While the term “radio frequency” may evoke memories of the days when excitable broadcasters announced the latest news and sports, RF entails much more today than simply what’s on the radio. In electronics, any application that requires transmitting and receiving radio signals above 100MHz is considered an RF circuit board, while those above 2GHz transitions into microwave PCBs. These boards are used for many different applications, including IoT and other wireless devices, radar, and cell phones.
Circuit boards that operate at these frequencies require specific RF design guidelines and PCB layout and circuit optimization above the needs of standard digital circuitry. RF designs begin with the laminates for board fabrication and may require material characteristics that exceed standard FR-4. From there, the designer will need to be aware of the RF-specific rules that may affect how the board is laid out. We will look at some of these rules and guidelines to prepare designers for their next RF design.
General RF Design Guidelines for PCB Layout and Circuit Optimization
RF circuit boards are much more sensitive to interference and demand tighter tolerances in their design. Specifically, noise, ringing, and reflections must be accounted for, requiring careful impedance matching when routing. Signal return paths are critical in the high-frequency circuits of an RF design. The signal returns currents will follow the path of least impedance, even if it isn’t the designated path. Those unexpected signal return paths can also introduce additional noise.
In RF PCB layout, circuit simulation and analysis become essential to find problem areas and correct them. Some deviations to standard PCB layout rules can be expected to achieve the best signal performance, including tighter placement clearances and reduced pad sizes. However, these modifications should only be used when necessary to maintain the desired DFM rules for error-free fabrication and assembly. RF PCB design isn’t a matter of black magic science, but instead, knowing what compromises are available to achieve the best performance possible. Let’s look at some of these unique requirements starting first with component placement.
RF Component Placement
When placing components on an RF circuit board, the needs of the circuitry often demand a tighter part placement. Modifying the component placement is one of those compromises that the designer must carefully manage to ensure that the board can still be built with standard automated assembly equipment to keep production costs from escalating. RF boards also tend to have more metal on the external layers for their power networks, and components are often placed directly on those areas. While this connection may be better for the performance of the RF circuitry, it can cause problems for PCB assembly.
Another aspect of RF PCBs to keep in mind is that the components may require altering during layout. These can range from changing the size and shape of the PCB footprint pads to incorporating specific metal features and shapes into the footprint. Work like this is usually done individually to components on the board instead of altering the base library part. Next, we’ll list some of the routing requirements you may encounter on an RF PCB design.
Trace Routing for RF Circuits
As with any high-speed routing, short, direct routing with varying widths will be necessary for controlled impedance routing and higher-current power traces. Here are some other points about RF routing to keep in mind:
- Most trace routing must be manually added for specific RF topologies.
- Setting up and using your design rules is critical to success.
- Routing features such as copy and paste may also prove useful when replicating areas of similar circuitry.
- Curved trace corners or mitered corners will help reduce impedance fluctuations.
- Vias provide fencing around areas of the ground plane for shielding.
In some RF designs, some areas of routing will be treated as components. This routing requires specialized tools that can accommodate parametric component routing to send these trace segments, which are now lumped components, into RF analysis systems.
For power and ground planes, prepare for higher power requirements in RF designs and ensure that the thermal reliefs can handle the current while facilitating automated soldering. Some copper pours, and planes in an RF design may need to be converted into lumped components for RF analysis. It may also be necessary to create unusually shaped metal areas for unique RF elements using the drafting capabilities of your CAD tools and then convert those areas into intelligent net objects.
As you can see, RF design may involve some new and exciting methods of laying out a circuit board. Now let’s look at what your manufacturer will need from you to build this circuit board.
Building Your RF PCB
You’ll begin by ensuring partnership with a PCB contract manufacturer that understands RF circuits’ needs and has proven experience building them. Some deviations from standard DFM rules may force your PCB CM to create special assembly processes, and the CM needs to understand how to work around these areas. The CM will also need to be provided with some additional information before building the board:
- Is the board a new design or a modification of an existing one? New boards will require additional review to ensure that nothing is missed while existing designs will typically have already gone through those steps.
- How much experience does the design team have with the target application of the board design, and how much general RF experience do they have? The answer here will give the PCB CM an understanding of what to look for during their reviews.
- What are the expected power/reflective waveforms in the application that this circuit board will need to support?
- What kind of critical RF issues has the design team encountered with earlier designs, and were they able to identify the root causes of the problems and find an approach that worked?
- Has the circuit board been designed with an adequate power delivery network to support the grounding methodology being used and the targeted voltage requirements?
At VSE, we have been helping our RF customers for over 30 years and are familiar with the challenges and issues you face in designing high-frequency circuit boards. Our engineering team can help you identify areas of concern and provide design recommendations as needed.