EMC for PCB design: Anything but Black Magic

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EMC problems are often responsible for re-design cycles in PCB design practice. Due to ever shorter innovation cycles of for example cell phones or IoT applications, such as fitness trackers or smartwatches, and many other electronic products, these time-consuming re-design cycles should be avoided under all circumstances.

Electromagnetic compatibility (EMC) is the branch of electrical engineering and physics that deals with the unintentional generation, propagation, and reception of electromagnetic waves (in the E and H fields). These can subsequently cause undesirable effects such as functional interferences, malfunctions, or even physical damage in electronic devices.

Generally speaking, there are two basic aspects to be considered. First, the emission referring to the unwanted generation of electromagnetic energy by a source and its transmission to the sinks and the countermeasures that should be taken to reduce such emission. And second, the respective susceptibility to interference relating to the operation of electrical/electronic equipment (or components) that become “victims” of such unintended electromagnetic interference.

To say this in a simple way, EMC is the ability of electronic systems to function in a common electromagnetic environment without, first of all, being affected by other systems and, second of all, interfering with other systems.

Know the potential EMI sources

On a printed circuit board, there are various potential sources of interference, that can cause a variety of potential effects from the following categories:

  • Signal and Power Integrity (conducted emission)
  • Radiated emission
  • Immunity to radiated and conducted emissions
  • ESD
Potential noise sources on a PCB

Often unnoticed by PCB designers, the ribbon cable on a PCB connector, for example, physically forms the arm of a dipole and thus forms a parasitic antenna. In this case, current and voltage peaks occurring during the switching process of the active components in the power supply can lead to an excitation of this parasitic antenna. This would result in an increased radiation pattern.

In addition, the signal-shapes in digital signaling are in theory considered ideal rectangles. Nevertheless, in reality, they don’t exist in such a form. Instead, the signals are more or less distorted and disturbed. The resulting voltage peaks of the reflections and crosstalk will also have a negative effect on the EMC behavior.

Integrating EMC-compliant design into product development

EMC-compliant design is crucial to the success of a product. Only by complying with the EMC regulations of the specific target market or application (for example the medical or automotive industry) can products be approved for customer deployment in the first place. However, problems are often only discovered during the prototype testing, which besides a lack of know-how is often due to a lack of integration of proper EMC verification procedures into the design process.

To manage EMC in the design process and to detect problems at an earlier stage there are several options. The first step should be the systematic definition and use of design constraint processes, especially for signal and power integrity issues, in the design process.

Tool-supported EMC design reviews

Tool-supported EMC design reviews can also be introduced to ensure that the relevant EMC guidelines are followed. CR-8000 Design Force comes with EMC Design Rule Checks, where specific rules can be selected or deselected as well as prioritized on a per-user or per-design base. The circuit designer can classify EMC-relevant signals for such checks as early as in the schematic design. The EMC rules suitable for an application are then selected and applied during the design phase.

The direct integration into the CAD process (2D and 3D) and the automatic generation of reports in the form of DRC checks which are familiar to any layout engineer, simplify the workflow. These reports with images and respective progress status or approval information can be exchanged as Excel documents between the members of the development team. This information is also stored in the design data for the joint work on EMC aspects in the design team.

The rules implemented in the EMC-Adviser for Design Force contain recommendations for various design issues that enable non-experts to solve signal integrity, power integrity, and electromagnetic compatibility problems and thus help to develop an error-free, EMC-compliant layout. No additional software is required to validate the identified potential EMC issues.

My “First-Aid-Kit” for PCB Designers

For an EMC-compliant PCB design, it is essential to consider (at least, but of course there are many more) the following four aspects:

  1. Identify and evaluate parasitic antennas

    It is essential to work out where parasitic antennas could form on the PCB. Parasitic antennas are developing electrical or magnetic monopole or dipole structures.

  2. Recognize and account for the current return paths

    An electric current inevitably returns to its source. Thus, visualizing the return paths and the return loops plays an important role. Depending on the application, the returning current either runs along the path with the lowest impedance or the path with the lowest resistance. In order to select a correct return path, lines should not be wired across slots if possible, not even in differential pairs.

  3. Understanding the various coupling effects

    Coupling paths between the source and the sinks can either occur depending on parasitic voltages or parasitic currents or be IO-related. In many cases, their root causes are not immediately recognizable in the layout.

  4. Understanding resonances as potential antennas

    Almost all electrical structures can become resonant. This includes single lines and differential signals, but also power supply structures, cables, packages, even vias. Fortunately, the resonance frequency for many structures can be easily calculated using this formula:fres=1/(2π√L∙C) However, knowledge of the values for the (parasitic) inductances (L) and capacitances (C) is not quite so easy to obtain and often requires complex analysis. Also, resonances cannot be completely erased. It is important to know and understand the effect and to avoid excitation – where possible.

You can find these and other tips for EMC-compliant PCB design explained in detail and practice-oriented in our webinar

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Ralf Bruening
Ralf Bruening
Product Manager/Senior Consultant
Ralf Bruening is a product manager involved in development and deployment of Zuken’s analysis and high-speed solutions for the past 20 years. Ralf works to improve customer design processes and solve complex technical challenges associated with high speed signal transmission, power integrity problems and EMI emissions. In his free time, Ralf likes to golf, cook for his family and listen to rock music.