Conformity Magazine - October 2008 - (Page 52)

While an anechoic chamber can be used either for interference or immunity testing, it has to meet the requirements of the appropriate standards. For interference, it has to be shown to have OATS-equivalent site attenuation (ANSI C63.4/ CISPR 16-1-4); for immunity, it must meet the field uniformity requirements of EN 61000-4-3. Things to consider in an anechoic chamber are: • Frequency range for desired performance, which depends partly on the anechoic material characteristics and partly on the size and geometry of the chamber; • Size of the chamber in relation to desired measuring distance (e.g., 3 m, or 5 m, or even 10 meters); • Equipment handling considerations –turntable capacity, door openings; • Power – to maintain chamber isolation, all power must be filtered; • Cooling – to control the environment and remove EUT-generated heat; • Access for signal cabling. Adding in Conducted Emissions Line conducted emissions, both power and signal, are required tests for a variety of equipment. The most common range for power line conducted emissions is 150 kHz to 30 MHz, although some tests (particularly NEBS, for telecom equipment) go lower. On the power mains, the key piece of equipment is the line impedance stabilization network, or LISN. Its job is to present a known impedance at the equipment under test, irrespective of the actual mains impedance. This allows a consistent measurement of the interference potential imposed on the mains. LISNs vary primarily in their current carrying capacity, the number of sections they have, since each power line and neutral must be measured, and the details of their internal circuitry. Smaller, but more complex circuitry must be added for measuring conducted emissions on telecom leads. The question of telecom lead line conducted measurement has been controversial. It has been discussed, introduced, postponed, and reintroduced into CISPR 22 over the past several years. However, on August 1, 2007, conducted emissions on telecom ports became a reality when EN 55022:1998 finally superseded its predecessor - a mere 9 years after its original publication. When shielded cables are used, the measurement is straightforward. However, when the cables are unshielded, a CDN (coupling/decoupling network) is a preferred measurement technique. From a measurement point of view, the coupling/decoupling network must not affect desired signal transmission, and must have a high differential to common mode rejection ratio (also called longitudinal conversion loss, or LCL). This is important so that no portion of the large, intended differential signal contributes to the measurement and obscures or is mistaken for the common mode signal that is to be measured. In addition, a number of different circuit types must be supported (e.g., 2, 4 and 8 wire, different data rates, different impedances, different wiring of pairs). In the absence of a CDN, as when an unusual custom circuit occurs for which one is unavailable, capacitive coupling with a high impedance FET measurement probe can be employed. Conducted Immunity For conducted immunity, either high-powered coupling/ decoupling networks must be used, or bulk current injection clamps. Some of the CDNs are simple, being little more than resistors and chokes. Others are more complex and expensive, which motivates the use of the bulk injection clamps. These function as transformers, with the driven wire functioning as the secondary. When choosing an injection clamp, consideration should be given to the operating frequency range and power handling capacity. Ideally, the clamp should cover the entire range of test frequencies to avoid the cost of multiple clamps and the additional time to swap clamps across every cable under test. Clamps not rated for high enough power (or fed by underpowered amplifiers) can be utilized by passing the cable under test through the clamp multiple times. As an aside, we note that current clamps also work in the reverse direction, for sensing current flow down cables. For Specification Type Frequency range Modulation Programmability RF signal quality Accuracy Dynamic range Comments Function generator, RF signal generator, sweep generator Function generators available at lower frequencies Internal vs. external; AM, FM, pulse; depth of AM modulation and width of FM GPIB or other interface, available drivers; Response and settling time; sweep vs. discrete step Frequency stability, harmonic distortion, spurious signals relative to fundamental signal Frequency and amplitude Maximum and minimum signal levels Table 3: Signal generators—relevant specifications 52 Conformity oCtober 2008

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Conformity Magazine - October 2008 Contents Editor's Note FCC Releases Annual Report on National Do-Not-Call Registry Commission Cracks Down (for the Fourth Time!) On Junk Fax Marketer FCC Releases Quarterly Reports on Consumer Inquiries and Complaints Greenpeace Ranks Electronics Manufacturers Standards List For The EU’s Energy Consumption Directive Published EU Commission Publishes Standards List for Directives on Pressure Equipment, Pressure Vessels Standards List for the EU’s Medical Devices Directive Available The IEEE Product Safety Engineering Society: The First Five Years ESD Open Forum Improving Election Security and Accuracy: The Future of Voting System Certification Conformity Assessment and Accreditation: Their Role in the Global Market Design Issues in Extreme EMC Environment Focus On... Test Equipment Buyers Guide Product News Updated Standards List for EU Directive on Active Implantable Medical Devices FDA Warns of Effects from CT Scans on Electronic Medical Devices FDA Provides Report Card on Its PMA Review Process CPSC Releases Import Safety Strategy Other CPSC Actions in the News IEC Standards Update Product Reviews UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - October 2008

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