RF PCB Routing Guidelines to Suppress Noise

Shield your RF signals with via arrays.

We’ve come a long way from tin cans and string to transmit messages. Communication has always been one of the pillars that sets humanity apart from the rest of the species in the world, but our ability to both find new ways to communicate and to optimize older ways of communicating is what impresses me the most.

From radar to WiFi and walkie-talkies, RF designs are a fixture in our daily lives making RF PCB routing guidelines a fixture in our daily designs. Frequencies falling in the RF range span from thousands to billions of Hz, and these frequencies are present in a huge number of analog/mixed signal devices. A board does not have to appear in a communication device to be classified as an RF circuit, but many of the same design rules will apply.

RF PCB Routing Guidelines: RF Signals and Your Layer Stack

Routing your RF board to ensure signal integrity is just as much about designing the right layer stack as it is about laying traces. You can suppress transmission line effects in your signal lines with the right layer stack in your PCB.

While normally discussed in terms of digital signals, signal reflection at an impedance discontinuity affects analog signals when traces operate as transmission lines. When the propagation delay along an interconnect is greater than one-quarter the oscillation period of the analog signal, then you will need to worry about transmission line effects and ensure that your traces are impedance matched.

Although there is some natural attenuation of a reflected signal in an analog signal trace, the analog traces are constantly pumped with a harmonic source, and a reflected signal can form a standing wave in a trace if reflected at an impedance discontinuity. The natural attenuation in the signal trace only dampens the maximum amplitude at resonance, it does not completely eliminate resonance.

Any analog signal resonance on a transmission line can form a standing wave along the trace (depending on the geometry), creating a high amplitude electric field that can induce noise in other areas of the board. You can eliminate this problem if your traces are impedance matched with your source and load components.

So how can you ensure that your traces always remain impedance matched? First, you should use impedance controlled design with your layer stack. This ensures that traces routed in the signal layers will have a defined value within a specific tolerance. You will only need to worry about matching the impedance of your source and load components to this value. In other words, if one component at the end of an interconnect has different impedance than your signal trace, you must compensate the impedance of the component, rather than the trace itself.

Ensure your boards can work through the intricacies of proper signalling

Some RF Routing Basics

Because the impedance of your traces is so important, your routing techniques should take account of anything that relates to:

• EMI from other traces/components, susceptibility to external oscillating magnetic fields, and EMI radiated from your board
• Decoupling between power and ground
• Preventing coupling between RF signal traces
• Anything that can increase the impedance mismatch between traces, sources, and loads
• Prevent resonances that can radiate strongly into other areas of your board or into external boards
• Avoid sharp angles of shapes to remove discontinuities of impedance
• Use shielding traces for RF signals
• Separate the RF traces by clearances on the same layer and other layers

With high frequency, RF signals may impact other circuits and could also be impacted by other signals. This is why it is important that the RF traces are protected. The key methods include good grounding, shielding and filtering.

While this is a tall order, you cannot perfectly satisfy every requirement at all times. Which of these points should receive more attention depends on the particular application for your board.

While the list of RF PCB routing guidelines is extensive, here are some important guidelines to consider:

To suppress radiation from your circuit to your power nets, you can surround the power plane with grounded vias. It is also a good idea to place the power plane between two ground planes as this will sufficiently decouple the power and ground planes throughout the board.

With higher frequency RF circuits, traces carrying your analog signals will need to be quite short in order to prevent transmission line effects. The separation between lines should be as large as possible, and they should not be routed close together over long distances. Coupling between parallel microstrip traces increases as the parallel routing distance increases and the separation distance decreases.

Once you’ve calculated the trace geometry you need for a given layer stack, you should try to minimize the use of vias on your traces as each via increases the impedance of your interconnect. Aside from increasing impedance, any stubs left on a via will act as high frequency resonators. Vias should be back-drilled in order to prevent standing waves from forming in the via stub as a resonating signal in a stub can act as a strong radiator or antenna.

If you need to route an RF signal line to a different layer, you can use two vias in parallel to minimize the total additional inductance and impedance. Two vias in parallel will have total impedance and inductance that is half the value of a single via. When you need to place a bend in an RF signal line due to routing constraints, you’ll want to use a bend radius that is at least 3 times the trace width. This will minimize impedance changes that arise from bending a trace.

RF Boards as Mixed Signal Devices

Unless your RF board is part of a multi-board system, your RF PCB is likely to be a mixed  signal device. Some devices are exceptions, such as RF amplifiers. As such, you will need to consider standard mixed signal design techniques when working with these systems. Some of these devices will include wireless capabilities, so wireless design rules will also play a part in your design process.

With RF devices, you will likely include other analog circuitry on your board that supports your RF components and provides greater functionality. If this is the case, you should try to segment your sensitive RF components from other analog components in order to avoid routing analog return signals below sensitive RF circuit blocks.

You’ll want to follow best practices for mixed signal design, including properly splitting your ground planes, taking care to place mixed signal ICs, and proper arrangement of your analog power and ground sections. Your goal should be to reduce noise in the digital section from coupling into the RF analog section, and vice versa. Pay attention to some basic rules for routing mixed signal PCBs if you are designing this type of board.

Maintaining security in your circuit board is vital

A great simulation package that takes data from your PCB design package makes it easy to determine the best design choices for your particular application. There are plenty of issues to consider in RF design as the right design choices depend heavily on the primary operating frequency, as well as whether your RF board is really a mixed signal board.

With strong layout and design software, you will never have to encounter the borderline ridiculous problems of poor simulation checking, inability to add proper rules or constraints, or requiring too much time investment for a simple design request. Cadence’s analysis tools are here to provide your RF boards with the integrity, analysis, and simulation they need to move into production.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.