Making Conducted and Radiated Emissions Measurements

Table of Contents

• Introduction to Radiated and Conducted Emissions Measurements
  • Pre-compliance versus full compliance EMI measurements
  • Systems for performing pre-compliance measurements
• Pre-compliance Measurements Process
  • European norms descriptions
  • Federal Communications Commission
• Emissions Testing
  • Introduction
  • Conducted emissions testing
  • Radiated emissions measurements preparation
  • Setting up the equipment for radiated emissions measurements
  • Performing radiated emissions measurements
• Problem Solving and Troubleshooting
  • Diagnostics testing setup
• Appendix A: Line Impedance Stabilization Networks (LISN)
  • Purpose of a LISN
  • Transient limiter operation
• Appendix B: Antenna Factors
  • Field strength units
• Appendix C: Basic Electrical Relationships
• Appendix D: Detectors Used in EMI Measurements
  • Peak detector
  • Quasi-peak detector
  • Average detector
• Appendix E: EMC regulatory agencies
• Glossary of Acronyms and Definitions

Introduction to Radiated and Conducted Emissions Measurements

The concept of getting a product to market on time and within budget is nothing new. Companies know electromagnetic interference (EMI) compliance testing can be a bottleneck in the product development process. To ensure successful EMI compliance testing, pre-compliance testing has been added to the product development cycle. This is a low-risk, cost-effective method to ensure your device will pass final compliance testing. In pre-compliance testing, the electromagnetic compatibility EMC performance is evaluated from design through production units. Figure 1 illustrates a typical product development cycle.

Many manufacturers use (EMI) measurement systems to perform conducted and radiated EMI emissions evaluations prior to sending their product to a test facility for full compliance testing. Conducted emissions testing focuses on unwanted signals that are on the AC mains generated by the device under test (DUT). The frequency range for these commercial measurements is from 9 kHz to 30 MHz, depending on the regulation. Radiated emissions testing looks for signals broadcast for the DUT through the air. The frequency range for these measurements is between 30 MHz and 1 GHz, and based on the regulation, can go up to 6 GHz and higher. These higher test frequencies are based on the highest internal clock frequency of the DUT. This preliminary testing is called pre-compliance testing.

Figure 2 illustrates the relationship between radiated emissions, radiated immunity, conducted emissions, and conducted immunity. Radiated immunity is the ability of a device or product to withstand radiated electromagnetic fields. Conducted immunity is the ability of a device or product to withstand electrical disturbances on AC mains or data lines. In order to experience electromagnetic compatibilities (EMC) problems, such as when your radio interferes with your mobile phone reception and affects call quality, there must be a source or generator, coupling path, and receptor. An EMC problem can be eliminated by removing one of these components.

European requirements pay special attention to product immunity. Product immunity is the level of the electric field that a receptor can withstand before failure. The terms immunity and susceptibility are used interchangeably. This document will not cover immunity testing.

1.1 Pre-compliance versus full compliance EMI measurements

Full compliance measurements require the use of a receiver that meets the requirements set forth in CISPR16-1-1, a qualified open area test site or semi-anechoic chamber, and an antenna tower and turntable to maximize DUT signals. Great effort is taken to ensure accuracy and repeatability. These facilities can be quite expensive. In some specific cases, the full compliance receiver can be replaced by a signal analyzer with the correct bandwidths and detectors as long as the signal analyzer has the sensitivity required.

Pre-compliance measurements are intended to give an approximation of the EMI

performance of the DUT. The cost of performing pre-compliance tests is a fraction of the cost of full compliance testing using an expensive facility.

The more attention to detail in the measurement area, such as a good ground plane and a minimal number of reflective objects, the better the accuracy of the measurement.

1.2 Systems for performing pre-compliance measurements

The components used in systems for pre-compliance measurements are as follows: signal analyzer with N6141C EMI measurement application, line impedance stabilization network (LISN), transient limiter, and antennas. To isolate problems after they have been identified, close field probes N9311X-100 are used.

The environment for pre-compliance testing is usually less controlled than full compliance testing environments. See Figure 3 for the components used for pre-compliance testing.