and RF performance.
Materials and substrate concerns
Selecting the right materials for the various layers—conductive, substrate, surface finish, solder mask, and silkscreen—is crucial for RF PCBs. Substrate materials’ dielectric constants (Dk), conductivity, permeability, and loss tangents affect propagation losses and noise.
Instead of common PCB materials like flame-retardant woven glass-reinforced epoxy resin (FR4), RF PCBs prefer polytetrafluoroethylene (PTFE) because its dielectric properties and lower loss tangent results in less HF signal attenuation.
PCB design software must support the modeling of all these aspects and facilitate dielectric constant matching for various stack-ups.
Layout and routing impacts
Mitigation of many HF phenomena involves careful design of layouts, routing, and geometries. These aspects include:
- arrangements of layers in a stack-up
- distance between components
- isolation between traces to prevent crosstalk
- coplanar waveguides
- curvatures in traces
These layout aspects are like a web — a minor change in one region can have ripple effects on everything else. So automatic checks and adjustments are essential for productivity.
What are the main features to look for in RF PCB design software?
Figure 2. Essential capabilities of RF PCB design software
How do you choose the best PCB design software for your wireless project needs? We outline the key features you should look for to streamline your RF PCB design workflows.
RF circuit synthesis features
Circuit synthesis capabilities must include the following:
- Component libraries: The software must include a comprehensive inventory of preverified RF components like amplifiers, mixers, and filters for different needs. It must be able to synthesize RF circuits that combine models for active and passive components. These features greatly accelerate design cycles.
- Impedance matching tools: The tools must facilitate automatic impedance matching, which is essential for signal integrity.
- Parametric analysis: RF PCB tools must support parameter sweeps to evaluate circuit performance under different conditions.
Schematic capture capabilities
Look for the schematic design features listed below:
- Mixed-signal support: The tools must have seamless support for analog and digital component integration to mitigate interference.
- Component customization: The tools must support customizing components with specific models and footprints.
- Design rule checks: The software must include automatic RF-specific DRCs that ensure compliance with component placement and connection geometry recommendations.
- Hierarchical organization: The tools must support partitioning and organizing complex designs in a structured way.
- Intuitive user interfaces: The tools must achieve high productivity through Intuitive graphical interfaces that allow easy component placement, routing, and extensive modifications.
Layout and routing features
Some essential layout and routing features are outlined below:
- Layer stack design: The tools must support advanced management and analyses of multi-layer stack-ups and their materials to isolate RF and non-RF signals.
- Precise control over impedance: The tools must allow precise control over trace routing and geometry to achieve ideal impedance.
- Advanced routing algorithms: Routing capabilities tailored for RF signals are essential to minimize crosstalk and interference.
- Layout visualizations: Users must be able to visualize PCB layouts and design parameters in 3D for optimizing component placement and trace routing.
- Layout verification tools: Verifying layouts against design rules and standards is essential to ensure operational integrity.
Signal integrity and simulation capabilities
Real-world modeling and simulation features like the ones below are crucial for good RF PCB designs:
- Integrated simulation models: The tools must be able to import various models, such as simulation programs with integrated circuit emphasis (SPICE), PSpice, LTSpice, and proprietary models created using popular commercial and open-source computer-aided design (ECAD) and EDA software like Altium Designer, Cadence Allegro/OrCAD, Autodesk EAGLE, KiCad, EasyEDA, and others.
- Three-dimensional electromagnetic (3DEM) simulation: Real-time 3DEM simulations and 3D modeling are critical for assessing parasitic effects and HF behaviors.
- Signal integrity analysis: Tools to analyze reflections, crosstalk, and timing issues are essential for ensuring signal integrity.
- Post-layout simulations: Engineers must be able to simulate and monitor metrics like crosstalk and reflection on completed layouts.
- Transient and frequency domain analysis: These tools enable circuit performance analysis in both domains to assess temporal and spectral characteristics.
- Thermal analysis: It’s essential to predict thermal effects due to power dissipation in RF components.
- Propagation characteristics: These tools analyze wave propagation through various circuit and layout configurations.
Fabrication output features
RF design software must generate the output formats required for PCB manufacturing. They include Gerber, Open Database (ODB++), and the Institute of Printed Circuits (IPC) standards like IPC-2581 for communicating design information to PCB manufacturers and IPC-D-356A for describing netlists.
What are the differences between RF and regular PCB design software?
Some key differences in functionality between regular and RF PCB software are listed below:
- Signal integrity features: RF PCB software tools comprehensively support analyzing and predicting HF signal integrity concerns like crosstalk and reflections. They include powerful electromagnetic simulation capabilities, while regular PCB tools provide only basic signal integrity checks.
- Impedance management: RF PCB software provides very precise impedance calculations and controls to minimize reflections and improve signal integrity.
- Material modeling: RF PCB software facilitates the selection of suitable materials like PTFE laminates based on electrical and thermal modeling. Since material selection is not crucial for regular PCBs, their tools often just default to FR4 and don’t include comprehensive analyses.
- Layout and routing features: RF PCB tools include advanced floor planners, layout engines, stack designers, and auto-routers optimized for HF signals. Regular PCB tools also include advanced layout and routing, but they lack the specific insights demanded by HF signals.
- Mixed-signal support: RF PCB tools enable accurate co-design of RF and digital circuits while managing interference issues. Regular PCB tools do not have the depth of integration required for high-speed HF PCBs.
- RF-specific design rule checks (DRCs): RF PCB tools have extensive DRCs for HF phenomena and regulatory compliance, while regular tools lack DRCs specific to RF designs.
What are the best practices for designing multi-layer wireless PCBs using RF PCB design software?
RF PCBs use more layers than regular PCBs. Multiple layers enable designs to isolate sensitive signal paths, reduce crosstalk, control impedance, reduce EMI from power and ground planes, and mitigate thermal effects.
Key best practices for their designs are outlined below.
Layer stack-up design
Optimize layer arrangement using the following guidelines:
- Place power and ground planes adjacent to signal layers to provide low-resistance return paths and minimize inductance.
- Ensure that trace widths and spacing achieve a common impedance (usually 50 Ohms) throughout the PCB.
- Use hybrid stack-ups with RF materials (like PTFE) for the outer layers and cheaper materials for the inner layers.
Layout and routing
Follow these guidelines:
- Use stitching vias for efficient ground connections.
- Avoid parallel routing with non-RF signals to mitigate crosstalk.
- Keep RF trace lengths short to reduce signal loss and interference.
- Maintain adequate spacing between sensitive traces to avoid EMI.
- Use smooth trace bends and avoid sharp corners to reduce reflections.
- Isolate any digital circuits from RF components to prevent grounding issues.
Material selection
Select materials for the substrate and other layers according to the following recommendations:
- Ensure dielectric constant matching across layers to avoid signal distortion.
- Use materials with low-loss tangents to ensure signal integrity, like PTFE laminates.
- Select materials with similar CTE and low moisture absorption to prevent failures due to thermal stress.
Simulation, DRC, and testing
Place test points carefully to avoid interference with RF paths.
SPICE models and other RF models are available. Use them along with advanced electromagnetic simulation tools to thoroughly analyze signal integrity and RF performance after every change. This includes reflection analysis, crosstalk analyses, pre-layout simulations, and post-layout simulations.
Use specialized PCB design tools with comprehensive RF-specific DRCs and net-by-net analysis capabilities to ensure layout integrity and standards compliance.
How does Keysight’s RF PCB design software facilitate wireless projects?
Figure 3. Keysight ADS
The Keysight Advanced Design System (ADS), a key part of the circuit design suite, is the leading solution for RF and microwave PCB schematic capture, layout, and simulations. It provides an easy-to-use design environment and promotes best practices through integrated design guides.
ADS provides a seamlessly integrated, all-in-one environment for conducting interactive signal integrity analysis, power integrity analysis, circuit simulations, electro-thermal co-simulations, and 3DEM simulations.
For accurate signal integrity analyses, ADS includes linear circuit, nonlinear harmonic balance, and circuit envelope simulations. It can run SPICE simulations of RF components like amplifiers. It implements EM simulation techniques like momentum and finite element methods to predict signal integrity challenges in high-speed designs. Powerful statistical tools can identify issues during design and improve manufacturing yield. It can generate X-parameters through accurate nonlinear modeling for HF designs.
ADS integrates digital signal processing capabilities with RF simulations for accurate mixed-signal co-simulations.
ADS includes advanced DRCs and layout vs. schematic (LVS) tools to identify RF bottlenecks and mistakes after each change.
ADS includes extensive RF component libraries and process development kits from leading foundries, ensuring a seamless transition from design to production.
Create flawless RF PCB designs with Keysight
Figure 4. Keysight RF circuit design tools
In this article, you obtained an understanding of the features necessary in RF PCB design software like Keysight’s ADS.
To go deeper, enroll in the RF Board Design Flow Educational Bootcamp.