A fun thing to do if you have access to a semi-anechoic chamber is use lots of duct tape to make sure absolutely nothing changes between measurements other than a certain design change--one accepted long ago that already went into production, which cost $$$ (cable ferrites, wrapping cables multiple times through ferrites, various black magic/witchcraft-based ideas)--and end up with evidence that a pass at a compliance lab years ago was misattributed to an expensive design change instead of a new test setup. Even all the duct tape in the world, however, doesn’t bring the uncertainty to 0.
The standard almost addresses this as you’re supposed to (as best as I can remember) adjust the EUT’s position on the turntable relative to the cables, which are also to be individually adjusted (position/orientation) to maximize emissions for each frequency “of interest” (along with mast height if I remember right). Try that with a console with 20+ cables at even one frequency… with a simplifying assumption that a cable can either be laid out in state “A” or state “B”, that’s still over 1 million combinations. Good thing for the lab techs (and whoever pays the labs’ hourly rates) to have some “uncertainty” cushion -Elliott From: Brian Kunde <bkundew...@gmail.com<mailto:bkundew...@gmail.com>> Reply-To: Brian Kunde <bkundew...@gmail.com<mailto:bkundew...@gmail.com>> Date: Tuesday, July 11, 2023 at 9:31 AM To: <EMC-PSTC@LISTSERV.IEEE.ORG<mailto:EMC-PSTC@LISTSERV.IEEE.ORG>> Subject: Re: [PSES] FCC Part B questions If I may pile on late, keep in mind that measurement uncertainty is Plus or Minus (±). Years ago when I was with a previous company, we had a buy/sell piece of junk product that we were selling with our company's brand/name on it. It was audited in Sweden as part of their surveillance program and it failed by 2dB. The test lab said they could not say it FAILED because 2dB was within their measurement uncertainty, so we could continue to ship and sell this product in Sweden. Has anyone else ever experienced this? Thanks, The Other Brian On Sat, Jul 8, 2023 at 8:28 AM Brent DeWitt <bdew...@ix.netcom.com<mailto:bdew...@ix.netcom.com>> wrote: Hi Brian. It's not entirely clear which measurement range you are asking about, but I'll assume conducted emissions in the range of 150 kHz to 30 MHz. Short answer: You can skip QP and Avg detection if the peak detection level is below the Avg detection limit. No margin is "required" to pass any emissions limit. Zero dB margin is still passing. That said, measurement uncertainty in that range is generally 3-4 dB, so having a passing margin greater than that gives you some confidence that a re-test at another time and lab will still pass. Hope that helps. respectfully, Brent DeWitt Milford, MA On 7/8/2023 12:38 AM, Brian Gregory wrote: Hi there, A question came up that I can't answer w/o a copy of Part 47. Does the FCC report require Quasi-Peak (QP) data, or just Avg and Peak. When do peak readings trigger the need to report QP? I'm pretty sure Part 15 has AVG and QP limits listed. Next was what sort of margin is expected in order to pass CE emissions requirements (CISPR 16 or 32)? Memory serves that one wants 3dB of margin, but memories can be imperfect! "Colorado" Brian 720-450-4933 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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