Hi, I take exception with the following quote from Amendment A1, 1995, to EN55022:1994:
"In the case of Printed Wiring Board Assemblies (PWBA), separately marketed for the enhancement of diverse host units, the PWBA (e.g. ISDN interface, CPU, adaptor cards, etc.) shall be tested in at least one appropriate representative host unit of the PWBA manufacturer's choice so as to ensure compliance of the PWBA with the entire population of hosts in which it is intended to be installed." It is indeed the case that testing in one representative host meets the requirements. But the statement, "...of the PWBA manufacturer's choice so as to ensure compliance of the PWBA with the entire population of hosts in which it is intended to be installed" is absurd. Often times, the host chosen by the PWBA manufacturer is the quietest device that the manufacturer can find that will allow the card to pass and is not representative of a typical host. This may allow the PWBA manufacturer to eliminate reasonable port filtering, minimize pcb layers, use unshielded connectors, avoid fixing poor layout on the card, all in the interest of cost reduction and time to market. Then, when the PWBA is installed in a real typical host, the thing fails emissions limits by wide margins. I can think of one video card that will remain nameless that is very popular. I have tested the thing in a variety of host systems with a variety of video displays. The card has an FCC Class B ID and CE Marking and is marketed as Class B. I have never seen it pass Class B in any combination of hosts, and have failed it for Class A on several. I know of ethernet cards with single and multiple ethernet ports that have this problem too. Likewise with 15" video displays. They are typically such a cheap commodity that most no longer use even a full shield on the video amp much less any enclosure shielding. Tested on a variety of video cards and in a variety of systems, they often fail Class B limits. I find two problems typically: poor video cable shield termination, and poor shielding of video emissions emanating from the cathode drivers in the displays. It is obvious to me that the 15" color display manufacturers are choosing quiet video sources to qualify these products. I conclude this assuming that they are staying within the law by manufacturing products that are typical of the original test sample. Then there are the keyboards and mouses that seem to display curious anomalies (to be kind) under ESD, EFT, radiated immunity and conducted immunity testing that are CE marked. They have essentially no enclosure shielding, little or no cable shielding and no common mode noise suppression, cheap PCB layout, and are CE marked. Again, I have tested certain models on a variety of hosts to draw this conclusion. >From a technical point of view, testing with a representative host/support device to show compliance with other hosts is a total "crock." The standard should be ammended to remove this bogus claim. >From a business and practical perspective, it makes perfect sense. To do the right thing technically (test on a typical host) would be way too big of a burden for industry to bear since "typical" changes rapidly and would be impractical to prove. Therefore, as much as I dislike it, the actual requirement in the standard should remain the same. Unfortunately, the burden falls on system integrators and importers trying to qualify the myriad combinations of components requested by their aggressive marketing/sales departments. But, nobody said it was an easy job! Regards, [email protected] ---------- From: Charles Blackham DD-KB To: emc-pstc; martin Subject: Re: Computer board tests List-Post: [email protected] Date: Wednesday, December 11, 1996 10:35AM Martin Amendment A1, 1995, to EN55022:1994 states: "In the case of Printed Wiring Board Assemblies (PWBA), separately marketed for the enhancement of diverse host units, the PWBA (e.g. ISDN interface, CPU, adaptor cards, etc.) shall be tested in at least one appropriate representative host unit of the PWBA manufacturer's choice so as to ensure compliance of the PWBA with the entire population of hosts in which it is intended to be installed." So according to the standard, the required minimum number of hosts in one. I believe that one is sufficient, especially where you have no idea what computer the end user will have. I would always check the Emissions from support equipment before adding an adaptor card, as combining CE marked computers,monitors, keyboards etc. does not always produce a combination that is still meets class B limits ! Charlie Blackham x31592 Approvals Engineer Madge Networks Limited
