Introduction
Electromagnetic interference (EMI) is a common challenge in modern electronic systems. From smartphones and computers to industrial machinery, automotive electronics, and wireless communication equipment, electronic devices can generate unwanted electromagnetic energy that affects the performance of other equipment. Understanding how to measure EMI is essential for identifying interference sources, improving product reliability, and ensuring compliance with applicable electromagnetic compatibility (EMC) requirements.
EMI measurement involves detecting and evaluating unwanted electromagnetic emissions produced by electronic devices. Engineers use specialised instruments, such as spectrum analysers, EMI receivers, antennas, and current probes, to determine the frequency, amplitude, and characteristics of interference. Accurate measurement helps manufacturers identify potential problems before products reach the market.
What Is EMI Measurement?
EMI measurement is the process of assessing electromagnetic noise generated or transmitted by an electrical or electronic device. This noise may travel through the air as radiated emissions or through cables and power lines as conducted emissions.
There are two primary types of EMI measurements:
1. Conducted EMI measurement
Conducted EMI travels through electrical conductors, including power cables, signal wires, and grounding connections. It is commonly measured at the power input of equipment using a line impedance stabilisation network (LISN) and an EMI receiver or spectrum analyser.
2. Radiated EMI measurement
Radiated EMI travels through the air in the form of electromagnetic waves. It is measured using suitable antennas and receiving instruments in a controlled environment, such as an EMC chamber or an open-area test site.
Both measurement types are important because a device may generate interference through its wiring, its enclosure, or both pathways.
Essential Equipment for Measuring EMI
Accurate EMI measurement requires the correct instruments and test setup. The equipment depends on the device under test, frequency range, applicable standards, and whether conducted or radiated emissions are being evaluated.
Common equipment includes:
EMI receiver: Measures electromagnetic emissions across specified frequency ranges and provides measurement capabilities suited to compliance testing.
Spectrum analyser: Displays signal amplitude against frequency and helps engineers identify interference peaks and dominant noise sources.
Antennas: Capture radiated electromagnetic emissions for evaluation.
Line impedance stabilisation network (LISN): Provides a defined impedance at the equipment's power connection and couples conducted noise to the measuring instrument.
Current probes: Detect high-frequency interference currents flowing through cables.
Near-field probes: Help locate noise sources on circuit boards and inside electronic enclosures.
EMC test chamber: Reduces external electromagnetic interference and supports controlled radiated-emissions testing.
Calibration accessories and cables: Help maintain measurement accuracy and repeatability.
Not every troubleshooting task requires a complete compliance laboratory. Near-field probes and a spectrum analyser may be sufficient for initial diagnostics, while formal compliance testing generally requires a more controlled and standards-specific setup.
Step-by-Step Guide to Measuring EMI
Step 1: Identify the Device and Applicable Requirements
Begin by identifying the equipment under test and the potential sources of interference. Consider its operating voltage, switching frequencies, clock signals, communication interfaces, cables, and connected peripherals.
Next, determine the applicable emissions requirements. For example, FCC Part 15 requirements may apply to certain unintentional radiators marketed in the United States. Other products may fall under CISPR standards or product-specific EMC requirements.
The applicable standard determines the measurement frequencies, detector settings, limits, antenna arrangements, and test distances.
Step 2: Prepare the Test Environment
Choose a suitable testing environment based on the measurement objective. Initial troubleshooting can take place on a laboratory bench, but accurate results may require a shielded enclosure, semi-anechoic chamber, or open-area test site.
Keep unnecessary electronic devices away from the measurement area and arrange cables consistently. External radio signals, nearby power supplies, and other equipment can affect readings.
For formal testing, follow the environmental and site requirements specified by the relevant standard.
Step 3: Configure the Measuring Instruments
Set up the EMI receiver or spectrum analyser according to the test requirements. Configure the frequency range, resolution bandwidth, detector type, input attenuation, and other relevant parameters.
For conducted emissions, connect the equipment under test to a suitable LISN and connect the measurement port to the receiver through the required cables and attenuation components.
For radiated emissions, position the appropriate antenna at the specified distance and height. Antenna polarisation and equipment orientation may need to be varied to identify the highest emission levels.
Always check instrument calibration status and follow equipment manufacturers' instructions.
Step 4: Operate the Device Under Realistic Conditions
Power on the device and operate it in a representative configuration. Test modes that may produce the highest emissions, such as maximum processing load, active wireless communication, motor operation, or high switching activity.
Connected peripherals, cable lengths, and operating modes can significantly influence EMI results. Record the test configuration so measurements can be repeated consistently.
Step 5: Scan the Frequency Range
Perform an initial frequency sweep to identify potential interference signals. A spectrum analyser displays detected signals as peaks across the selected frequency range.
Look for prominent peaks, broadband noise, unexpected harmonics, and signals that may approach applicable emission limits. Use preliminary scans to identify frequencies that require closer investigation.
For compliance testing, conduct the prescribed final measurements using the correct detector, bandwidth, and measurement procedure. A quick spectrum scan alone does not establish regulatory compliance.
Step 6: Measure and Record Emission Levels
Record the frequency and amplitude of relevant emissions, together with the detector type, bandwidth, antenna or probe configuration, and operating conditions.
EMI measurements commonly use units such as:
dBµV: Voltage level expressed relative to one microvolt.
dBµV/m: Electric field strength expressed relative to one microvolt per metre.
dBµA: Current level expressed relative to one microampere.
dBm: Power level expressed relative to one milliwatt.
Compare the measured results with the applicable limits, ensuring that measurement corrections, cable losses, antenna factors, and other required factors are included.
Step 7: Identify and Reduce EMI Sources
If measurements reveal excessive emissions, investigate the circuit or system components responsible. Common sources include switching power supplies, fast digital signals, poorly routed cables, inadequate shielding, and grounding problems.
Potential corrective actions include improving PCB layout, adding appropriate filtering, reducing cable-loop areas, improving enclosure shielding, and correcting grounding or bonding arrangements.
After implementing changes, repeat the measurements using the same configuration to determine whether the modifications have reduced emissions.
Common Challenges in EMI Measurement
EMI measurement can be affected by several factors. External radio-frequency signals may produce misleading peaks, while inconsistent cable arrangements can change emission levels. Incorrect detector settings, unsuitable antennas, inadequate grounding, and uncalibrated instruments can also lead to unreliable results.
Another common mistake is confusing near-field troubleshooting with formal compliance testing. Near-field probes are valuable for locating noise sources, but their readings do not directly establish compliance with far-field radiated-emissions limits.
Following the relevant standard and documenting each test configuration helps improve repeatability and confidence in the results.
Why Professional EMI Testing Matters
Professional EMI testing provides access to calibrated instruments, controlled environments, suitable antennas, and experienced EMC engineers. Testing laboratories can evaluate conducted and radiated emissions, investigate unexpected interference, and recommend practical design improvements.
For manufacturers preparing electronic products for the market, professional testing can help identify problems early, reduce redesign costs, and support applicable regulatory compliance activities. The required testing depends on the product category, destination market, and relevant standards.
Conclusion
Measuring EMI involves identifying potential interference sources, selecting suitable instruments, preparing a controlled test environment, scanning relevant frequencies, recording emission levels, and comparing the results against applicable requirements. Conducted and radiated measurements provide complementary information about how unwanted electromagnetic energy travels through a device and its surroundings.
Whether troubleshooting a circuit board or preparing a commercial product for compliance evaluation, a systematic measurement process helps engineers improve electromagnetic compatibility and product reliability. For dependable results, use calibrated equipment, follow the appropriate test standard, and seek support from an experienced EMC testing laboratory when formal compliance measurements are required.
Frequently Asked Questions
1. What instrument is used to measure EMI?
An EMI receiver or spectrum analyser is commonly used to measure electromagnetic emissions. Antennas, LISNs, current probes, and near-field probes are selected according to the measurement type and objective.
2. How is conducted EMI measured?
Conducted EMI is typically measured using a LISN connected to an EMI receiver or spectrum analyser. The setup measures unwanted high-frequency noise travelling through power or signal conductors.
3. How is radiated EMI measured?
Radiated EMI is measured using an appropriate antenna and receiving instrument. Testing takes place at a specified distance in a suitable environment, following the applicable measurement standard.
4. What units are used for EMI measurements?
Common units include dBµV for voltage, dBµV/m for electric field strength, dBµA for current, and dBm for power. The correct unit depends on the measurement method.
5. Can EMI be measured without a professional laboratory?
Yes. Engineers can perform preliminary investigations using spectrum analysers and near-field probes. However, formal compliance testing may require calibrated equipment, a controlled test site, and procedures specified by the applicable standard.
6. What is the difference between EMI and EMC testing?
EMI testing evaluates unwanted electromagnetic emissions or interference. EMC testing is broader and assesses whether equipment can operate acceptably in its electromagnetic environment, including relevant emissions and immunity requirements.
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