How to Measure EMI: A Complete Guide to Electromagnetic Interference Testing

Electromagnetic interference (EMI) can cause electronic devices to malfunction, communicate incorrectly, or interfere with nearby equipment. As modern products increasingly rely on wireless communication, high-speed electronics, switching power supplies, and digital circuits, measuring EMI has become an essential part of product development and compliance testing. How to measure EMI depends on the type of interference, the device under test (DUT), the applicable standards, and whether you are measuring conducted or radiated emissions.

This guide explains the fundamentals of EMI measurement, the equipment required, testing procedures, common standards, and practical considerations for obtaining reliable results.

What Is EMI?

Electromagnetic interference is unwanted electromagnetic energy generated by an electrical or electronic device that can affect the operation of another device or system.

EMI can occur through two primary paths:

• Conducted EMI: Interference travels through cables, power lines, signal wires, or other conductive paths.
• Radiated EMI: Electromagnetic energy travels through the air from the source to nearby equipment.

For example, a switching power supply can generate high-frequency electrical noise that travels through a power cable. At the same time, poorly controlled circuit traces or cables can act as antennas and radiate electromagnetic energy.

Measuring both types of emissions helps determine whether a product meets applicable electromagnetic compatibility (EMC) requirements.

Why Is EMI Measurement Important?

EMI measurement is important for several reasons. First, many electronic products must meet regulatory requirements before they can legally be marketed in particular countries or regions.

Second, excessive emissions can cause real-world problems. A device might interfere with:

• Wireless communication systems
• Radio and television reception
• Medical equipment
• Navigation equipment
• Industrial control systems
• Computers and networking equipment
• Automotive electronics
• Aircraft or aerospace systems

EMI testing also helps engineers identify noise sources early in product development, reducing the risk of expensive redesigns later.

What Equipment Is Used to Measure EMI?

Professional EMI measurements typically require specialized test equipment and a controlled test environment.

1. EMI Receiver or Spectrum Analyzer

An EMI receiver or suitable spectrum analyzer measures the frequency and amplitude of electromagnetic signals.

An EMI receiver designed for compliance testing may provide features such as:

• Quasi-peak detection
• Average detection
• Peak detection
• Frequency scanning
• Appropriate resolution bandwidths
• Automated measurement functions

Spectrum analyzers are widely used for troubleshooting and pre-compliance measurements, while compliance laboratories often use dedicated EMI receivers.

2. Antennas

Radiated EMI is measured using calibrated antennas. Different antennas are used depending on the frequency range and test requirements.

Common antenna types include:

• Biconical antennas
• Log-periodic antennas
• Horn antennas
• Broadband antennas

The antenna receives electromagnetic energy emitted by the DUT and sends the signal to the measurement receiver.

3. LISN

A Line Impedance Stabilization Network (LISN) is commonly used for conducted emissions testing.

A LISN provides a defined impedance to the equipment under test and couples unwanted RF noise from the power line to the measurement receiver. This creates repeatable testing conditions.

4. Current Probes

Current probes can measure unwanted RF currents flowing through cables and conductors. They are particularly useful during troubleshooting and development testing.

5. Test Site

Radiated emissions measurements require a controlled electromagnetic environment. Compliance laboratories may use:

• Semi-anechoic chambers
• Fully anechoic chambers
• Open-area test sites

The purpose is to reduce reflections and external electromagnetic signals that could affect the measurement.

How to Measure Conducted EMI

Conducted emissions testing generally evaluates unwanted RF energy traveling through power or other conductive connections.

A typical procedure involves the following steps.

Step 1: Prepare the DUT

Configure the product in its intended operating mode. Connect required peripherals, cables, loads, and power supplies according to the applicable test standard.

The DUT should operate in a mode that represents a realistic worst-case emission condition.

Step 2: Connect the LISN

The AC power input of the DUT is connected through the appropriate LISN. The LISN establishes the required impedance and provides a measurement port for the EMI receiver.

Step 3: Connect the EMI Receiver

Connect the LISN measurement output to the EMI receiver or spectrum analyzer.

The receiver scans the required frequency range and records the noise generated by the DUT.

Step 4: Perform a Frequency Scan

Perform an initial peak scan to identify frequencies where emissions are highest.

This scan helps engineers quickly locate potential problem areas.

Step 5: Perform Required Detector Measurements

Depending on the applicable standard, emissions may need to be measured using peak, quasi-peak, average, or other specified detectors.

The measured levels are then compared with the applicable limits.

How to Measure Radiated EMI

Radiated emissions testing measures electromagnetic energy emitted from the DUT into the surrounding environment.

A typical radiated emissions test includes these steps:

Step 1: Place the DUT on the Test Table

The DUT is positioned on a non-conductive or appropriately configured test table at the specified distance from the measurement antenna.

Common test distances include 3 meters and 10 meters, depending on the standard and facility.

Step 2: Configure the DUT

Operate the device in its normal or worst-case operating condition.

Cable arrangement can be particularly important because cables may act as unintended antennas.

Step 3: Scan the Frequency Range

The antenna receives emissions from the DUT while the EMI receiver scans the required frequency range.

Step 4: Rotate the DUT

The DUT may be rotated to identify the orientation producing the highest emission level.

Step 5: Adjust Antenna Height and Polarization

The antenna height and polarization are varied according to the test procedure.

Both horizontal and vertical polarization may be evaluated because an emission can change significantly depending on antenna orientation.

Step 6: Record the Maximum Emissions

The highest relevant emission levels are recorded and compared with the applicable limits.

Understanding EMI Measurements

EMI measurements are commonly expressed in dBµV, dBµV/m, or other units depending on the measurement method.

For conducted emissions, voltage levels are often expressed in dBµV.

For radiated emissions, field strength is commonly expressed in dBµV/m.

The basic conversion for voltage in decibels relative to one microvolt is:

dBµV = 20 log₁₀(V / 1 µV)

For example, a larger dBµV value represents a larger measured voltage.

Radiated measurements also require factors such as antenna factors and cable losses to be incorporated into the final field-strength calculation.

EMI Measurement vs. EMC Testing

EMI measurement is one component of broader electromagnetic compatibility (EMC) testing.

EMC evaluates whether equipment:

1. Does not produce excessive electromagnetic interference, and
2. Can continue operating properly when exposed to electromagnetic disturbances.

EMI measurements primarily focus on emissions, while EMC testing can also include immunity testing.

Therefore, a product can have acceptable emissions but still experience problems when exposed to external electromagnetic fields.

Common EMI and EMC Standards

The appropriate standard depends on the product, market, technology, and intended application.

Common standards and regulatory frameworks include:

• FCC Part 15 for many unintentional radiators in the United States
• CISPR standards for emissions from various types of equipment
• EN standards used for European compliance
• IEC standards covering various EMC test methods and equipment categories
• MIL-STD-461 for applicable military equipment
• Automotive EMC standards for vehicle and automotive electronic systems

The applicable limits and measurement methods can vary considerably. Engineers should identify the correct standard before beginning formal testing.

How to Measure EMI During Product Development

EMI testing does not need to begin only after a product is completely finished. Pre-compliance testing can identify problems much earlier.

Engineers can use:

• Near-field probes
• Current probes
• Spectrum analyzers
• LISNs
• E-field and H-field probes
• Temporary shielding
• Ferrites
• Different grounding configurations

Near-field probing is particularly useful for locating noisy components or circuit areas. Once a source is identified, engineers can investigate PCB layout, grounding, filtering, shielding, and cable routing.

Common Sources of EMI

Several components and design features frequently contribute to EMI.

Switching Power Supplies

Rapid switching transitions can produce significant high-frequency noise.

High-Speed Digital Circuits

Fast clocks, processors, memory interfaces, and communication buses can generate broadband emissions.

Motors and Relays

Inductive loads can create voltage transients and electromagnetic noise.

Poor PCB Layout

Large current loops, inadequate ground planes, and poorly routed high-speed signals can increase electromagnetic emissions.

Cables

Cables can conduct interference or act as antennas that radiate unwanted energy.

How to Reduce EMI After Measuring It

If a product fails an EMI measurement, engineers can investigate several mitigation techniques.

Improve PCB Layout

Reduce high-frequency current-loop areas and provide appropriate return paths.

Add Filtering

Common-mode chokes, capacitors, ferrites, and other filtering components can reduce unwanted noise.

Improve Shielding

Metal enclosures or appropriately designed shields can reduce radiated emissions.

Control Cable Routing

Keep noisy cables away from sensitive circuits and minimize unnecessary cable loops.

Improve Grounding

A carefully designed grounding strategy can reduce unwanted currents and improve EMC performance.

Reduce Switching Noise

Optimizing switching speed, adding appropriate snubbers, and controlling transient behavior can reduce high-frequency emissions.

Common EMI Measurement Mistakes

Accurate EMI testing requires careful attention to setup. Common mistakes include:

• Using an incorrect antenna factor
• Ignoring cable losses
• Incorrect LISN configuration
• Poor cable placement
• Testing in an uncontrolled environment
• Using incorrect detector settings
• Failing to test different DUT orientations
• Measuring only one operating mode
• Using unsuitable frequency ranges
• Confusing peak measurements with required compliance measurements

These errors can produce misleading results and make troubleshooting considerably more difficult.

Best Practices for Accurate EMI Measurement

For reliable results:

1. Identify the applicable standard first.
2. Use calibrated measurement equipment.
3. Follow the required test configuration precisely.
4. Test the product in representative operating modes.
5. Evaluate both conducted and radiated emissions when applicable.
6. Investigate peak frequencies rather than relying only on overall scans.
7. Maintain proper cable configuration.
8. Use a controlled test environment for formal measurements.
9. Repeat measurements when unexpected results occur.
10. Keep detailed records of equipment, configuration, frequencies, and results.

Final Thoughts

Knowing how to measure EMI is essential for designing reliable electronic products and achieving EMC compliance. Conducted EMI is typically evaluated through a controlled connection such as a LISN, while radiated EMI is measured using calibrated antennas and an appropriate test environment. EMI receivers, spectrum analyzers, probes, antennas, and other specialized equipment allow engineers to identify unwanted electromagnetic energy and determine whether emissions meet applicable limits.

The most effective approach is to combine formal compliance testing with pre-compliance measurements throughout product development. Early identification of EMI sources makes it easier to improve PCB layout, filtering, grounding, shielding, and cable design before the final certification stage.