AppNote56 - Application Note #56 Proper Use of RF Field...

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1 of 6 111909 Figure 1: Closed Loop Method Application Note #56 Proper Use of RF Field Probes Used in EMC Radiated Immunity Testing By: Jason Smith, Applications Engineer Supervisor, Jason Galluppi Software/systems Engineering Supervisor & Pat Malloy, Senior Applications Engineer RF field probes are perhaps one of the most underrated components of a radiated immunity test system. While we rely heavily on data sheets to select all the other devices required to generate a given field level, only a live test using a trusty field probe will measure the resultant field level and thus, validate the RF test system. While we assume that all measurements meet the accuracy published in the probe’s data sheet, it must be noted that these indispensible tools can be a source of considerable error if not used correctly. This application note will review the underlying concepts of field measurement with an emphasis on the proper application of field probes. Broadband Isotropic RF field probes are commonly used in immunity testing applications. Insuring the accuracy of these very precise measuring devices over their entire frequency range can be challenging for both the probe designer and the user. To minimize errors, the user should understand how these devices operate and what influences could affect their resultant readings. Test methods There are two generally accepted methods used to determine field strength; the closed-loop method and the substitution method. While each method has its advantages and disadvantages, ultimately the test standard will determine the method used. Closed Loop Method : In this method an RF field probe is positioned in front of or on top of the EUT during susceptibility testing. The field is adjusted to the intended field strength for each frequency step across the entire test frequency band. Since the commonly used diode-type field probe can not accurately measure a modulated RF signal, one must either apply correction factors to the probe readings or take measurements using only a CW signal and only apply modulation later during the actual test run. Closed loop testing provides a “real-time” reading of field level as well as a direct correlation between instantaneous field level and the operation of the test device. This Immediate feedback adds confidence in both the system performance and the immunity of the EUT. Closed loop testing also proves to be the fastest method since no additional time is required to calibrate and level the RF field. Some argue that this approach is a better representation of the actual RF levels seen by the EUT. This method is a good choice for testing small EUTs where reflections from a relatively small reflective surface area have a minimal affect on the RF field. As the size of the EUT increases a point is reached where reflections from the larger reflective surface area of the EUT will have a noticeable affect on the field. The resultant standing waves create an unstable situation whereby the probe readings can vary dramatically when moved about the room or as a
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This note was uploaded on 06/07/2011 for the course EE 11 taught by Professor D during the Spring '11 term at Central Lancashire.

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AppNote56 - Application Note #56 Proper Use of RF Field...

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