How to prepare for successful EMI and EMC Testing

Sometimes called radio frequency interference (RFI), electromagnetic interference (EMI) occurs when electromagnetic energy disrupts the operation of an electronic device. The source of EMI can be man-made, such as other electrical devices like switch-mode power supplies, personal computers, or naturally occurring, such as electrical storms, solar radiation, or even cosmic noises.

Electromagnetic compatibility (EMC) is the ability of a device to operate as intended in an environment with other electrical devices or sources of EMI without affecting those other devices. A device is said to be EMC-compliant when it does not influence the electromagnetic environment to the extent that other devices and systems are negatively affected.

To ensure compliance with EMI and EMC national and international regulations, many companies employ the services of a specialized testing facility. Since these tests are expensive—even if the product fails—several companies perform EMC pre-compliance testing in-house. If they have a sophisticated enough lab, companies may also perform EMC compliance testing in-house.

EMI (Electromagnetic Interference) pre-compliance testing is a series of tests and measurements conducted on electronic devices or equipment to assess their electromagnetic emissions before undergoing formal compliance testing for regulatory certification. The primary purpose of EMI pre-compliance testing is to identify and address potential electromagnetic interference issues early in the product development cycle, helping manufacturers ensure that their products meet electromagnetic compatibility (EMC) standards and regulatory requirements.

When electronics are submitted to EMC testing labs, there are many types of EMC tests these labs run. Generally speaking, EMC testing is divided into two categories: immunity testing and emissions testing.

Immunity testing

Immunity testing is the process of transmitting RF energy onto a device under test (DUT) (also referred to as equipment under test or EUT) to determine if the DUT/EUT operates correctly when in such an environment.

Emissions testing

Emissions testing is the process of measuring the RF emissions – both radiated and conducted – of a DUT/EUT to determine if its emissions levels do not exceed the limits defined by the appropriate standard. Emissions testing includes both radiated and conducted emissions tests.

• Radiated emissions: Radiated emissions are the intentional and unintentional release of electromagnetic energy from an electronic device. A radiated test is performed to ensure emissions emanating from the DUT or EUT comply with the applicable limits.
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• Conducted emissions: Conducted emissions are the coupling of electromagnetic energy from a device to its power cord. Like radiated emissions, the allowable conducted emissions from electronic devices are controlled by different regulatory agencies and testing is performed to ensure emission levels are below the applicable limits.

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EMC-compliance tests are commonly done off-site prior to the production of a device. Open-air test sites, or OATS, are the reference sites used for most standards. They are especially useful for emissions testing of large equipment systems. However, RF testing of a physical prototype is more often carried out indoors, in a specialized anechoic chamber. Types of chambers include anechoic, reverberation, and the gigahertz transverse electromagnetic cell (GTEM cell).

Roughly 50 percent of products pass their first EMC compliance test when pre-compliance isn’t considered, so it’s essential to understand the process and standards a device is tested to in order to increase your chances of success.

1. Know your standards: There are a variety of EMC standards that specify test procedures and limits for radiated and conducted emissions. These vary by country and type of equipment under test. The first step in testing is determining which standards apply to your product, and researching the testing procedures and limits required for compliance.
2. Conduct pre-compliance EMC testing: EMC compliance testing can take up to two weeks to complete—not including the time it takes to get your product into the test queue—and can cost up to $20,000 per submission. A failure in EMC compliance can result in expensive redesigns and product launch delays. And since nearly 50 percent of products fail their first EMC compliance test when pre-compliance isn’t considered, it’s likely that you’ll need to repeat your visit to the test house, multiplying costs over time.
3. Find an EMC testing lab: Once a product has gone through pre-compliance testing and passed the test with a sufficient margin, it needs to be formally certified by an EMC testing lab. Accredited labs are the gold-standard for EMC testing and choosing an accredited lab is always recommended—though not necessary—to ensure your device is ready to go to market. There are, however, some instances that require certification from an FCC-accredited lab, including devices that fall under the 'certification' authorization procedures.

The goal of EMC pre-compliance testing is to mimic the compliance test set up within an acceptable margin to uncover potential problems and reduce risk of failure prior to the expensive compliance test. EMC pre-compliance testing typically involves:

• Spectrum analyzer with quasi-peak detector
• Preamplifier (optional)
• Antenna with non-metallic stand for radiated emissions
• Line impedance stabilization network (LISN) for conducted test
• Power limiter for conducted test
• EMC near-field probes for diagnostics (optional)
• Oscilloscope with frequency and time correlation capabilities to assist in debugging (optional)
• EMC testing software

When selecting a test site, rural areas, conference rooms, and basements are good options because they minimize external signals that might mask the DUT emission levels you are trying to measure. Other considerations for improving accuracy include having a good ground plane and reducing the number of reflective objects around the test area.

EMC pre-compliance testing is a fast and affordable way to ensure products pass EMC tests the first time In the early development stages, design-for-EMC techniques are combined with diagnostics to build products with low susceptibility to both external and internal interference. Later in the development cycle, pre-compliance testing is used to catch compliance problems and improve the probability of a successful first pass of full EMC-compliance testing.

Additional benefits of pre-compliance EMC testing, include:

1. Detects errors early, fixing potential issues
2. Lowers testing and design costs
3. Projects become more agile
4. Lowers risk of failure and leads to assured compliance
5. Addresses both over and under design and engineering
6. Full EMI Compliance Chamber vs Tektronix Spectrum Analyzer