Reasons for Conducted Emissions Testing Failures

Key Takeaways

• What conducted emissions testing is and how to set up a test.

• What measurement uncertainty is and why it causes tests to fail.

• Common measurement uncertainty standards. 

To test whether electromagnetic interference is affecting a device or piece of equipment, manufacturers conduct EMC testing. Electromagnetic interference can be classified as either conducted or radiated emissions. EMC testing measures conducted emissions with conducted emissions testing and radiated emissions with radiated emissions testing.

The testing procedure and measurement techniques of emissions testing must follow EMC testing standards. A violation of these standards can result in test failures. The measurement techniques and instruments used for conducted emissions testing play an important role in determining the successful completion of conducted emissions tests. Measurement uncertainty is one reason why conducted emissions testing can fail. In this article, we will discuss conducted emissions testing procedures and the standards that help designers navigate measurement uncertainty. 

Conducted Emissions Testing 

Electromagnetic disturbances conducted by the equipment under test are measured through conducted emissions testing. The measurement of conducted emissions within the range of 30 kHz to 30MHz that get coupled to the equipment under test through power cables and signal lines are identified through conducted emissions testing. The equipment under test receives a pass or fail certificate depending on the emissions measurement and the EMC standard followed in the region where the test is conducted.

When the conducted emissions measurement is within the acceptable limits stated in the EMC standard, the equipment under test receives a pass certificate. Passing the conducted emissions test successfully assures the manufacturer and customer that the utility power supply network will not be polluted by plugging in the device. It also ensures that electrical and electronic devices operating in close proximity do not interfere with the device.

How to Set Up a Conducted Emissions Test 

The conducted emissions test setup consists of:

1. Device under test
2. Line impedance stabilization network (LISN)
3. Spectrum analyzer or EMI receiver
4. Ground Plane

The utility power mains is supplied to the device under test through LISN.  The spectrum analyzer or EMI receiver measures the noise. The device under test, spectrum analyzer, and LISN are placed over and connected to a common ground plane.

Measurement Uncertainty: A Reason Why Conducted Emissions Test Failures Happen

There are many possible reasons for conducted emissions test failures—an improper EMC connection port in the device under test, a test setup arrangement that violates EMC standards, or measurement uncertainty are just a few examples.

Measurement uncertainty is a complex failure problem. If the measurement uncertainties are above certain specified limits, the conducted emissions test is not accepted by the accreditation body and the test is considered a failure.

There are many sources of measurement uncertainty in conducted emissions testing:

• LISN impedance
• Impedance mismatch
• Attenuation of LISN-EMI receiver
• LISN voltage division factor
• EMI receiver reading
• Receiver pulse repetition
• Receiver pulse amplitude
• Receiver sine wave

The LISN is the largest source of measurement uncertainty. The LISN impedance deviation reflects as high decibels in combined standard uncertainty and expanded uncertainty, and causes wrong emissions test results.

Measurement Uncertainty Standards 

Today, EMC regulatory bodies quote the measurement uncertainty limits for electromagnetic emissions during conducted emissions testing. The measurement uncertainty in conducted emissions testing is subjected to ISO/IEC guide 98-3, which is the reference for all measurement uncertainty documents. This guide provides a clear picture of measurement errors of all sorts, which are also applicable to conducted emissions testing. The standard IEC CISPR 16-4-2 addresses uncertainties in EMC measurements under the specification for radio disturbances as well as immunity measuring apparatuses and methods. Another measurement uncertainty standard from the United Kingdom Accreditation Service is LAB34, which focuses on the measurement errors and calibration requirements in EMC testing. 

Understanding the reasons for conducted emissions test failures is the first step to getting your design to pass the test. Cadence’s software offers assistance in creating designs that will pass conducted emissions testing and meet EMC standards. 

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