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Test & Measurement World, July/August 2012

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me is if just installing the probe can be enough to change the value of the common-mode current that is seen by a significant amount. Answer: Current probes are a very As the global leader in precision, programmable power supplies, AMETEK Programmable Power offers the product breadth and expertise test engineers rely upon. • The industry's broadest selection of AC and DC programmable power supplies and loads • Deep application expertise for industries ranging from HBLEDs and electric vehicles to electronic components • Able to deliver clean, low-noise power even at high densities • The test engineer's most reliable and trusted power brands: Elgar, Sorensen, California Instruments and Amrel valuable tool, and a do-it-yourself probe will work in a pinch (Ref. 2), but I'd highly recommend investing in a good commercial probe, such as the Fischer Custom Communications F-33-1 probe (see similar probes in Figure 2). This will serve you well. Current probes do affect the com- mon-mode cable currents to a minor de- gree. In fact, my colleague, Doug Smith (www.emcesd.com) has determined that at high frequencies, the cable currents can couple into the shielded case of the probe and be diverted along the outside of the probe-cable shield. This can be at least partly blocked with clip-on ferrite chokes. The important point, however, is to use the probe to monitor reductions in common-mode currents, which gen- erally equate to reductions in measured radiated emissions. Chassis design Question: When designing a chassis, what are the allowable hole and slot sizes, and what is the attenuation of dif- ferent sizes? I've read several sources that recommend keeping slots and holes to 1/20 of a wavelength of the maxi- mum frequency of concern. At my last company, we made high- power (1 kW to 10 kW) RF and DC power supplies, and we always used very small holes (about 0.1 in.) in the chassis for the air holes. It seems to me, though, that based on the 1/20 of a wavelength rule, I should be able to have a 2-in. slot for up to 300 MHz, but does this de- pend on how powerful the source is? Answer: After the discussion of wave- length and what constitutes an efficient radiator, you now know that 1/20 of a wavelength is not efficient, and slots, seams, gaps, and traces where the domi- nant harmonic is 1/20 wavelength will not tend to be an issue. At the same time, when considering the shielding effec- tiveness of an enclosure, a seam measur- ing 1/20 wavelength, or less, should not be an issue. In fact, the general rule of thumb for shielding effectiveness of a slot is 1/20 wavelength provides about 20 dB of shielding effectiveness. An array of slots (for example, for a ventilation port) will be less effective than small holes to some degree. There have been studies on the shielding effec- tiveness of slots versus holes, and these studies led to the development of the corresponding empirical equations for shielding effectiveness. This is why most ventilation ports are often a matrix of small holes, rather than an array of slots. Jeremy is correct that for very-high- power RF sources, a chassis requires small holes for ventilation, and I'd suggest designing the enclosure for harmonics up to at least 1 GHz, which equates to approximately 1/2 in. maximum slot length. Again, Ott has handy charts in his FIGURE 2. Current probes measure common-mode currents that tend to cause cable radiation. book for determining the shielding properties of slots, holes, and seams. These are just a few hints that can help you prepare a design for formal EMC testing. To see further discussion of these points, see the online version of this article in "The EMC Blog" on www.tmworld.com/4376432. Plus, feel free to add your own questions to the discussion. T&MW REFERENCES 1. Ott, Henry, Electromagnetic Compati- bility Engineering, 2009. www.hottconsultants.com/EMCE_book_ files/emce_book.html 2. Wyatt, Kenneth, "The HF Current Probe: Theory and Application" Interfer- ence Technology, March 20, 2012. www.interferencetechnology.com/the-hf- current-probe-theory-and-application. Kenneth Wyatt is the founder of Wyatt Tech- nical Services and a specialist in EMC design, test, and troubleshooting. He is the author of "The EMC Blog" on TMWorld.com (www. tmworld.com/electronics-blogs/4376432/ Test & Measurement World | JULY/AUGUST 2012 | www.tmworld.com –26–

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