Conformity Magazine - November 2007 - (Page 38)

(e.g., IEC 61000-4-16) only test common-mode, whereas differential-mode tests may also be required to better simulate the effects of EM environment on the equipment. Above 30kHz, the test signal at each frequency step has an unmodulated period, followed by “chirp” modulation at least over the range of “especially susceptible frequencies” below 30kHz, then is pulsed OFF for one second then back ON again using an unmodulated CW signal. Such “CW, chirp, plus OFF/ON” tests must be slow enough to be sure of detecting any errors, malfunctions or damage given the response times of the functions being monitored. If necessary, time may be able to be saved by monitoring critical internal signals to avoid having to wait for long time-constants to respond. Special fiber-optic probes are available for such monitoring, but careful test planning might avoid the need to use them. If the “especially susceptible frequencies” have previously been identified, the testing time might be able to be reduced by modulating only at those frequencies, instead of a full chirp. Where exposure to pulsed sources is possible (e.g., radars, pulse weapons, etc.) their relevant frequency range should be covered using appropriate pulse modulation waveforms, especially any waveforms with a frequency content that includes any of the “especially susceptible frequencies”. At each tested RF frequency, a CW test with a one-second pulse off and then on again is usually required. During immunity testing, all variations in functional performance should be recorded, and analyzed afterwards to see if they had any relevance for safety. Physical Environment, Wear and Tear, HALT The physical environment over the lifetime of the system can degrade its EM performance. Shock and vibration, bending forces, temperature extremes or cycling, wear and tear and many other lifetime mechanical, physical, climatic and biological influences can affect the radio-frequency (RF) stability of some types of circuits, and degrade the performance of EM mitigation measures such as shielding, filtering and transient suppression, for example by corrosion. There are well-established test methods for most physical phenomena. HALT test experts combine physical test methods to quickly discover likely end-of-life characteristics. But some physical stresses might occur that are not covered by established standards, for example the use of abrasive cleaners, or the repetitive opening and closing of a door or inspection panel. It may be necessary to devise realistic tests for such physical lifetime stresses. To verify that the EM design is adequate requires EM testing during the application of the physical stresses, such as mechanical forces, temperature extremes, etc. Appropriate close-field probing techniques can detect whether the EM performance of a shield or filter is significantly degraded by the physical stress. If degradation is to be permitted, more sophisticated quantitative measurements may be needed. However, EM testing is only needed before and after HALT tests that simulate ageing and wear-and-tear. Where electronics are protected from the physical environment by an external means, such as an enclosure, physical tests can be carried out on the enclosure itself, as shown in Figure 1, maybe using close-field probing of its seams, joints and other apertures, instead of the antenna shown. This has the advantage that the enclosure can be proved to be adequate as a parallel activity to the electronic and/or software design, helping to shorten timescales and reduce overall project costs. HALT test plans should always be designed by HALT testing experts, based on the system’s physical environment specification. Adding EM tests to these HALT tests need not add significantly to the overall time or costs, if they are designed appropriately. Where suitable data exists or can be calculated for a particular EM design aspect, and when it is fully documented in the project’s records, combined EM and physical testing may not be necessary. For example, tests such as those depicted by Figure 1 might not be needed where an enclosure manufacturer has already applied appropriate physical and HALT tests and measured their effect on EM performance (having first checked that the manufacturer’s claims can be relied upon). Figure 1: EM testing during physical stress testing 38 Conformity noVEmBEr 2007

Table of Contents Feed for the Digital Edition of Conformity Magazine - November 2007

Conformity - November 2007 Contents Editor’s Note Commission Sets Date, Terms for 700 MHz Auction FCC Reaffirms Roaming Obligations Commission Moves to Improve USF Management FCC Proposes Fines for Violations of E911 Handset Requirements Commission Levies $1 Million Plus Fines for Unsolicited Fax Advertisements GR-3150 - Lithium Battery Certification Levels Based on Criteria for General Product, Safety, and Performance, Part 2 ESD Open Forum Machinery Field Evaluation for the United States Validation, Verification, and Immunity Testing Techniques for EMC for Functional Safety Thermal/EMC Simulation Addresses New Mechanical Design Challenges Product Safety Equipment Buyer’s Guide FCC Releases Report on Telephone Numbering Resource Utilization Canada Still Has PCS Spectrum to Assign Updated Standards List for the EU’s Medical Devices Directive Updated Standards List for EU Directive on Active Implantable Medical Devices Call for Papers for the 2008 EOS/ESD Symposium Announced Mobile Phone Use in Hospitals May Be Dangerous CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Product Reviews Looking Back: Items from Past Issues of Conformity Upcoming Events Advertisers

Conformity Magazine - November 2007

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