Ed, I don¹t recall why the specific value of 20 dB was selected, but you have hit on all the reasons a pad is necessary.
The first reason is the issue of protecting the EMI receiver/analyzer, and I suspect the largest pad was chosen for that job that would maintain the required sensitivity. As you say, modern EMI receivers/analyzers are a good match to 50 Ohms as longs as there is some attenuation in front of the mixer; if zero attenuation is selected there is a problem with match, but that isn¹t the case for CE102, but only the stricter RE102 limits. The 20 dB pad is important in converting the LISN EMI port to 50 Ohms; I have been complaining for a number of years that the 20 dB pad needs to be placed at the LISN EMI port rather than somewhere else. See Figure CE102-3 between 461E vs. -461F as a result of such complaints. All MIL-STD-461 limits/test methods are tailorable, so if there is some good reason to reduce the 20 dB pad to a lower value, that may be suggested to the customer. Finally, while I sympathize with and share your desire to use EMI test data not just for pass/fail compliance but for information, that is most assuredly not the official reason for MIL-STD-461, and any assertion otherwise will be met with withering condemnation, from any official source. Ken Javor Phone: (256) 650-5261 From: Ed Price <[email protected]> Organization: ESP Labs Reply-To: <[email protected]> Date: Thu, 12 Apr 2012 05:27:46 -0700 To: <[email protected]> Subject: 461 CE102 Attenuator Value Ken: Why did the committee settle on the value of 20 dB for the CE102 attenuator? I know that transient current changes in the LISN load can cause energy to be coupled into the analyzer, possibly damaging the analyzer input, so some protection is a good idea. And, if you do have a high signal level, it¹s better practice to reduce it externally rather than risk the attenuators within your analyzer. OTOH, I hate to see a ³flat line² instead of the real analyzer noise floor plus very low-level signals, when measuring really quiet test specimens. I realize that contemporary analyzers have a noise floor way, way below the limit value, but that¹s not a good reason to give away 20 dB of sensitivity. I realize that there is also a question of impedance matching of the analyzer input to the 50-Ohm feed system. True, that used to be a problem with the equipment we now have on our collection shelves (like an NF-105 or NM-50), but is impedance error still a real issue with contemporary analyzers? MIL testing is not just a ³pass / fail² concept; you are also gathering data that other engineers may need to evaluate for future system integration (possibly at limits more stringent than your current testing). So, why is 20 dB specified; why not 10 dB or 6 dB or 3 dB or even 0 dB? Could the standard allow attenuation to be optional? If some attenuation is felt to be necessary, could a 3 dB attenuator be specified as the minimum, with possibly larger values ³recommended²? Ed Price El Cajon, CA USA From: Ken Javor [mailto:[email protected]] Sent: Wednesday, April 11, 2012 6:58 AM To: [email protected] Subject: Re: [PSES] FW: inquiry concerning MIL STD 461E, CE102 test. As a member of the committee that writes MIL-STD-461, I second Ed¹s advice: option 3 is the only correct technique. Ken Javor Phone: (256) 650-5261 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/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: Jim Bacher <[email protected]> David Heald <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/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: Jim Bacher: <[email protected]> David Heald: <[email protected]>
