Re: [PSES] FCC Part B questions

Wed, 12 Jul 2023 16:57:13 -0700

I think we're bundling independent errors here.  The lab's 17025 measurement uncertainty is independent of the EUT.  That said, you are certainly correct that measurement _variability_, due to the complexity of the EUT, generally swamps the lab MU, especially in the case you mention when multiple long cables exist. 

On 7/12/2023 4:17 PM, Elliott Martinson wrote:



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>
*Reply-To: *Brian Kunde <bkundew...@gmail.com>
*Date: *Tuesday, July 11, 2023 at 9:31 AM
*To: *<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> 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

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