The Basics of EMC Testing for Electronic Devices

Electromagnetic and radio frequency interference shielding is concerned with the “noise” and electromagnetic emissions from signals and currents inside electronic devices. Electronic devices can impact how nearby devices function and they can be susceptible to emissions from other neighboring devices.

The effects of interference range from annoying (e.g. static on the radio) to life-threatening (e.g. malfunctioning aircraft controls or electronic breaking signals).

OEMs and their suppliers need to design and test electronics for electromagnetic compatibility (EMC). In general, testing verifies if a device falls within acceptable frequency ranges and meets limits on amounts of radiation emitted. Testing also indicates if a device is susceptible to radiated or conducted emissions from neighboring devices. All new electronic devices must pass EMC tests in order to be brought to market.

EMC includes international, national, and local standards. Definitions, tests, and standards are developed by two bodies within the International Electrotechnical Commission (IEC): Technical Committee 77 and the International Special Committee on Radio Interference (CISPR). There are different tests and standards based on the type of product/device and whether it’s intended for residential or industrial use. In the US, standards are governed by the American National Standards Institute (ANSI), Department of Defense (DoD), Federal Communications Commission (FCC), Radio Technical Commission for Aeronautics (RTCA), and the Society of Automotive Engineers (SAE). Local standards are usually based on these and the CISPR standards.

Here’s a quick look at how testing works.

EMC testing basics

Electronic devices must pass formal testing by an accredited EMC testing lab. These facilities do various tests in carefully constructed and shielded rooms called anechoic chambers, designed to prevent illegal transmission of radiofrequency radiation generated during testing. They also prevent radiation from entering the room from external sources.

While stamped components, such as shield boxes or cages and cable connectors, are not necessarily closely inspected, they can have an effect on a device’s EMC if they leak radiation or contribute to electrical noise. Ancillary equipment used with the device under testing (DUT) can also contribute to testing success or failure, including USB drives, power or adapter supply cables, and device housings.

Testing covers these areas:

• Electromagnetic emissions from the device
  • Radiated emissions or the strength of emissions radiated through the air
  • Conducted emissions, or emissions along coupling paths of power, signal, data cables and into other devices
• Immunity from external sources of emissions
  • immunity from continuous or transient (e.g. electrostatic charges, power surges, or pulsed magnetic fields) radiation
    • Radiated immunity refers to how susceptible the device is to emissions radiated through the air
    • Conducted immunity refers to susceptibility to radiation directly from other sources (e.g. power, signal, or data cables)

Pre-compliance EMC testing

One testing company estimates up to 50 percent of products fail testing on the first try. There are many possible causes for failure because electronic devices contain so many individual components and circuits. Anyone or several components together can contribute, and it takes time to determine exactly where problems originate.

Due to the high cost of formal testing and subsequent redesign, and considering the fines associated with non-compliance, it is in your interest to include EMC in your designs from the earliest stages. While only a certified testing facility can verify a device officially meets all EMC requirements, you can also do some of your own pre-compliance testing to find and fix problems before formal testing. These strategies save time and money during product development and beyond.

You probably won’t have access to a true anechoic room, but you can still get good results by testing in an area with minimal signal exposure such as a basement or building away from others and without any other electronic devices nearby.

Radiated emissions test equipment includes an EMI receiver, spectrum analyzer, EMI antenna, and E field, H field, and current probes. Immunity testing calls for RF signal generators and radiating antennas.

Look for the following potential points of failure:

• circuit loops (especially those with large surface area)
• coupling paths like slots, seams, apertures, and gaps in components
• emissions “escape” points on device housings like cable connectors, buttons and switches, display screens
• ancillary equipment that will be used with the device under normal operating conditions, which may not pass testing even if your device does (e.g. USB drives, power adapters)

One of the most effective ways to save time on EMC compliance is to include it in your electronics designs from the start. Another is to use top-quality components manufactured to print. At CEP Technologies, we specialize in precision progressive stampings for EMI/RFI shields at tight tolerances. Get in touch to see how we can help with your next project.