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Conducted Emissions Testing of Electrical and Electronic Devices

Key Takeaways

  • Conducted emissions are the electromagnetic energy that gets coupled into an electrical or electronic device, its power cables, and associated circuits.

  • Conducted emissions testing measures the levels of electromagnetic disturbances conducted by the equipment under test. 

  • Conducted emissions testing consists of a line impedance stabilization network (LISN), a spectrum analyzer or EMI receiver, and a ground plane. 

 Utility power supply

Conducted emissions directly influence the power quality of a utility grid

In product development, ensuring that a product’s design adheres to EMC regulations improves the reliability and safety of the product. EMC regulations determine the allowable limits for emissions in electrical and electronics equipment, and there are specific EMC regulations and testing standards for conducted emissions as well as radiated emissions. In this article, we will focus on conducted emissions: what they are, the standards that guide these emissions, and the testing procedure for measuring them.

What Are Conducted Emissions?

Electromagnetic energy that gets coupled into an electrical or electronic device, its power cables, or associated circuits is considered conducted emission. Conducted emissions can be either common-mode interference or differential-mode interference. 

Common mode interference appears in a live wire, neutral wire, or signal wire with respect to earth connections. Common-mode conducted emissions often affect signal and control lines. Common-mode interference is normally associated with high-frequency switching.

Differential-mode interference is between the live wire and neutral wire or between signal wires. Differential-mode interference results from low-frequency switching.

There are a few standards that provide guidance on the allowable limit for these emissions. 

Standards for Conducted Emissions 

As with any other electromagnetic disturbance, there are standards issued by regulatory commissions to limit conducted emissions. The standards set for each product category are different and depend on the electromagnetic environment where the product will be in service. Two examples of conducted emissions standards are the FCC standards for conducted emissions in the 450kHz to 30MHz frequency range and the CISPR 22 standards limiting the conducted interferences in the frequency bandwidth of 150 kHz to 30MHz. In order to ensure products meet these standards, emissions testing is conducted.

The Importance of Conducted Emissions Testing

Electrical and electronic equipment must pass conducted emissions testing in order to be legally authorized to be sold on the market. It is important for devices to pass conducted emissions testing, as devices will influence the power quality of the utility grid where they are plugged in. When electromagnetic energy coupled to a product reaches the utility grid, it can interfere with other devices connected to the same utility supply. There, emissions can be conducted or radiated to other devices, depending on the paths available for the spread of electromagnetic disturbances.

Conducted emissions testing measures the level of electromagnetic disturbance conducted by the equipment under test. These tests look at coupled emissions in the range of 30 kHz to 30MHz that are coupled to equipment through power cables and signal lines. Conducting coupled emissions testing ensures the utility power supply network is not polluted by plugging in a device and that devices operating in the vicinity are not damaged by interference from the device under test.

Conducted Emissions Testing Setup

The typical setup for conducted emissions testing consists of:

  1. Line impedance stabilization network (LISN) - supplies the utility power mains to the device under test.
  2. Spectrum analyzer or EMI receiver - measures noise.
  3. Ground Plane - the device under test, spectrum analyzer, and LISN are placed over and connected to a common ground plane.

A line impedance stabilization network is a three-port network connected to the power mains, spectrum analyzer, and device under test. This network provides standardized impedance at radio frequency for the device under test at its measurement points. The LISN connects the measurement points of the device under test to the spectrum analyzer. The unwanted electromagnetic signals from the supply side are attenuated by the LISN and prevent the disturbance caused by such signals during test execution. As the frequency ranges in the FCC and CISPR standards differ, the LISN impedance value differs as well. The noise measured by the spectrum analyzer is compared with conducted emissions standards to check whether it is within acceptable limits. 

Conducted emissions from electrical and electronic devices are detrimental to their operation as well as the operation of nearby devices. In fact, conducted emissions are one of the common reasons for poor power quality in utility mains. Conducted emissions testing is one of the most practical ways to address these damaging emissions. Cadence’s suite of design and analysis tools can help designers simulate electromagnetic disturbance generation during the design stage of a product. 

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