Thanks Michael.
I agree completely. Even though my comments probably didn't reflect it too well(hastily bashed out late on a Friday afternoon), changing one thing at a time is how I'd normally work on these problems.
At 10:59 PM 15/09/2000, michael.sundst...@nokia.com wrote:
I might add that the BEST way to do this is to only change one thing at a time, then retest. It's hard to tell what single change of the multiple changes attempted actually did the change. Michael Sundstrom Nokia Mobile Phones, PCC EMC Technician cube 4E : 390B phone: 972-374-1462 mobile: 817-917-5021 michael.sundst...@nokia.com amateur call: KB5UKT -----Original Message----- From: EXT Peter Poulos [mailto:pet...@foxboro.com.au] Sent: Friday, September 15, 2000 1:38 AM To: marti...@appliedbiosystems.com; emc-p...@majordomo.ieee.org Subject: Re: Near Field Versus Far Field Hi Joe. You asked for an explanation as to why the difference between the near and far field results. I think the replies so far have probably answered that question. I've tried here to give some help with the real problem of solving the excess emissions. From my own experience and discussions with colleagues, I've found you definitely need to do some (if not most) of the trouble-shooting while at the test site. Finding a problem then just returning to the lab to solve it usually leaves you with a lot of questions unanswered. That might not be much help this time but perhaps next time? The following is how I'd go about tackling the problem. I'm curious to see if there's anyone in the group who disagrees with my approach. As with any EMC problem, you've got to consider the source, the transmission medium and the victim. Obviously there's nothing you can change about the victim (the test antenna) but you should be able to narrow it down to work out the real source, and the means by which it is being radiated. For clues to the problem's cause to begin with I usually ask: (1) For the problem frequency, what's the most likely source? (2) For the problem frequency, what's the most likely source antenna? At 400MHz the wavelength is a bit under 1m (3x10^8 / 400x10^6 = 75cm) so any short cables (or at this frequency, maybe even long PCB track - like back-plane tracks?) that might make nice 1/2 wavelength or 1/4 wavelength dipole antennas would be the first I'd check out. Could also be a slot antenna effect in your enclosure - any seams or gaps in the box that are in this ball-park? Usually I'd try isolating the source by either disconnecting cables, turning off or unplugging cards, attenuating cable emissions with copious amounts of ferrite clamps etc and get the test engineer to do a spot check at the problem frequency as I tried eliminating each suspect. This is where the buckets of ferrite cable clamps, rolls of aluminium foil, shielding mesh and earthing straps come in to play. Here's where that near-field probe might come in handy too. This kind of troubleshooting though often requires a fairly intimate understanding of the way the equipment under test works so you can be confident about your assumptions and the conclusions you draw from the observed results. If the design engineer isn't actually at the test site, she/he should at least be accessible by phone to discuss the problems and make suggestions as to what to try. Using this technique, you can usually narrow it down fairly quickly to the source and antenna. If there's time, and its practical then I'd try some quick modifications to the problem circuit that's the source of the noise in order to get some reference of what changes cause what kind of reduction in the emission levels. Quite often though, you have to be aware that a change may solve the emission problem at the frequency you're working on, but result in the energy appearing elsewhere in the radiated spectrum causing the equipment to exceed the limit at some other frequency, especially if you've just modified the source antenna and not the signal causing the emission. Also note - although its difficult when you're rushing to get the problem fixed, it pays to make good records of what you change and what the results are - can help a lot later on. If you have the time at site to try a few different options (that are repeatable later), and get the highest 3 or 4 emission levels for each option at site, then if you can't find a solution you're happy with at the test site, it gives you a reference to work with back in the lab. For example, say that you found that: Design Change #1 resulted in 6dB reduction in the emission at 400MHz with other peaks (below the pass/fail limit) at 200MHz (3dB under), and 800MHz(8dB under) Design Change #2 resulted in 20dB reduction in the emission at 400MHz but caused the peak at 200MHz that went over the limit by 6dB with the peak at 800MHz reducing to 10dB under the limit. Design Change #3 resulted in 3dB reduction in the emission at 400MHz with other peaks (below the pass/fail limit) at 200MHz (9dB under), and 800MHz(7dB under) Then when you get back to the lab to try and find a good permanent fix, by repeating the changes you made at site, and comparing the emissions levels you observe for each in the lab with the results at the test site, you can get a reasonable feel to tell if your measurements in the lab are going to be indicative of what you'll see at site. If you have access to a spectrum analyser and an antenna that covers the frequency range you're interested in you can probably get a better feel for the effect of your changes than when using a near-field probe. If the emissions are high enough to fail the test, there's a good chance you can get a reasonable indication of the result of changes by comparing the emission results measured in someone's back yard(know anyone who owns a farm/ranch?) or perhaps the roof at work, to those measured at the test site. Use the antenna and analyser to measure the peaks with your equipment under test switched off first so you have an idea of what the ambient peaks are, then measure the emissions from the unmodified equipment, set up as close as practical to how it was at site. Although far from ideal, if you can see a decent correlation between what you measure and what was measured at site for the same equipment under test, then I'd have a lot more confidence in the results of this kind of measurement than for those taken in the near field. Although the results would only be ball-park, that's often all you need anyway. Given the expense of official OATS testing (unless your company happens to won its own site), I'd want to be pretty confident of a fix before booking in again. Hope that's of some help. Regards, Peter Poulos ------------------------------------------------------------------------- Please note: The views, opinions and information expressed and/or contained herein do not necessarily reflect the opinions or views of Foxboro, the organisation/s through which this communication was transmitted nor any other third party, unless explicitly stated so. Peter Poulos (Hardware Design Engineer) Foxboro Australia At 03:07 AM 15/09/2000, marti...@appliedbiosystems.com wrote: >I am having a difficult time answering the following question for a >non-technical person. Hopefully, someone can put the answer into a language >that a non-technical person can understand. > >We have a 400 MHz clock and are failing radiated emissions at 10 meters by >10 dB >at 400 MHz. We bring the product back to our lab and start making >modifications >on the clock circuit and taking measurements with a near field probe. With >these modifications and measuring with a near field probe, we realize a 10 dB >reduction in emissions at 400 MHz. Why would we not see the same >reduction when >taking the product back to a 10 meter site? > >Your help is appreciated. > >Regards > >Joe Martin >marti...@appliedbiosystems.com > > > >------------------------------------------- >This message is from the IEEE EMC Society Product Safety >Technical Committee emc-pstc discussion list. > >To cancel your subscription, send mail to: > majord...@ieee.org >with the single line: > unsubscribe emc-pstc > >For help, send mail to the list administrators: > Jim Bacher: jim_bac...@mail.monarch.com > Michael Garretson: pstc_ad...@garretson.org > >For policy questions, send mail to: > Richard Nute: ri...@ieee.org ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org