B&W photo of computer peripherals and devices

A Visit to the EMC Lab: Radiated Vs. Conducted Emissions and Immunity

At home, at work, and at play, our lives are filled with electronic devices. We (very fairly) expect these devices to work properly in each of these contexts, and usually, they do, thanks to the keen eye of the electromagnetic compatibility (EMC) testing lab. But EMC testing is a multi-faceted and often complex endeavor in which every yang has a yin, and every action reacts. Your yin or reaction might be the added expense of extra protection components or a slight compromise on functional performance. Whatever the trade-off, all aspects of a product must be brought into harmony before it goes to market.

The testing lab evaluates four aspects of EMC: Radiated emissions, radiated immunity, conducted emissions, and conducted immunity. Together, these tests illustrate how well a product can withstand interference from potential sources of electromagnetic energy in its intended environment, as well as how the product might, in turn, affect those other electronics (hopefully not very much, or you may have some re-designing to do!).

Before visiting the EMC lab, it helps to understand what each test aims to evaluate. What is the difference between radiated and conducted emissions? Why must your product have both types of immunity? This post, the second in my "Visit to the EMC Lab” miniseries, breaks down the differences between radiated and conducted immunity and emissions.

Emissions Vs. Immunity: Why We Test Both

Why so many ways to measure EMC? Unfortunately, undesirable energy can leak in many ways (this is what we call “emissions”) and missed product vulnerabilities can have devastating consequences (immunity). Therefore, it’s important to test how much energy is entering and leaving your system via hardware (conducted) as well as over the air (radiated). Testing all four aspects gives your product the best chance to meet EMC standards and thrive in its intended environment. Fortunately, a product that is well designed in one aspect tends to perform well in the others, and improving either emissions or immunity performance can often give the other aspects a boost as well.

Conducted Vs. Radiated: Two Ways for Electromagnetic Energy to Travel

Conducted emissions testing is the measurement of energy leaving your product via supply and data ports as well as the cables through which the current flows. This is generally a lower frequency measurement. Conducted immunity is the reverse: It measures how robust your product is to RF energy entering the system through the very same ports and cabling.

In contrast, radiated emissions testing shows what electromagnetic waves propagate from the operating product—typically a higher frequency measurement. Radiated immunity testing shows how robust the product is when faced with electromagnetic waves at specific field strengths.

How EMC Labs Measure Emissions and Immunity

Conducted emissions and immunity are usually measured in Volts and Amps, but they are often expressed in decibel (dB) form. In contrast, radiated emissions limits and immunity levels are measured in Volts and Amps per meter—but they are often still expressed in dB form. This can be a common source of confusion, so pay close attention during your EMC lab visit.

Why the difference? Radiated testing concerns the field strength of electromagnetic waves, using antennas to measure or transmit those electromagnetic waves respectively. The “per meter” qualifier is a fundamental aspect of describing wave behavior. Without it, the measurements mean nothing.

To learn more about electromagnetic waves, I invite you to visit one of my previous blog posts, “What is EMF? Electromagnetic Fields Explained.” To better understand how an emissions limit measured 10 meters from the product correlates to the energy source levels at the product, stay tuned for a deeper dive in a future blog post.

EMC Standards, Levels, and Limits

There are a variety of EMC standards committees responsible for determining acceptable immunity levels and emissions limits. This decision is based on a mix of known and expected electromagnetic environmental conditions in which the product is expected to operate seamlessly.

The harsher the environment, the stricter the requirements. However, the same product can have different EMC emissions and immunity requirements if it is used in more than one environment. In such a case, you can simply target the most stringent requirement; it’s always better to have exceptional performance in less stringent environments than to fall short of the standard where it matters most.

EMC Testing Challenges

While the measurement objective is simple, EMC testing can be quite complex from the lab’s perspective. Let’s touch on some of those complexities:

  • Cable length: It may seem like a minor detail, but all cable lengths must be known and disclosed. Not all tests are necessary for shorter cables. However, if a customer plans to plug in longer cables later, then it’s important to test these at the same time as the rest of the product. The product should be tested exactly as it will be released into the world. The EMC testing results only apply to the exact setup measured, including cable type/length.

  • Communications channels: Considerations must be made to ensure that test connections (coupling) do not stop or impede the product’s communications channels from functioning normally. Before testing even begins, the lab must (for example) calibrate and verify antennae and coupling devices, meet noise floor requirements, and ensure amplifiers and preamps are operating within their linear ranges.

  • Test equipment selection: There are many detector types including Peak, Average, and Quasi Peak detectors. Even when emission limits are well understood, labs must choose the right tool for the job. Each type of detector has its limits since each one processes undesirable product emissions differently.

  • Monitored product parameters: The criteria that decide a successful/unsuccessful outcome must be fair and relevant to potential product sensitivities. While category limits, levels, and standards are firm, other aspects of the evaluation process can be more flexible and should be viewed with a holistic and equitable lens.

These are just a few of the challenges and complexities an EMC lab must consider when evaluating products. Fortunately for us engineers, we don’t have to worry about every little detail; that is why we have EMC testing labs to set us straight! And you can count on them to do so, for the lab must stand behind its results after the product goes to market, for better or (hopefully not) for worse. That’s why their stamp of approval carries so much weight. When the lab says all four EMC aspects are in harmony, you can be reassured that your product will perform as expected, even when surrounded by all the other devices in its intended electromagnetic environment.

Find the third blog in this miniseries here.