Re: Near Field Versus Far Field/Troubleshooting
Chris, I've been doing a similar test for years, only I use a pin solderd to about 2 feet of wire. This way I don't actually touch the circuit myself. FYI, Derek. --- 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
R: Near Field Versus Far Field
I try to re-send this today to the emc-pstc group. Yesterday it got kicked back
for unknown reasons.
Paolo Roncone
Hello Martin,
I read a number of excellent replies to your question. I would just like to add
my 0.02 cents based on my hands-on experience. I have been doing radiated
emissions testing and troubleshooting for 11+ years now. Up to date I was lucky
to have a full compliant test site (10m and 3m) available for both
qualification testing and for troubleshooting. I managed to solve quite a
number of emissions problems by using both near-field probes ("sniffers") and
antennas in the far field.
As others have already explained, there are many variables (both related to the
EUT and the test setup) that influence readings in the far field. That's the
reason why in many cases you may not get a correlation between relative
readings with a near field probe and in the test site. Plus (I would add) near
field measurements are more "tricky" and less easily repeatable than far field
measurements (very sensitive to probe position, test operator, external sources
etc.).
Any emissions (or immunity) process has three essential elements (or categories
of elements):
1. Source (electronic components, ICs..)
2. Coupling path (PCB traces, connectors, chassis elements ...)
3. Antenna (PCB traces at high enough frequencies, cables, slots in metal
enclosures...).
All these elements usually play a part in the overall readings in the far
field.
Many times the best (and cheapest) solutions are implemented at the source
level because you block the emissions as much "upstream" as possible. On the
other hand in many other cases the effects of the other 2 elements (coupling
paths and antennas) can dominate (especially when they trigger resonances at
some frequencies).
In my opinion, as a general rule near field probes should only be used to help
locate the most critical element for your specific problem, NOT for
measurements, because the "sniffer" (by definition) cannot pick-up the overall
picture. It usually works best for locating sources and coupling paths (noisy
circuit components and PCB traces). When you have located the "hottest" areas
(either by correlating frequencies of emissions with known signals or by
choking off cables or with "sniffers" or with anything else that works in your
case) you try fixes - one at a time as already mentioned - then after each
modification take readings ALWAYS in the far field, either in a full compliance
test site or in other locations where you can put an antenna far enough from
the EUT and get a repeatable setup. In this way you keep checking all the
elements (sources, copupling paths, antennas) at every step of your
troubleshooting work.
Hope this helps...
Paolo
-Messaggio originale-
Da: marti...@appliedbiosystems.com [SMTP:marti...@appliedbiosystems.com]
Inviato:giovedì 14 settembre 2000 19.08
A: emc-p...@majordomo.ieee.org
Oggetto:Near Field Versus Far Field
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
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RE: Near Field Versus Far Field
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
RE: Near Field Versus Far Field
Hi Don. Thanks for the example. Hearing about these kinds of experiences makes troubleshooting easier for everyone I think. Thanks to Chris Maxwell for his contribution too. It'd be great to hear more stories from the experience of some other members of the group. I think your approach was great for the problem you had, but wanted to say it doesn't always yield the quickest answer. To add to the examples: Most of the equipment we test has a large number of (mostly long) communication cable attachments. We had a recent experience where the equipment was over the limit due to emissions being conducted onto one such cable. Although with the cable attached the emissions from the cable were high, the actual emissions from the source in the near field were low (most of the noise was directly conducted onto the cable rather than radiated from the problem circuit board). Near field probing wouldn't have told us any quicker what the source was than the educated guessing + far-field measurement approach we took. In our case though, just from the frequency of the emission we already knew which circuit board was the culprit so it was just a matter of finding out what the coupling mechanism was to the cable. Under different circumstances though, I certainly agree that if you can use near field probes to home in on an unknown source, that would be likely to yield an answer quicker than trial and error. I guess I'm trying to say near field probing is useful, but doesn't always work. Because there can be such differences in the emissions measured in the near field as opposed to the far field it isn't always the best method. Regards, Pete. - 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 12:16 AM 16/09/2000, umbdenst...@sensormatic.com wrote: Hello Joe, Consider the following -- in the far field (3 or 10 meters), a plane wave is monitored. In the near field, using either commercial or lab built near-field probes, either E field or H field emissions will be monitored separately. The E, H components will be isolated. The emission may be identified with a probe, but the effective radiator (culprit antenna) for that emission might be missed. The above is an answer to the question. If you would like a real life experience describing the difference, read the example that follows. On a recent product we had a band of frequencies of non-compliant emissions that were somewhat polarity sensitive. We observed a particular signature of the emission (modulation on a pulse) at 3 meters using a bilog antenna. Using a direct contact E field probe, the pulse frequency showed up at high levels around the processor and DSP chip, but not with the signature. We were able to find a trace of the corresponding polarity that was suspect and had a similar signature, and at a lower level than we found around the processor and DSP chip. Looking at the schematic, we identified a reasonable fix. But that only helped part of the profile. We then sniffed with a non-contact magnetic loop probe and found another viable culprit. The fix implemented brought the product into compliance with reasonable margin. Neither fix by itself brought the product into compliance. Both were necessary, required a minimum amount of components and contributed to rationale "source suppression". We did not introduce "balloon squeezing", i.e., beat down an emission at one frequency and see it pop up at another frequency. This kind of isolation is more effective than monitoring the far field emission, hypothesizing the culprit antenna while analyzing the schematic. We have done it both ways. The near field approach takes a little more time to set up but saves time in the long run. Or maybe we were just lucky! Best regards, Don > -- > From: > marti...@appliedbiosystems.com[SMTP:marti...@appliedbiosystems.com] > Reply To: marti...@appliedbiosystems.com > Sent: Thursday, September 14, 2000 1:07 PM > To: emc-p...@majordomo.ieee.org > Subject: Near Field Versus Far Field > > > > > 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 modif
RE: Near Field Versus Far Field/Troubleshooting
I wanted to throw in an emissions trouble shooting technique that I saw Jon Curtis do one time when I was at his lab. I'd like to call it the "Jon Curtis Wet Finger Test" We had a signal failing at 200Mhz. We had narrowed the problem down to either the GPIB cable or the GPIB interface circuitboard connected to the GPIB cable. We were able to open up the unit and gain access to the GPIB board while it was running. Jon wet his finger and ran it over the pins of a few suspect IC's. When his finger touched one particular pin, the spectrum analyzer reading changed radically. It was a 40Mhz clock line. We reduced our emissions by cutting the run and putting a resistor in line with it. OK being a compliance guy, the lawyer in me is saying the following: 1. Please don't try this on AC or hazardous voltages (for obvious reasons). 2. Also, know your IC's. As you run your finger over some IC's, shorting adjacent pins may cause trouble. In our case it didn't. 3. Also, be careful of any hot IC's or heatsinks. 4. Wash your hands afterwards. (Lead's poisonous you know.) What I'm trying to say is: it's a neat technique and may be helpful, but I don't want someone to electrocute themselves, hurt their DUT or burn a finger trying it. Please use caution See ya's later Chris Maxwell, Design Engineer GN Nettest Optical Division 6 Rhoads Drive, Building 4 Utica, NY 13502 PH: 315-797-4449 FAX: 315-797-8024 EMAIL: chr...@gnlp.com > -Original Message- > From: michael.sundst...@nokia.com [SMTP:michael.sundst...@nokia.com] > Sent: Friday, September 15, 2000 8:59 AM > To: pet...@foxboro.com.au; marti...@appliedbiosystems.com; > emc-p...@majordomo.ieee.org > Subject: RE: Near Field Versus Far Field > > > 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
RE: Near Field Versus Far Field
Hello Joe, Consider the following -- in the far field (3 or 10 meters), a plane wave is monitored. In the near field, using either commercial or lab built near-field probes, either E field or H field emissions will be monitored separately. The E, H components will be isolated. The emission may be identified with a probe, but the effective radiator (culprit antenna) for that emission might be missed. The above is an answer to the question. If you would like a real life experience describing the difference, read the example that follows. On a recent product we had a band of frequencies of non-compliant emissions that were somewhat polarity sensitive. We observed a particular signature of the emission (modulation on a pulse) at 3 meters using a bilog antenna. Using a direct contact E field probe, the pulse frequency showed up at high levels around the processor and DSP chip, but not with the signature. We were able to find a trace of the corresponding polarity that was suspect and had a similar signature, and at a lower level than we found around the processor and DSP chip. Looking at the schematic, we identified a reasonable fix. But that only helped part of the profile. We then sniffed with a non-contact magnetic loop probe and found another viable culprit. The fix implemented brought the product into compliance with reasonable margin. Neither fix by itself brought the product into compliance. Both were necessary, required a minimum amount of components and contributed to rationale "source suppression". We did not introduce "balloon squeezing", i.e., beat down an emission at one frequency and see it pop up at another frequency. This kind of isolation is more effective than monitoring the far field emission, hypothesizing the culprit antenna while analyzing the schematic. We have done it both ways. The near field approach takes a little more time to set up but saves time in the long run. Or maybe we were just lucky! Best regards, Don > -- > From: > marti...@appliedbiosystems.com[SMTP:marti...@appliedbiosystems.com] > Reply To: marti...@appliedbiosystems.com > Sent: Thursday, September 14, 2000 1:07 PM > To: emc-p...@majordomo.ieee.org > Subject: Near Field Versus Far Field > > > > > 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
RE: Near Field Versus Far Field
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
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 wh
Re: Near Field Versus Far Field
Several excellent ideas have been put forward on this phenomenon. Here is my $.02 worth on the subject. I have often been able to reduce the far-field emissions, based on the reduction in near-field emissions. The important thing is to co-relate the far-field and near-field emissions, by comparing their frequency spectrums. It is possible that the 400 MHz clock you worked on with near field probe may not be the real culprit. Another trace may be coupling the 400 MHz clock noise to some cable, or one of the other signals, which is driven by the 400 MHz clock, may be the real source. Regards, Ravinder PCB Development and Design Department IBM Corporation - Storage Systems Division Email: ajm...@us.ibm.com *** Always do right. This will gratify some people and astonish the rest. Mark Twain marti...@appliedbiosystems.com@ieee.org on 09/14/2000 10:07:36 AM Please respond to marti...@appliedbiosystems.com Sent by: owner-emc-p...@ieee.org To: emc-p...@majordomo.ieee.org cc: Subject: Near Field Versus Far Field 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
RE: Near Field Versus Far Field
Consider how the reflected wave from the ground plane at the 10m site might add or subtract to the incident wave depending on frequency and antenna height. Also, the near-field probe measurement is taken in the near-field (obviously) while the antenna at the 10m sight is in the far field. Also, your results with the near-field probe will depend on probe orientation and distance held from the product. -Original Message- From: Bronaugh, Edwin [mailto:edwin.brona...@icn.siemens.com] Sent: Thursday, September 14, 2000 5:10 PM To: 'marti...@appliedbiosystems.com'; emc-p...@majordomo.ieee.org Subject: RE: Near Field Versus Far Field Mr. Martin, have you thought about the fact that the near-field probe does not integrate the whole radiation sphere from your product? On the 10 m site, your antenna sees radiation components from all parts of the EUT, including any cables; while your near-field probe in the lab cannot pick all of these up at the same relative levels and phases as does the antenna on the OATS. This may not be your problem, but in my opinion, it invites investigation. Regards, EdB -Original Message- From: marti...@appliedbiosystems.com [mailto:marti...@appliedbiosystems.com] Sent: Thursday, September 14, 2000 12:08 PM To: emc-p...@majordomo.ieee.org Subject: Near Field Versus Far Field 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
RE: Near Field Versus Far Field
Mr. Martin, have you thought about the fact that the near-field probe does not integrate the whole radiation sphere from your product? On the 10 m site, your antenna sees radiation components from all parts of the EUT, including any cables; while your near-field probe in the lab cannot pick all of these up at the same relative levels and phases as does the antenna on the OATS. This may not be your problem, but in my opinion, it invites investigation. Regards, EdB -Original Message- From: marti...@appliedbiosystems.com [mailto:marti...@appliedbiosystems.com] Sent: Thursday, September 14, 2000 12:08 PM To: emc-p...@majordomo.ieee.org Subject: Near Field Versus Far Field 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
RE: Near Field Versus Far Field
Joe, Let me try this one, The near field is composed of a sum of terms 1/r, 1/r^2, etc. and the far field is only composed of the 1/r term. If your 10dB reduction was solely due to a reduction in the higher order terms, 1/r^2 etc, then in the far field no reduction will have taken place. I hope this helps, William D'Orazio CAE Electronics Ltd. Electrical System Designer Phone: (514) 341-2000 (X4555) Fax: (514)340-5552 Email: dora...@cae.ca -Original Message- From: marti...@appliedbiosystems.com [mailto:marti...@appliedbiosystems.com] Sent: Thursday, September 14, 2000 1:08 PM To: emc-p...@majordomo.ieee.org Subject: Near Field Versus Far Field 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
Near Field Versus Far Field
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
