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joi, 2 februarie 2012

EMC Test Facilities

EMC Test Facilities

by Lynda-Ross Vega

Electromagnetic measurements must be conducted under specialized conditions for both emission and immunity testing. Whether the purpose of the test is for certification to a regulatory standard, customer requirement, or troubleshooting, testing must be performed in an environment that allows for measurement to occur without disruption from external ambients. This chapter provides insight into various types of facilities available to the test engineer, regardless of the type of measurement to be taken. It is difficult to troubleshoot a problem area when undesired signals from broadcast stations (AM/FM radio, televisions, personal communication equipment, etc.) occur at the same frequency as the signal of interest being measured in an open or uncontrolled environment.

It is not expected that those using this book for the purpose of testing and troubleshooting for EMC compliance will build the facilities detailed herein. Material is provided that allows one to understand the test environment that products are placed within and necessary steps toward achieving a test environment that provides correlated test results.

For EMC compliance, we are concerned with attempting to measure the amount of electromagnetic field strength being radiated by a device under test. For immunity, we attempt to determine if a specified amount of electromagnetic energy will couple into the unit and cause degradation of performance. For both emissions and immunity, it is important to have a reflection-free area as well as eliminate all external ambients from the test environment. Each proscriptive standard (i.e., military or commercial) specifies in detail requirements and test procedures to be followed. Included are open-area test sites (OATSs), screened rooms (anechoic or semianechoic), and specialized test cells.

This overview of different facilities, both indoor and outdoor, used for both emission and immunity testing. For each facility, a basic review on how to perform tests is presented. The reader is referred to the vendor of each product for specific details along with appropriate application notes. Not every possible item that can be used for EMC testing is detailed herein. Only the more commonly used facilities are presented.

An OATS is the prescribed facility for performing the majority of radiated emission testing. It provides the most direct and universally acceptable approach. An OATS requires a calibrated receive antenna, a proper ground plane, and quality coaxial cables and must be located a significant distance from metallic objects and high-ambient electromagnetic fields such as broadcast towers and power lines. This allows accurate radiated emissions from a EUT to be measured. Similarly, using a calibrated transmitting antenna for susceptibility tests on specialized equipment can be investigated under specific field and test conditions, especially if the frequency to be transmitted does not fall within a frequency range designated for communication purposes.

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