Conformity Magazine - October 2007 - (Page 44)

Conducted Emissions Commercial conducted emissions tests are performed over the 9 kHz to 30 MHz frequency range on power and data lines. Power line testing offers some unique challenges, depending on the type of power supply used in the DUT. The traditional power supply, for example, which uses a transformer and rectifiers along with filtering, has a tendency to conduct harmonics of 60 Hz onto the power lines (see Figure 1). This type of emission concentrates most of the energy in the area below 1 MHz. It is interesting to note that not all conducted emissions are produced by the DUT power supply. Energy from other areas of the DUT can be coupled to the power lines though the power supply as well. Placing some temporary shielding around the power supply elements can help the engineer understand if there is any coupling into the power supply. For a closer look at the conducted emissions of this type of power supply, the engineer can reduce the stop frequency so that the emissions are spread out over the visible span as shown in Figure 2. The signals can be stabilized by switching the trigger to “line” mode. Another class of power supplies is the switcher power supply. Its frequency is usually above 20 kHz to avoid any annoying hum. Because the switcher’s emissions tend to be broadband in nature and therefore require different measurement techniques, they pose an additional measurement challenge for the engineer. Figure 3 provides an example of a switcher broadband signal. The goal is to make sure that the conducted emissions are below the appropriate limit lines. Most conducted emissions have two limits, quasi-peak and average. Today’s spectrum analyzers display the limit line, plus any margin the operator may wish to set as shown in Figure 3. The display must be corrected for any loss that the transducer (e.g., the line impedance stabilization network or current probe) may have. Since the quasi-peak detector is a very slow responding detector, measurements are performed using peak detection Figure 1: Conducted emissions Figure 3: Broadband emissions Figure 2: Reduced stop frequency display 44 Conformity oCtober 2007 Figure 4: Zone sweep and measure-at-marker

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

Contents Editor’s Note Commission Proposes Changes for 57-64 GHz Spectrum Rules FCC Continues Actions Against Jamming Device Distributors Radio Operator Fined for Failing to Make Equipment Available for Inspection Commission Fines Unsolicited Fax Advertisers Firm Offering Quality Standards Training Also Fined Breaking New Ground with Lithium-Battery Generic Requirements for Use in Telecommunications Facilities ESD Open Forum EMC UK 2007 Conference Set Staying Abreast of U.S. E-Waste Legislation EU Commission Establishes RFID Expert Group Update on China's CCC Compliance Program System Level ESD Testing: The Waveforms An Alternative Approach to Surge Suppression Testing Equipment Buyer's Guide EU Scientific Committee Releases Updated Data on EMF Risks Updated Standards List For The EU’s EMC Directive Updated Standards List For The EU’s Directives on Measuring Instruments Call for Presentations for 2008 International ESD Workshop Announced CPSC Actions In The News IEC Standards Update Product Reviews UL Standards Update New Products From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Upcoming Events Advertisers

Conformity Magazine - October 2007

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