Geetha,
1. RF semi-anechoic chambers are acceptable for testing as long as they meet the volumetric normalized site attenuation requirements in the standards. The FCC does accept chamber measurements as long as these requirements are met. 2. 1.6 dB margin is pretty slim. As noted previously, labs may vary by 6 dB and nobody is terribly surprised. A signal 1.6 dB under the limit in one lab could be 4 dB over the limit in another. 3. You list 6 frequencies, and only six frequencies from the first test. This is one of my objections to the current standards which only require that the 6 worst signals be listed. A difference between two labs can cause different lists to be provided with minor changes in signal amplitude. Our labs provide a much larger list. 4. You question the frequencies from the lab in Taiwan, but not the frequencies from the lab in the US. How would 43 MHz and 52.38 MHz come from 4 MHz and 12 MHz clocks if harmonics are the only way to generate emissions (which, by the way, they are not). I think you got bit by the problem of repeatability between labs. Also, there is a significant time difference between the two measurements and you could have changes in the product that you are completely unaware of. Have your parts vendors changed any of their processes? Have there been die shrinks on any of your semi-conductor parts? Too much information is not available in this message to make a solid determination, but passing by only 1.6 dB in the first instance leaves you very vulnerable to failing at a later date simply due to differences in the labs (facilities, precise techniques, etc.). Unless the FCC and VCCI read these messages they are unlikely to find out about your BSMI test results. However (and I’m not a lawyer, nor do I play one on TV, nor did I stay in a Holiday Inn Express last night), if your product were found to cause harmful interference in the US and the FCC found that you had non-compliant data and knew of a fix that you weren’t shipping, they might not look kindly on your actions. You don’t need to go retrofit product in the field, but I would recommend (and this is a business decision for your company to make) that you ship the ferrite with future product. Hope this helps. Now, go make some ferrite vendor happy. Ghery S. Pettit (note, the opinions expressed are my own and not necessarily those of my employer) _____ From: [email protected] mailto:[email protected]] On Behalf Of Geetha Balasubramanian Sent: Tuesday, March 22, 2005 7:52 AM To: Grasso, Charles; 'Bob Richards '; '[email protected] '; '[email protected] ' Subject: RE: Radiated Emission Testing Hi Gurus, As per the US lab report, the frequency at which the device had minimum pass margin is 68 MHz. At 68 MHz in vertical polarisation the margin is 4.7 dB & in horizontal polarisation the margin is 1.6 dB. Measurable emissions were found only in 40 , 43, 52.38, 64 & 68 MHz. But as per the Taiwan lab report no measurable emission is found >from the device at the above frequencies. The device failed in 122.61 MHz at horizontal polarisation. There are 2 clock sources in the device, one runs at 12 MHz and the other runs at 4 MHz. Frequencies showing up in the US lab report are atleast the harmonics of these 2 frequencies. But frequencies mentioned in the Taiwan lab report are looking funny (111.27, 135.3, 285.69 MHz. etc.) US lab tested in OATS but the TAIWAN lab tested in a anechoic chamber. I heard that chambers are not the correct way to measure ( always the results will be wrong) that is why FCC never accepts chamber measurement. Is it so? Thanking you all for the time & efforts spent on this. Thanking you all in anticipation Geetha "Grasso, Charles" <[email protected]> wrote: Hi Bob, Don't you ALWAYS maximize cables. I found that with cable maximization my test repeatabilty improved dramatically. HAs cable maximization fallen out of favour? From: [email protected] To: [email protected] Sent: 3/22/2005 6:58 AM Subject: Re: Radiated Emission Testing It is not an unusual occurance. This is one good reason why you want a good margin under the limit. Most people like to see a margin of at least 6dB. One variable is the site itself. A good OATS is the best. Sites can have up to 4dB variation from theoretical, and one chamber I've been involved with had a problem in the 50-60MHz range, where it was 3.9dB out -- barely acceptable. You might want to ask to see the site attenuation data from the labs involved. There can ! also be a big difference depending on the measuring distance. Some standards allow extrapolation to be done when measuring at some distance other than the distance spelled out in the standard, but looking at the difference in theoretical site attenuation between a 3m and a 10m site will show that linear extrapolation is not accurate through much of the frequency range. It is not unusual to get different results at the SAME lab, depending on the person conducting the test. Like it or not, some technicians are better or more diligent when it comes to maximizing the emission by playing with cable placement. When I worked at a compliance test lab, I ticked off a few clients when their product was passing by 2dB until I moved the cables and rescanned only to fail them by 1dB. Bob Richards Square D. --- Geetha Balasubramanian wrote: > Dear Experts, > > One of our products had b! een tested by an > accredited lab at US on May 2004 and found to be > compliant with FCC, CE & VCCI. We started mass > manufacturing the product and it is being sold in > the US, Europe & Japan markets for the past 6 > months. Then came a requirement for that product in > Taiwan market. Since the May 2004 testing at the US > lab was not run in a BSMI approved system it became > necessary for us to retest the same product. This > time the product has been tested in an accredited > lab in Taiwan. The Taiwan lab tests are showing that > the product is not compliant to the CISPR22 and need > a ferrite core in the USB cable to make it compliant > to the standards. Product tested by the US lab & > tested by he Taiwan lab are absolutely identical. > As an engineer responsibled for compliancy I > am a bit confused now. How to resolve this issue? I > request you experts to g! uide me. > > Sincerely > > Geetha Balasubramanian > This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
