Conformity Magazine - October 2007 - (Page 46)

first. The engineer then zooms in on signals above the quasi peak (QP) limit and performs a measure-at-marker measurement as shown in Figure 4. The measure-at-marker shows the peak, quasi-peak and average value. To measure broadband signals, the engineer must reduce the span down so that the signal covers most of the display and switch on the QP detector. As an alternative method, the engineer can go to zero span and set the step frequency size to the bandwidth specified by the EMI band. In this case, the bandwidth is 9 kHz for band B. The engineer simply switches on the QP detector, selects center frequency and uses the up/down key to step through the broadband signal to compare the signal to the limit line or margin. Radiated Emissions The goal of pre-compliance radiated measurements is to emulate, as close as possible, full-compliance measurements. This means that the DUT must be placed on a table at the specified height. The antenna, whether biconical, log periodic or broadband, must be placed at the prescribed distance usually three meters. Then, if possible, the DUT must be operated in worst case mode. The technique for measuring radiated emissions is similar to conducted measurements with one major exception. If the ambient environment in which the radiated emissions measurements are being made is minimal (e.g., well below the regulatory limit), then the engineer must make emissions measurements without recording the existing ambient environment. Figure 5 provides an example of the local ambient environment. If the ambient environment is such that signals are above the limit, then the engineer will need to make a copy of the screen over the span of the antenna with the DUT turned off. This will provide something to compare against the emission with the DUT turned on. If necessary, the engineer can then switch the DUT off and on to be sure of a signal. Figure 6 shows a typical measurement with the DUT turned on. Note that, once the clock frequencies generated within the DUT are known, the engineer has a good place to start looking for emissions. All sides of the DUT must be measured. It is recommended that the DUT be rotated 45 degrees for each measurement, over 360 degrees, copying the screen for each position. Different positions may have different frequency and amplitude radiation emissions; therefore the engineer will need to record the frequency and amplitude of suspect signals for each position tested. After the initial scan is completed, the engineer simply returns to each of the suspect signals and performs a measure-atmarker. If, after the measure-at-marker is performed, the signal is still above the QP limit or established margin, then signal must be reduced via a redesign. Figure 5: Ambient environment Isolating Problem Emissions The next step in pre-compliance measurements is to isolate the signals marked earlier. To do this, the engineer can use close field probes attached to an amplifier and a spectrum analyzer. One technique is to set the spectrum analyzer to the signal frequency of interest and move the probe slowly over the surface of the DUT until the point of emission is found. Once the signal has been identified the engineer can attempt to find the source within the DUT. Solving the problem at the source is less expensive than adding shielding to each unit sold. Some places to look for the source include ground paths, shielded cable grounds and unshielded pigtails. Cabinet vent holes can also allow emissions if not designed correctly -- that is short narrow holes versus one long narrow vent. Short holes do a much better job of shielding. Figure 6: Ambient and DUT signals 46 Conformity oCtober 2007

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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