Hi Doug, you cant make choices for manufacturers. If their literature doesn't say use high quality cables, and the product sells for $10, what consumer is going to spend $45 on a nice cable?
We disagree on used cables, Several of my clients make hubs, and we do not test with brand new cables. We simulate anticipated bends and also clamping ( eg Ty-Wraps ) like we would expect in a machine. Personally I believe this is appropriate have burnt much midnight oil on products that may have passed as you describe in a lab, but are crap when deployed. Power supplies are my personal pet peeve. Deploying ferrites on Ethernet cables would make me very nervous. I think you have a very good point about POE injectors, please keep up the good work. I welcome any change in emissions test methods, what we have currently are pretty crap. Cheers, Derek. -----Original Message----- From: Doug Smith <[email protected]> To: Derek Walton <[email protected]> Cc: Michael_Sundstrom <[email protected]>; bstammerjohan <[email protected]>; EMC-PSTC <[email protected]> Sent: Tue, Jul 26, 2011 1:24 am Subject: Re: [PSES] ESD Spacer Hi Derek, On cables, yes use the cables supplied, but if they are not then use good quality cables bought from a store. One new cables, let me use Ethernet cable as an example. As these cables are bent and kinked with age, the balance will be degraded by the wires and twists being disturbed and common mode currents can be formed. I carefully measured this effect while I was at Bell Labs years ago. This has nothing to do with the product tested and there is nothing a product designer can do about degraded cables. It happens. No amount of system redesign can help here so why not use new cables? Using old kinked cables can make any product fail! The only thing one can do is to throw out the old cables and don't use them for testing. The common mode currents can be generated anywhere along the cable driving as a dipole from that point, so putting a ferrite at the connector will not always help, in fact it may make things worst by tuning the antenna. I described such a tuning effect in 1999. See http://emcesd.com/tt120199.htm . Separately, I have been doing some balance measurements on POE injectors for Ethernet. Of three I tested, two would cause any product to fail by disturbing the balance in the Ethernet cable. With no other significant contributions to the measurement (common mode current 20 dB lower without injector) one of the injectors would cause a 10 dB failure of Class B emissions by generating common mode current. May be publishing the data soon. Don't know how anyone claims to pass EMC class B using that POE injector in a test setup. The normal EMC test setup is not useful for tracking down this type of emission accurately. Will also publish the test method I developed. Doug On 7/25/11 7:27 PM, Derek Walton wrote: Hi All, I'd just like to point out here that the EN does not give any criteria for the dielectric withstand nature of the spacer. So, while we can argue best practice, if a lab meets the thickness, all is permissible. I also disagree with Doug's statement on cables. You must test with whats supplied by the manufacturer, or with ones that he states you must use. Why, well because cables can make a huge difference in performance. I would call messing with cables like this part substitution and fraud. I also disagree about using new cables. The reason for this is that cables will degrade with some flexing and you should be assuring a product meets the EMC requirements for it's design lifetime, not on a good day in a lab, with the wind behind it and rubber boots on. Just trying to keep the common sense hat on compliance testing Cheers, Derek Walton. -----Original Message----- From: Doug Smith <[email protected]> To: Sundstrom, Michael <[email protected]> Cc: Bryce Stammerjohan <[email protected]>; EMC-PSTC <[email protected]> Sent: Mon, Jul 25, 2011 8:59 pm Subject: Re: [PSES] ESD Spacer If the poster board is 0.5 mm thick as required, you may be failing equipment that might otherwise pass, which I have seen happen. A spark through the poster board will likely be worse than the one you supply from the ESD simulator in terms of peak current and di/dt. In any case, unless the poster board in 0.5mm thick and you have confirmed there is no spark through, than you cannot say you have tested to IEC 61000-4-2! Maybe OK for a pretest but not for CE marking. It's hardly worth it to save a few cents on test materials (the insulating layer we are talking about, USB cables, Ethernet cables, and many other things) and waste thousands of dollars in the end. I see this happen a dozen times per year. I also recommend using new, high quality cables for any performance test. This is especially important for USB and Ethernet cables. Doug On 7/25/11 10:32 AM, Sundstrom, Michael wrote: Guys, I just use poster board, I get 20 sheets at a local office supple house for $5-$6 ? Am I missing something? Do you need plastic sheets? Michael Sundstrom OHD / TREQ Dallas Electronic Lab Analyst, EMC Lead 2170 French Settlement Rd, Suite B Dallas, Texas 75212 (214) 579 6312 (940) 390 3644c KB5UKT Albert Einstein once said, "The definition of insanity is doing the same thing over and over again and expecting different results". From: Bryce Stammerjohan [mailto:bstammerjohan@THO - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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