Tips for High Frequency PCB Design in Embedded Systems

Today's high-speed embedded systems incorporate diverse functionality, components, digital interfaces, and of course, wireless/RF signaling. If you're designing embedded systems with any level of computing power that also includes an analog front end, then you have multiple mixed-signal design challenges to overcome. Whether it's a simple sub-1 GHz radio connection, Wifi/BLE, or multi-gigabit Ethernet, embedded systems need some way to interface with the outside world that does not rely on pure digital signaling. This applies to systems like backplanes, single-board computers, IoT products, and much more.

With the high frequencies used in today's embedded systems, board designers have a responsibility to confront these problems early in the design process. Successful high-frequency PCB design comes down to success in three areas:

• Stackup design and grounding
• Component placement
• Routing and impedance control

This might sound obvious and it's the kind of fundamental design tips we push in many articles on this blog. However, these points should be underscored here as high-frequency PCB designs with some digital functionality, particularly modern embedded systems, rely on success in all three areas to ensure systems function as designed. To help you get started on the floor plan and layout of your next PCB, let's look at each of these areas to establish some useful PCB design rules for high frequency in embedded systems.

Challenges in High Frequency PCB Design

The challenges involved in high-frequency PCB design and digital systems all focus on signal integrity, and to some extent power integrity as I'll briefly discuss. The types of PCBs we're considering here are boards for embedded systems that will also include some RF functionality. The RF portion can be implemented in a standard wireless protocol like WiFi or BLE, while the digital section could be just about anything that interfaces with an MCU, MPU, or FPGA.

Isolating High Frequency Signals

Probably the most important design consideration in these systems is to isolate digital and analog signals. If your system has an analog section, antenna feedline, etc., you'll need to isolate the analog section away from the digital section as crosstalk can interfere with the analog signal. This is one reason it's preferable to use waveguide routing as it provides natural isolation, as I'll explain in more detail below.

If you can do this portion successfully, you're closer to producing an analog section with greater EMI immunity in the analog section. As I'll discuss below, it's best to start thinking about this at the stack-up stage (the beginning of the design) and NOT during the routing stage (towards the end of the design). First, there are some peculiarities in embedded systems that you should consider as you floorplan the design.

What Makes Embedded Different?

The difference between an embedded system and many other digital systems with high-frequency PCB design is the use of high-speed digital interfaces. Although high-speed computing interfaces use differential pairs for data transmission, there is still the possibility of common-mode noise (crosstalk) induced between digital and analog sections. In addition, signals with faster rise time concentrate their signal power out to higher frequencies, so it is more likely to appear as noise on a single-ended RF interconnect.

Since you're basically dealing with a miniaturized computer running at high speed, you also have power integrity concerns, particularly ground bounce/rail bounce. The two effects are basically the same, they just manifest themselves in different ways.

The table below briefly summarizes the challenges involved in designing embedded systems with an analog section. We can generally link design challenges in each area back to our three fundamentals of stack-up design, routing/impedance control, and isolation.

Obviously, there are a lot of areas to examine when trying to design high-speed embedded systems with an analog/RF section. The other problematic area is that the solutions in each area are frequency-dependent. Since this is a high-level article, there's not enough time to hit every area when it comes to high-frequency PCB design rules for high frequency, but I'll link out to other articles where relevant.

It Starts With Ground Plane and Stackup Design

Most power integrity and signal integrity problems can be resolved with the right PCB stack-up. The stack-up will also affect solvability, routability, impedance control, EMI/EMC, thermal control, and isolation. If you're designing an embedded system with high-speed digital interfaces and an analog section, then the stack-up is the place to start thinking about your design.

I've shown an 8-layer stack-up below that will provide room in the interior for low-speed signals and additional planes as needed, but I've only labeled the important signal and plane layers at the top and bottom of the board. If you don't need this additional space, you could certainly stop at 4 layers with a Signal/Plane/Plane/Signal arrangement. The important point here is that I've allowed 2 reference layers to be adjacent to each other, regardless of the other layers. The goal of this type of stack-up is to keep reference planes close to each other to prevent the types of power integrity problems that can turn into signal integrity problems. 

This type of stack-up will give you two component layers if needed, plenty of ground between each side of the board, and adjacent plane layers that can be used for a power/ground plane pair to provide plenty of interplanar capacitance. The reference planes on Layer 2 and Layer (N - 1) are the critical layers for ensuring impedance control, isolation, and consistent reference throughout the board. This type of high-layer count stack-up (at least 6) is common for more advanced systems with small footprints.

If you are working with a purely analog or mixed signal device, there are a number of important points to consider in your stack-up and grounding floor plan. However, you've also got to put high-speed/high-frequency components onto the board, so it's natural to look at component placement before you start routing.

Component Placement Above the Top Reference Plane

First, you should separate your ground planes into digital and analog sections, but leave these two sections physically connected to provide a consistently low reactance path back to the ground. If you are working with frequency RF signals at multiple frequencies, consider sectioning off a third analog ground for these signals and components. If you stack the ground and power as I've shown above, then you've already done the same with the power section and, ideally, prevented multi-port excitation from high transfer impedance in the PDN.

Routing Your High Frequency PCB Design With Isolation

The next point to consider is how traces are routed between components. However, you might also need some strategies for isolation between different board sections and around individual interconnects to prevent interference. Some strategies include:

• Coplanar waveguide routing on the surface layer, particularly for tracks between components, tracks to feedlines or routing to coaxial connectors.
• If you have a high enough layer count, consider setting one layer aside for stripline routing. Be careful with this as vias can create problems at ultra-high frequencies (mmWave).
• Placing a grounded fence around the RF power plane provides a good level of suppression as it creates two out-of-phase radiators.
• Consider using band gap structures to provide isolation as these can be printed directly onto a high-frequency PCB.

How you design the interconnect to ensure consistent impedance matching is less important than how it is routed to prevent interference. Probably the most important point here is to keep track of return paths and route the RF interconnect away from the digital section. As long as you don't have a big return current passing near your RF interconnect, then you can be less concerned about crosstalk. Still, there are plenty of points to evaluate in the design before prototyping, and a field solver can help spot these problems

Evaluate Your Board Before Prototyping With Field Solvers

Anytime you can cut out a prototyping run and spot signal/power integrity problems at ultra-high frequencies, you've saved yourself time and money. The right field solver can help by taking your PCB layout and calculating the important signal integrity metrics needed to ensure an RF product will work as designed. In high-frequency PCB design, some of the important points to examine are network parameters, radiated EMI, and interference between different board sections. If you've placed an antenna directly on the board, you'll also want to examine the radiation pattern and radiated power.

When you need to calculate impedance and other parameters to determine transmission line losses, you can use the integrated field solver in the Layer Stack Manager in Altium Designer^®. For more advanced calculations involving S-parameter extraction, Altium Designer users can use the EDB Exporter extension to import their design into Ansys field solvers. This pair of field solver applications helps you verify your design before you begin a prototyping run, and are a very important part of the high-frequency PCB design rules for high frequency.

When you’ve finished your design and want to release files to your manufacturer, the Altium 365^™ platform makes it easy to collaborate and share your projects. We have only scratched the surface of what is possible to do with Altium Designer on Altium 365. You can check the product page for a more in-depth feature description or one of the On-Demand Webinars.