What is EMI?
Electromagnetic interference (EMI) is the disruption of operation of an electronic device when in the vicinity of electromagnetic energy of an external electrical source – either man-made or naturally occurring. Sometimes called, radio frequency interference (RFI), EMI can cause devices to not operate correctly or how they were intended to.
Sources of EMI are usually man-made, coming from other electrical devices such as switch-mode power supplies, internal circuitry of personal computers, cathode ray tube (CRT) displays, to just name a few, but can also be naturally occurring, coming from things such as electrical storms, solar radiation, or cosmic noises.
What is EMC?
Electromagnetic compatibility (EMC) is the concept of enabling a device to operate correctly in its intended environment, with or in the presence of other electrical devices or sources of EMI. Likewise, a device is said to be EMC compliant when it does not influence the electromagnetic environment to the extent that the functions of other devices and systems are negatively affected.
Why test for EMI or EMC?
Anyone designing, manufacturing, or importing products with electronics inside needs to care about EMC compliance. EMI and EMC regulations are in place throughout the world to provide improved reliability and safety to users of electrical and electronic equipment. These regulatory bodies have set specific limits on the amount of unwanted emissions that can be emitted from a device.
While there are many safety and reliability reasons, the primary goal of these commercial limits is to prevent interference to nearby television and radio receivers. Failing to pass EMC compliance can result in fines, seizures, or even product recalls.
What is EMI or EMC testing?
To ensure EMC/EMI compliance with regulations, many companies have historically employed the services of a specialized test facility, or test house, to perform EMC pre-compliance and compliance testing.
EMC compliance testing requires methods, equipment and measurement sites in compliance with national or international standards. Testing is required to confirm that a device meets the required standards. Compliance with national or international standards is defined by individual nations. It’s important to note, different nations can require compliance with different standards.
Testing is typically divided into two categories: emissions testing and immunity testing.
Emissions testing is the process of measuring the RF emissions – both radiated and conducted – of a device or EUT to determine if its emissions levels do not exceed the limits defined by the appropriate standard.
Immunity testing is the process of transmitting RF energy onto a device or equipment under test (EUT) to determine if the EUT operates correctly and without fault while under such an environment, according to the appropriate standard.
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 EMC test chamber (Figure 2). Types of chambers include anechoic, reverberation and the gigahertz transverse electromagnetic cell (GTEM cell). Compliance tests are commonly done prior to production of a device. Emissions testing includes both radiated and conducted emissions tests.
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 Device Under Test (DUT) or the Equipment Under Test (EUT) comply with the applicable limits.
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.
What is EMC pre-compliance testing?
An affordable and convenient alternative to using fully accredited compliance labs is investing in a pre-compliance test solution. EMC pre-compliance testing offers a fast, affordable method for evaluating your deign and any modifications made as you go through the design process. At the early stages of development, design-for-EMC techniques are combined with diagnostics to produce low susceptibility to both external and internal interference. Later in the development cycle, pre-compliance testing may be used to catch compliance problems early and improve the probability of a successful first pass of full EMC compliance testing without additional re-design.
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 stage. EMC pre-compliance testing does not require compliant test equipment and 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
- Near field probes for diagnostics (optional)
- Oscilloscope with frequency & time correlation capabilities to assist in debugging (optional)
EMC pre-compliance testing may be done using fast measurement techniques intended to give a ‘quick look’ at problem areas and is typically done at a temporary site by engineering personnel. More information on the ins and outs of EMC pre-compliance testing can be found on our Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC) solutions page.
When selecting a test site, it is best to pick a location that will minimize external signal sources. Rural areas, conference rooms or basements are good because they minimize signals that might mask the DUT emission levels you are trying to measure. Other considerations for improving accuracy involve having a good ground plane and reducing the number of reflective objects around the test area.
Why perform EMC pre-compliance testing?
Overall, EMC pre-compliance testing can save you time, money, and effort.
Full EMC compliance testing can be exhaustive, time consuming and expensive. A failure in EMC compliance at this late stage of product development can result in expensive re-designs and product introduction delays. And since nearly 50% of products fail EMC compliance their first time 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.
The full EMC compliance test in a certified lab can be expensive with costs ranging from $1,000 to greater than $20,000 per submission depending on the device and the number of countries you are selling your product into. Full EMC compliance testing can also be time consuming with emissions and immunity testing taking 2 to 6 days and generation of the test report taking another 3 to 10 days. Note, this doesn’t include the time to get your product into the test queue.
To prepare for EMC compliance testing, many companies have historically rented time in full compliance test facilities. This too can be expense with costs ranging from $700 for a half day to $10,000 per day depending on the services needed. As an alternative, some companies have chosen to set up fully compliant EMC test facilities in-house. The benefit of this is testing in an environment that duplicates actual testing. The disadvantage is costs can range from 100s of thousands to millions of dollars.
Benefits of pre-compliance EMC testing:
- Detects errors early, fixing potential issues
- Lowers testing and design costs
- Projects become more agile
- Lowers risk of failure and leads to assured compliance
- Addresses both over and under design and engineering
- Reduces future warranty issues
We discuss more of the financial benefits of pre-compliance testing in our Financial Case for an EMI/EMC Pre-Compliance Test Solution blog.