A few loose comments and additions on your approach: The CISPR committee is like you seeking for repeatability in test set ups. Therefore an effort is made to create a common set up for immunity and emission test set up. The idea is a (vert) cylindric volume of 1m60 miniumum diameter for left and right cables to extend 80 cm and then leading 80 cm down to ground surface where each cable is clamped using an (yet) undefined ferrite clamp. (on rotating table) Several propositions are into the voting chain of which one is an amendment to CISPR 16-1. The 1m60 cylinder does not cope for EUT sizes, so in order to create a workable test setup 2 m cylinder is required and still allow for moderate EUT sizes. In addition to the 2 m cylinder a 1m distance to the walls of the FAR would be required, and sufficient distance is required to the antenna for it to "illuminate" the full cylinder. Calibration procedures are also in the propositions.
However, a full compliant set up cannot be made in a 3 m room, due to size restrictions. Several aspects may however lead to substantial different radiations results as before: The fixed radiating lead length of 2 x 80 cm + ferrite behaving as 150 Ohms Common Mode load) makes the lead look like infinite to the EUT disturbanc voltage source (at least that's the idea). For frequencies below 1/2 lambda lead lenght ( 1m60 = 92 Mhz the wires (left + rigth) begin to behave like a shortened dipole with (of course) reduced emssions compared to real life where cables CAN be infinite. The procedure is now to find out in what combination of left right cable lay out creates maximum emissions. If you forget that, different phase steered cables may compensate emissions. Think of the cables as being current fed from a matrix of interference sources with arbitrary voltage where each cable is connected to a different point of the matrix and you get the idea. Just putting cables where you like is *not* sufficient. A FAR differs from an OATS in that no height variation of the antenna is required. For small sources or line radiating sources such as a dipole cable set (!!), the low frequency results may be up to 24 dB higher then at the OATS (with HOR polarization) this is due to the fact that on an OATS the reflected ground wave is in opposite phase and attenuates the incident wave. For this to compensate the hight variations were invented. travelling ways differ when rising and lowering the antenna until a maximum was found. Unfortunately at low frequencies one will never never rise the antenna as high as required for a full compensation of this effect. (I have a Mathcad 2001 model for those who want to experiment: infinite height would be required but signal topsat 15m). As there is no ground reflection in the FAR, the low frequency response is much better then on the OATS. Though antenna signal is 5 dB (theoretic 6) lower in a FAR because the absence of the reflected wave the effect of the insufficient OATS model are not accounted for. At vertical polarization this OATS effect does not happen as the reflected wave is not in opposite phase to the incident wave. Larger or 3D EUTS do behave better due to more variations of way length between radiator and antenna, and large receiving antennas have less problems then dipoles (especially active dipoles : small !!)./ Just a few notes to explain for the many differences that may happen between set up variations. Gert Gremmen -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of Alex McNeil Sent: dinsdag 20 augustus 2002 18:00 To: '[email protected]' Subject: Radiated Emissions setup Hi Forum, As we all know test setups are absolutely critical to EMC results. I have the good fortune of having a good pre-compliance radiated emissions site (3m OATS, Fully or Semi Anechoic Chamber). From using this site I see all sorts of "funnies" but still the most annoying is how much you can change the emission results just by changing cable positions. I carry out regular EMC audits and could never quite get the same results as 6 months previous, when that particular product was last tested (I keep the master product and was still getting different results due to setup). I now use an "Audit" board where I bundle the cables (30-40cm) in a particular "figure of 8" fashion via fixed wooden pegs. Thru' time I have convinced myself that this not only gives me worst case emissions but also that much sought after REPEATABILITY! I believe the setup is as per EN55022. I have a photograph* of this Audit board to allow this forum to comment on the good and the bad points of using such a system. Also, is there any reason I cannot take this Audit board to a Test House for Product Compliance testing and at the same time get correlation between a Compliance site and my own site? Photograph details (EUT with 3 X Serial Ports, PSTN and DC SELV power connection) 1. The 1M, 2M and 3M indicate the Horizontal bundling of cables from RS232 ports which would go to a tabletop EUT peripheral (or leave open circuit). The 3 lengths are connected to similar ports. This covers the different lengths issues. 2. The P is the inline power supply O/P cable going to the EUT. This is bundled vertically as it represents the power supply sitting on a customers floor. 3. The T is the PSTN telecomms cable. Again bundled vertically as it represents the cable going from the EUT to the wall socket. Finally, I have never understood the EN55022 figure 10 note 1 "If cables which hang closer than 40cm to the horizontal ground plane cannot be shortened to appropriate length, the excess shall be folded back and forth forming a bundle 30cm to 40cm long". Does this mean you can cut the cable to 1m or ? even though you will be shipping say 2m cables with the product? *I have not attached the photograph, 212Kb. This will be sent to those requesting it i.e. to those people who are thinking of commenting on this email. I look forward with interest to this forums comments. Kind Regards Alex McNeil Principal Engineer Tel: +44 (0)131 479 8375 Fax: +44 (0)131 479 8321 email: [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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