Design Guidelines for better EMC Standards

EMC is a measure of a device's ability to operate as intended in its shared operating environment while, at the same time, not affecting the ability of other equipment within the same environment to operate as intended. Evaluating how a device will react when exposed to electromagnetic energy is one component of this, known as immunity (or susceptibility) testing. Measuring the amount of EMI generated by the device’s internal electrical systems – a process known as emissions testing – is another.

Both aspects of EMC are important design and engineering considerations in any system. Failing to properly anticipate the EMC of a device can have a number of negative consequences, including safety risks, product failure and data loss. As a result, a wide range of testing equipment for EMC and EMI has been developed to give engineers a clearer picture of how a device will operate in real-world conditions.

Importance of EMC: Ensures devices operate without interference from other devices and do not cause interference themselves.

Growing Need: With increasing complexity and interconnectivity of devices, EMC is crucial for compliance and functionality.

1.    What is EMC?

EMC stands for Electromagnetic Compatibility. Every electronic device/machine on the market must be EMC compliant, meaning it must fulfill the EMC regulations and standards for the intended use of the product. Which EMC regulations and standards are applicable for which product is defined by the country where the product is sold (e.g. EU or USA etc.).

An EMC-compliant product must fulfill one or several of these points:

• Not interfere with other devices/machines in its environment (emission).
• Not be upset by other devices/machines in its environment (immunity).
• Not be upset or destroyed by an Electrostatic Discharge (ESD) event.
• Not be upset or destroyed by an Electromagnetic Pulse (EMP).

An additional point that can be mentioned here in this context: a product should not interfere with itself. This topic refers not to EMC in particular, it refers to signal integrity. However, signal integrity and EMC often go hand in hand with each other.

2.    EMC vs EMI:

EMI stands for Electromagnetic Interference and is often mixed up with EMC. EMI means that one electronic device/machine A is causing disturbance to another electronic device/machine B, which is in the surroundings of device/machine A.

What is the difference between EMC and EMI? Now, an EMC-compliant product has to be tested on EMI during its development. For an EMC-compliant product, EMI should not happen anymore. This is due to the fact that EMC-compliant products proved their electromagnetic immunity to be high enough and their electromagnetic emission to be low enough to work seamlessly in their predefined environment.

3.    EMC Compliance:

EMC Compliance means that an electronic or electromechanical product is compliant with the laws, directives, and regulations of the country where it is sold. First, every government issues its own EMC regulations (laws, directives) for its country. Whereas these national regulations often refer to multi-national regulations (e.g. countries in the European Union refer to the EMC directive 2014/30/EU).

Second, the government usually builds, appoints, or chooses an organization, commission, or committee which is responsible for defining the applicable EMC standards. Such organizations or committees define the applicable EMC standards in a way that products, which pass the tests defined in the applicable EMC standards, are then compliant with the EMC regulations (laws, directives).

4.    What are EMC Standards?

1) What are EMC Standards?

EMC Standards and norms define terms, rules and test methods for EMC. Furthermore, they specify limits and minimum test levels for electric and electromagnetic emissions and immunity of electromechanical and electronic products.

2) Why do we need EMC Standards?

EMC Standards help to make measurements comparable and repeatable by defining the test methods and the test equipment and the test environment. And most importantly, EMC Standards have the purpose of bringing harmonization to EMC testing, in the best case: a global harmonization. This lowers trade barriers and as a most important consequence for the society: harmonized EMC Standards help to increase global prosperity and wealth.

3) Who writes EMC Standards?

Norms and standards in EMC are either defined and worded by recognized international and national or regional organizations (like the EU delegates the wording of EMC Standards to CENELEC, or the administrative and/or regulatory bodies word the EMC Standards and regulations themselves.

4) What types of EMC Standards are there?

It is to be distinguished between the following classes or types of EMC Standards:

Basic EMC Publications: The Basic EMC Publications specify the terms and conditions for EMC testing, they define the rules necessary for achieving electromagnetic compatibility, they specify test methods (testing techniques, test setup, test equipment and environment) and so on. Basic EMC publications are the EMC standards to which other EMC standards (EMC Product Standards, Generic EMC Standards, etc.) refer to.

EMC Product Standards: The EMC Product Standards apply to particular products, such as electric road vehicles or coaxial cables.

EMC Product Family Standards: The EMC Product Family Standards apply to a group of products that have common general characteristics, that may operate in the same environment and have similar fields of application.

Generic EMC Standards: The Generic EMC Standards are for products operating in a particular EMC environment (residential/industrial), where there does not exist a specific Product (Family) EMC Standard.

5.    Measuring and Monitoring EMC?

Emissions testing requires the use of EMI measurement equipment such as receiving antennas, amplifiers and spectrum analyzers. Working together, these tools provide an accurate measurement of the amount and type of noise generated by a device. This can be done either on an open area test site or in a shielded, anechoic (or semi-anechoic), test chamber.

Immunity (or susceptibility) testing involves determining the ability of a device to tolerate noise from external sources. In order to do this, it is necessary to have tools that can simulate and measure electromagnetic energy specific frequencies. EMC testing equipment may be used to subject a device to electromagnetic noise at various frequencies, to simulate a power surge or to assess the effectiveness of a device's power supply. Ultimately, the nature of the device, its intended application and any regulatory requirements will determine which type of testing equipment is required.

6.    Design Guidelines for Better EMC standards:

        1.    Minimize the Loop Areas

        2.    Don't Split, Gap or Cut the Signal Return Plane

        3.    Don't Locate High-Speed Circuitry Between Connectors

        4.    Control Signal Transition Times

Rule #1: Minimize the Loop Areas

This simple rule is on nearly everybody's list of EMC guidelines, but it often gets ignored or compromised in favor of other guidelines. Often the board designer doesn't even know where the signal currents flow. Digital circuit designers like to think of signals in terms of their voltage. Signal integrity and EMC engineers must think of signals in terms of their current.

There are two things that every good circuit designer should know about signal currents.

      1.    Signal currents always return to their source (i.e. current paths are always loops)

       2.    Signal currents take the path(s) of least impedance.

At megahertz frequencies and higher, signal current paths are relatively easy to identify. This is because the path of least impedance at high frequencies is generally the path of least inductance, which is generally the path that minimizes the loop area. Currents return as close as possible to the path of the outgoing current. At low frequencies (generally kHz frequencies and below), the path of least impedance tends to be the path(s) of least resistance. Low frequency currents are more difficult to trace, since they will spread out. Significant current return paths may be relatively distant from the outgoing current path.

Rule #2: Don't Split, Gap or Cut the Signal Return Plane

Sure, there are some situations where a well-placed gap in the return plane might be called for. However, these situations are relatively rare and always involve a need to control the flow of low-frequency currents. The safest rule-of-thumb is to provide one solid plane for returning all signal currents. In situations where you expect that a particular low-frequency signal is susceptible or is capable of interfering with the circuitry on your board, use a trace on a separate layer to return that current to its source. In general, never split, gap or cut your board's signal return plane. If you are convinced that a gap is necessary to prevent a low-frequency coupling problem, seek advice from an expert. Don't rely on design guidelines or application notes and don't try to implement a scheme that "worked" in someone else's "similar" design.

Rule #3: Don't Locate High-Speed Circuitry Between Connectors

Among board designs that we have reviewed or evaluated in our lab, this is one of the most common problems we've encountered. Many times simple board designs that should have had no trouble at all meeting EMC requirements at no additional cost or effort, wind up being heavily shielded and filtered because they violated this simple rule.

Why is the location of connectors so important? At frequencies below a few hundred megahertz, wavelengths are on the order of a meter or longer. Any possible antennas on the printed circuit board itself tend to be electrically small and therefore inefficient. However, cables or other devices connected to a board can serve as relatively efficient antennas.

Signal currents flowing on traces and returning through solid planes result in small voltage differences between any two points on the plane. These voltage differences are generally proportional to the current flowing in the plane. When all connectors are placed along one edge of a board, the voltage between them tends to be negligible. However, high-speed circuitry located between connectors can easily develop potential differences of a few millivolts or greater between the connectors. These voltages can drive currents onto attached cables causing the product to exceed radiated emissions requirements.

Rule #4: Control Signal Transition Times

A board operating with a clock speed of 100 MHz should never fail to meet a radiated emissions requirement at 2 GHz. A well-formed digital signal will have a significant amount of power in the lower harmonic frequencies, but not so much power in the upper harmonics. Power in the upper harmonic frequencies is best controlled by slowing the transition times in digital signals. While, excessively long transition times can cause signal integrity and thermal problems, an engineering compromise must be reached between these competing requirements. A transition time that is approximately 20% of a bit period results in a reasonably good-looking waveform, while minimizing problems due to crosstalk and radiated emissions. Depending on the application, transitions times may need to be more or less than 20% of the bit period, however transitions times should not be left to chance.

7.    Methods for controlling rise and fall times

1. Use a logic family with a controlled slew-rate.
2. Put a resistor or a ferrite in series with a device's output.
3. Put a capacitor in parallel with a device's output.

The first choice, controlled slew-rate logic, is often the most effective option, especially when driving a matched termination. However, the majority of the logic circuits on most well-designed boards will have a capacitive termination. Using a series resistor to control the risetime of these circuits gives the designer more control and usually costs less. Ferrites can also be effective, but cost more and provide less control than resistors. Capacitors can actually increase the amount of high-frequency current drawn by the source device and in most cases are not an appropriate choice for transition time control.

Note that it is never a good idea to try to slow down or filter a single-ended signal by impeding the flow of current in the return path. For example, one should never intentionally route a low-speed trace over a gap in a return plane or put a ferrite on a ground in an attempt to filter out the high-frequency noise.

8.    Board Layout Design Guidelines:

1) Trace:

Increase Trace Width: Wider traces decrease radiated emissions.

Avoid 90-Degree Angles: Especially in high-speed applications to reduce reflections.

Consider Return Current Paths:  To Reduce radiated emissions and common-impedance coupling.

2) Vias:

Add Stitching Vias Around High-Speed Signal Vias: Adding stitching vias around high-speed signal vias improves the integrity of the ground plane and reduces emissions.

3) Area:

Separate Critical and Non-Critical Areas: High-frequency circuits and low-level analog circuits are critical.

4) Grounding:

Direct Grounding Path: Ensure low impedance and direct paths for current return.

Use Ground Planes: Solid ground planes on top and bottom layers help shield inner layers.

Avoid Split Ground Planes: Prevents large loops and high emissions.

Filling Ground pour: Filling the top and bottom layers of your PCB with circuit ground helps reduce radiated emissions by providing a solid return path for currents.

Utilize Ground Planes for Shielding: Solid ground planes on PCBs act as shields, separating noisy signals from sensitive ones. However, they are more effective against electric fields and high-frequency magnetic fields.

5) Decoupling:

Capacitor on Every Connector Pin: To reduce radiated emissions and improve conducted immunity. Adding a capacitor to every pin of a connector helps filter out unwanted signals and improves the device's immunity to external interference.

Conclusion:

Electromagnetic Compatibility (EMC) is a critical aspect of electronic design that ensures devices function effectively in their intended environments without causing or experiencing interference. By understanding the principles of EMC, including emissions and immunity, engineers can design products that comply with regulatory standards and perform reliably in real-world conditions.

Adhering to EMC standards is essential for reducing electromagnetic interference (EMI) and ensuring compliance with national and international regulations. Proper testing methodologies, such as emissions and immunity testing, help evaluate a device’s performance and identify potential vulnerabilities. Furthermore, implementing EMC-conscious design practices such as minimizing loop areas, optimizing grounding strategies, and controlling signal transition times can significantly enhance a product’s robustness against electromagnetic disturbances.

As electronic devices continue to evolve and integrate into increasingly complex environments, EMC remains a key consideration for engineers. By prioritizing EMC compliance from the early design stages, manufacturers can ensure product reliability, regulatory approval, and seamless operation in modern electronic ecosystems.