Universal Language: PCB Design to Minimize EMI in Communications Systems

Can you imagine what a world without lines would be like? At first, it seems wonderful to not have to wait around anymore, trusting that people will figure their business out. Disneyland, happiest place on Earth, receives about 44,000 people per day. Imagine a thousand people all rushing to get on the next available log flume. No longer just a tower of terror, but a whole theme park.

When designing PCBs, your traces are the lines and each has a particular signal it should be carrying. When this is not maintained, you have confusion or noise. In communication systems, PCBs are usually called upon to generate and process digital and analog signals with varying frequencies, constant DC power signals and do so according to timing regimens. All of these can cause signal integrity issues, such as electromagnetic interference (EMI).

EMI is unwanted noise that can cause erratic behavior and in some cases overloading which may lead to component damage. It is not possible to eliminate all noise from your design; however, by employing a few simple rules for component placement, trace routing and grounding the signal integrity issues associated with EMI can be greatly reduced.

How Does Component Placement Affect Signal Integrity Issues?

Rule one for reducing EMI effects is to partition your board components according to the types of signals that they use. Typically, this will lead to at least of the following component sections on your PCB.

• Digital Section - includes switching circuitry
• Analog Section - includes amplifiers and audio components
• RF Section - includes filters and antennas

Whenever possible component sections should include external inputs and outputs. For components that includes multiple signal types, such as processors and converters (A/D or D/A), place component across borders for the partitions and keep signal types and components separated as much as possible.

Routing Traces to Minimize EMI in Your Communications Systems

When routing traces, it is recommended to use short wide traces instead of long ones. Long traces increase resistance and power dissipation. Another good rule for inputs and outputs is to use differential routing, where one trace is for the signal and one is for the return.

Differential inputs and outputs

When using differential routing, make sure the traces are the same length and as close together as possible. This will cause noise cancellation between them. Keeping careful attention to your routing will always be rewarding for your designs; however, you’ll see that with EMI related issues specifically, routing can take you a long way.

Ensure Signal Stability with Grounding

Probably the most important rule to reduce EMI is to employ good grounding techniques. Not doing so can lead to a number of signal integrity issues, such as crosstalk, distortion and of course, noise. Here is a list of rules that you should follow to minimize these signal integrity issues.

1. Create a digital ground layer adjacent to the signal layer.
2. Keep digital and analog ground current separate.
3. Use the star grounding system, where all circuits ground at a single point, for analog circuits.
4. Liberally use bypass capacitors. Connect them directly to the signal pin they are being used to bypass and ground.
5. Use unused board area as ground.

Following these guidelines will help route stray signals to ground and away from signal traces.

Signal integrity issues can significantly affect the performance of your board and should be taken into account when designing your board as part of a good design for manufacturing (DFM) practice. It is even more critical when designing PCBs for communications systems that you consider the various sources of noise that can distort your signals. EMI, which can be generated by a number of communications PCB components, is a major signal integrity issue.

Incorrect trace routing can perpetuate interference throughout your system and degrade your signal quality. Appropriate grounding is another signal integrity issue that must be addressed during your PCB design. Successfully addressing these signal integrity issues during your PCB design is the first step to ensure your communication device and your customer is happy.

Don’t let a crack in your grounding keep EMI popping up in your designs.

Incorporating a good design strategy for communications systems can be difficult. However, with an advanced PCB design software package, such as Altium Designer^®, and the analysis capabilities of PDNA you have the tools to design a board that is optimized to minimize signal integrity issues.

For more information about signal integrity issues and how to design your PCB to minimize them, contact an Altium PCB design expert.

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