EMC Measurement vs. EMI Measurement: Key Differences Explained

This article clarifies the distinction between EMC (Electromagnetic Compatibility) measurement and EMI (Electromagnetic Interference) measurement. While often used interchangeably, there are subtle differences. It’s crucial to understand these distinctions, especially when dealing with compliance testing. For commercial applications, receivers must adhere to CISPR Part-16 standards, while military applications require compliance with MIL-STD-461. CISPR 16-1-1 specifies that a receiver should operate within a frequency range of 9 kHz to 18 GHz, with an absolute accuracy of about +/-2dB, among other requirements. Companies like Keysight Technologies develop specialized EMI/EMC equipment to meet these needs.

Types of EMC Measurements

Figure 1 illustrates the four main types of EMC measurements:

• Radiated Emission: This measures a device’s ability to avoid generating electromagnetic waves that could interfere with nearby wireless devices. The goal is to ensure the device’s emissions stay within acceptable limits.
• Radiated Immunity: This assesses a device’s capacity to withstand external electromagnetic waves without malfunctioning or degrading in performance.
• Conducted Emission: This focuses on a device’s ability to avoid generating conducted disturbances or noise on power or data lines. Ideally, noise should be eliminated, or at least reduced through effective circuit design.
• Conducted Immunity: This tests a device’s resilience to electrical disturbances and noise that may be present on power or data lines.

Importantly, EMI measurement falls under the broader umbrella of EMC measurement.

Conducted Emission Measurement: A Closer Look

The following steps are generally followed during a conducted emission EMC measurement (as shown in Figure 2):

1. Connect the Device Under Test (DUT) to the test system, including a Line Impedance Stabilization Network (LISN) and a limiter.
2. Set the desired test frequency.
3. Measure and record ambient emissions with the DUT turned off.
4. Switch on the DUT and measure signals using a peak detector. Record these readings.
5. Compare the recorded measurements against the applicable standards to determine if the DUT passes or fails.

EMI Measurement: Key Guidelines and Radiated Emission Testing

Here are general guidelines for conducting EMI measurements on a DUT:

1. Determine the Product’s Usage Location: This is vital for identifying the relevant regulatory agencies and standards for testing in the country where the product will be used.
2. Establish Test Limits: Determine the specific limits for conducted and radiated tests based on the appropriate regulations.
3. Select Frequency Bands: Choose the specific frequency bands that will be tested.
4. Calibration: Ensure all equipment is calibrated correctly, accounting for any cable losses or module gains.
5. Define Evaluation Signals: Clearly define which signals will be measured and the pass/fail criteria for each.

Both Radiated Immunity and Conducted Immunity tests are considered under EMI measurements.

Figure 3 illustrates a typical radiated emission test setup. The process generally involves:

1. Connecting a biconical antenna to an EMI receiver, positioning the antenna at a specified distance from the DUT.
2. Setting the test frequency and bandwidth.
3. Calibrating the system, including cable and antenna correction factors.
4. Measuring ambient emissions with the DUT turned off.
5. Switching on the DUT and recording the new measurements.
6. Comparing the recorded measurements against the standards to establish a pass/fail result.

In summary, while the terms are related, EMC encompasses a wider range of testing and compliance, while EMI focuses more specifically on identifying and mitigating sources of electromagnetic interference.