I strongly disagree with portions of this posting. Namely that the solution to test-related problems is to impose a production fix (over-braided cables) that adds cost and weight and has serviceability impact on every item sold and delivered to the customer. This is hardly a reasonable solution, except >from the narrow point-of-view of the EMC test engineer trying to keep his test schedule on schedule.
Also it is based on what I believe is flawed logic. While it is true that filters alter the impedance of the cable, and the requirement is that the devices on either end of that cable see the correct impedance, it is implied but not stated that the intentional frequency spectrum of the desired signal is what needs to be properly driven/loaded. If an audio signal is passed through a low pass wall filter with a 3 dB stop band above 150 kHz, the intentional signal has not been disturbed, unless this is hi-fi audio and you are a purist. It is certainly ideal to snub EMI at the source, and this means proper PCB layout. Perhaps as signal integrity as a science and rules of thumb for PCB layout become well-accepted, this form of design will be more prevalent within military procurements. But as it stands, military procurements are very small compared to consumer electronics, and engineering hours cannot be spread over large production runs. In the past it has definitely been the practice to not snub the source but provide band-aid fixes in terms of solid metal enclosures, and copious usage of filtering and shielding. But let's not add even more, just to make the testing easier! on 2/19/03 6:16 PM, [email protected] at [email protected] wrote: > > All, > > I do not recommend that you use heavy filtering at the chamber egress wall > to control emissions for a couple of reasons: > > 1) In good faith, you need to duplicate the actual *system* cabling and > loading of the end-target platform equipment or user. With filtered > connectors installed at the chamber egress wall, you certainly have isolated > the exercise equipment emissions away from the Equipment-Under-Test) EUT, > but that is not real world. While you may pass test limit line requirements, > when the product is used in the final application it may either radiate or > be susceptible. While you can always say you met the specification, it isn't > worth losing a customer or degrading your reputation. > > 2) Filter Connectors can (depending on type and values), present a very low > impedance at the chamber egress wall. This is undoubtedly NOT the end of the > cable (cabling connected to exercise equipment in the Ante Room). This may > cause unforeseen reflections on cables and shields creating worse emissions > or degraded susceptibility (alternate return paths) during testing. > > One way to solve your problem is to specify shielded (overbraided) cabling > if you have the ability to. This would have to be something agreed upon > between you and your customer. Be careful of product safety concerns in some > countries with ungrounded power cabling. > > The real way to solve the problem is to fix the common mode noise on the > cable by source suppression. Sometimes a painful solution but often the best > in the long run. This is painful if you are out of schedule (many do EMC > testing at the end of their development schedule), working with a difficult > OEM device, or cost is an issue. If you can't source suppress it, you're > back to containing it with the shielded cabling. Be careful about cable > shield pigtails or ferrules and wires. You will almost always have better > success with 360 degree overbraid shielding to EMI backshells. > > Philip Ross Wellington > Mgr. Signal Integrity & EMI > L-3 Communications CSW > > > > -----Original Message----- > From: Cortland Richmond [mailto:[email protected]] > Sent: Wednesday, February 19, 2003 1:25 PM > To: [email protected]; ieee pstc list > Subject: Re: RE02 cabling problem > > > > I was looking over Dries' post again and note that I overlooked something > important: He says that short shield goes on the *wall plate* -- which I > presume to be the shielded room wall. This doesn't change my recommendation > he test with unshielded wires. But it serves as a reminder that support > equipment must be protected from immunity stresses of its own. I hope there > are filters between the support equipment outside, and signal lines from > the chamber. > > > Cortland > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > Archive is being moved, we will announce when it is back on-line. > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > Archive is being moved, we will announce when it is back on-line. > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > -- Ken Javor EMC Compliance Huntsville, Alabama 256/650-5261 This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
