Evaluation Engineering - December 2008 - (Page 46)

EMC TEST 45 40 35 30 25 2006 20 15 10 5 0 0 500 1,000 2,500 Frequency (MHz) 2,000 2,500 3,000 3,500 2007 2008 Pre-Repair 2008 Post-Repair Figure 2. History of Calibration for a Biconilog Antenna Maintaining Calibration The importance of maintaining calibration for antennas used frequently for product compliance measurements cannot be over-emphasized. For example, if the calibration changes due to antenna damage or misuse, the compliance data will be suspect, and the compliance measurements may well have to be repeated. Figure 2 shows the history of calibration for a biconilog antenna over time and the effect the lack of maintenance had on the antenna’s performance. New Emphasis on Measurement Uncertainty Many factors impact the accuracy of antenna calibration, hence contributing to the term measurement uncertainty. Examples include properly matching the connection of the antenna to the cable and the cable to the receiver as well as taking into account cable insertion loss related to temperature. According to Bob DeLisi, senior staff engineer at Underwriters Laboratories and chair of the C63.23 Working Group in ASC C63® on Measurement Uncertainty, the ambient temperature impact of testing at an open area test site (OATS) can be very important. For example, if testing is done at a time of year that provides a signiﬁcantly varying temperature range over the antenna calibration interval, the heating or cooling of the cable can introduce errors. These errors must be factored into the uncertainty budget. For the ﬁrst time, the proposed revision to ANSI C63.5 will contain text 4 6 • E E • December 2008 on the elements of uncertainty and the deﬁnitions and equations describing how a lab calculates its measurement uncertainty and provisions on how to reduce the overall error contributions. It will be presented in a format enabling those performing antenna calibrations to readily enter data necessary for the calibration. The importance of measurement uncertainty when applied to antenna calibration sometimes is underestimated or ignored according to Dennis Lewis, lead engineer for RF/microwave and antenna metrology for The Boeing Company. For instance, if two laboratories calibrate antennas, you expect to have nearly the same result within the measurement uncertainty of their test setups, instrumentation, and procedures. If their measurement uncertainties are not comparable, you will get different results on that fact alone. How can you decide which calibration lab to use? Industry is moving toward the use of global suppliers, which makes it more critical to harmonize global antenna calibration standards. It also is important to understand the traceability path to a recognized national standards lab. For many companies, using an accredited calibration lab is all that is required. However, it is important to understand the lab’s scope of accreditation and verify that the scope is adequate for the service being requested. Accredited labs are required to participate in interlab comparisons (ILC) where a standard artifact, in this case an antenna, is sent around to various labs to be measured. The results are shared among the labs in an effort to validate each lab’s calibration process and help identify areas for improvement to processes and standards. Most Antenna Factor (dB) Golden Antennas by Daniel D. Hoolihan, Hoolihan EMC Consulting and NIST/NVLAP Lab Assessor Reference standards are called out in ISO/IEC 17025: General Requirements for the Competence of Testing and Calibration Laboratories. The term reference standards appears in Clause 5.6, titled Measurement Traceability, speciﬁcally, Clause 5.6.3.1. The laboratory is required to have a program and procedure for calibrating its reference standards. They shall be calibrated by a body that can provide traceability as described in Subclause 5.6.2.1. These reference standards of measurement should be held by the lab and used for calibration only and for no other purpose unless it can be shown that their performance as reference standards would not be invalidated. Finally, reference standards must be calibrated before and after any adjustment. For EMC testing laboratories, antennas used to qualify radiated emissions test sites for NSA, including volumetric SA, shall be considered reference standards. Whether biconicals, log-periodics, or complex combinations of biconicals and log-periodics, these antennas must be calibrated by an accredited calibration laboratory to satisfy the traceability requirement and reserved for verifying site attenuation of radiated test sites such as OATS, semi-anechoic chambers, and similar test arenas. The antennas should not be used for daily testing to prevent physical damage to them from unexpected shock impulses and vibration occurrences. Antennas used as reference standards should be treated as golden antennas. www.e v al u a ti o n e n g i n e e r i n g . c o m http://www.evaluationengineering.com

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Evaluation Engineering - December 2008 Contents Editorial Product Briefing Test Software C-V Measurements Nanoelectronics Test Product Guide Company Guide Machine Vision EMC Test Index of Advertisers

Evaluation Engineering - December 2008

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