Re: Characterizing a screen room
A reverberant chamber CAN be made to do service for precompliance; we did at a former employer. However, one must rely on experience and trickery. If you see a bump, move the antenna sideways a few feet. A reverberant chamber will have few peaks more than 6 dB above the actual value, but many deep, deep nulls, so you should move the antenna anyway. Test close; 1 meter. We ended up doing the testing with the operator inside the chamber, which was convenient for rotating manual tables, and using an older HP 141-T mainframe analyzer as the digital ones required modification to make them quiet enough. However, an 8590 only took a screen over the CRT and refurbishing the existing gasket by turning it around. We had to draw lines on the table for cable routing. Moving them away from the standard placement caused large difference in radiated fields. but this is PRE compliance, not the real thing. By using a standard cable set and routing, we were able to do good delta testing with results that proved out on later certification. A few cones or tiles strategically placed can _reduce_ though not eliminate resonances in the area of interest and this is always a help in reproducing readings. Just don't move the cones after you get them set up. Cortland == Original Message Follows Date: 05-May-99 08:52:12 MsgID: 1068-2224 ToID: 72146,373 From: Patrick Lawler INTERNET:plaw...@west.net Subj: Re: Characterizing a screen room Chrg: $0.00 Imp: Norm Sens: StdReceipt: NoParts: 1 Thanks for the responses, everyone. I was probably misleading when I used the phrase 'characterize a screen room'. It sounds too much like 'calibrate'. I was (and still am) pessimistic that we can any get decent information from a screen room. I was hoping to at least figure out a way to mark frequencies where the room was unreliable. Enough said. In light of comments made (especially the one below that mentions 20dB variations), it sounds like pre-scans to _discover_ problem spots are invalid as well. - If you saw a bump, it may be a reflection/resonance. - If you saw a quiet area of spectrum, it may be a null. It sounds like the process should be: 1) Discover frequencies by going to an OATS and doing a valid scan. 2) Put the same test setup in the screen room, and move it around until you saw the same relative amplitudes (ignore the absolute amplitudes). Mark the location of that exact test setup! 3) Work on the unit to reduce the relative amplitudes. BTW, I am going to try measurements in the parking lot. If a screen room isn't purchased, there should be lots of money available for lounge chairs and sun screen (I live in Southern California). On Wed, 28 Apr 1999 11:14:38 -0500, Patrick Lawler plaw...@west.net wrote: At 04:21 PM 4/27/99 GMT, Robert Bonsen rbon...@orionscientific.com wrote: My company is planning to purchase a screen room for radiated emissions precompliance testing. I'm aware that reflections can cause resonances and drastically influence readings. What kind of testing could I do to characterize the room (aside from simple experience)? The simple answer would be this: don't even try. You're much better off using the company parking lot to do pre-compliance radiated emissions testing. For conducted emissions/immunity, and to a certain extent radiated immunity, a shielded room is great. But not for RE. The reason for this is the reflections/resonances you get from the walls and the ceiling. You can get higher than 20 dB ripples on your measurements in an untreated (no absorber materials on walls/ceiling) shielded room. And these ripples are not very repeatable, they will change considerably with position (eg, moving your antenna or EUT less than an inch may result in field variations of much more than 10 dB). Because of these huge variations, testing cannot help you characterize your room and take these reflections into account in your emissions measurements. If you absolutely need to use a shielded room, try lining it with absorber materials. Even a few absorbers are better than none at all. Or try using another type of pre-compliance device like a GTEM or something similar. Another alternative would be to turn the shielded room into a mode-stir chamber. By rotating the properly designed mode stirrer, you will even out the variations which will result in fairly usable, repeatable numbers. The size of the room determines the usable frequency range. -- Patrick Lawler plaw...@west.net - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). == End of Original Message = - This message is coming from the emc-pstc discussion list
Re: Characterizing a screen room
Thanks for the responses, everyone. I was probably misleading when I used the phrase 'characterize a screen room'. It sounds too much like 'calibrate'. I was (and still am) pessimistic that we can any get decent information from a screen room. I was hoping to at least figure out a way to mark frequencies where the room was unreliable. Enough said. In light of comments made (especially the one below that mentions 20dB variations), it sounds like pre-scans to _discover_ problem spots are invalid as well. - If you saw a bump, it may be a reflection/resonance. - If you saw a quiet area of spectrum, it may be a null. It sounds like the process should be: 1) Discover frequencies by going to an OATS and doing a valid scan. 2) Put the same test setup in the screen room, and move it around until you saw the same relative amplitudes (ignore the absolute amplitudes). Mark the location of that exact test setup! 3) Work on the unit to reduce the relative amplitudes. BTW, I am going to try measurements in the parking lot. If a screen room isn't purchased, there should be lots of money available for lounge chairs and sun screen (I live in Southern California). On Wed, 28 Apr 1999 11:14:38 -0500, Patrick Lawler plaw...@west.net wrote: At 04:21 PM 4/27/99 GMT, Robert Bonsen rbon...@orionscientific.com wrote: My company is planning to purchase a screen room for radiated emissions precompliance testing. I'm aware that reflections can cause resonances and drastically influence readings. What kind of testing could I do to characterize the room (aside from simple experience)? The simple answer would be this: don't even try. You're much better off using the company parking lot to do pre-compliance radiated emissions testing. For conducted emissions/immunity, and to a certain extent radiated immunity, a shielded room is great. But not for RE. The reason for this is the reflections/resonances you get from the walls and the ceiling. You can get higher than 20 dB ripples on your measurements in an untreated (no absorber materials on walls/ceiling) shielded room. And these ripples are not very repeatable, they will change considerably with position (eg, moving your antenna or EUT less than an inch may result in field variations of much more than 10 dB). Because of these huge variations, testing cannot help you characterize your room and take these reflections into account in your emissions measurements. If you absolutely need to use a shielded room, try lining it with absorber materials. Even a few absorbers are better than none at all. Or try using another type of pre-compliance device like a GTEM or something similar. Another alternative would be to turn the shielded room into a mode-stir chamber. By rotating the properly designed mode stirrer, you will even out the variations which will result in fairly usable, repeatable numbers. The size of the room determines the usable frequency range. -- Patrick Lawler plaw...@west.net - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
Re: Characterizing a screen room
At 04:21 PM 4/27/99 GMT, you wrote: My company is planning to purchase a screen room for radiated emissions precompliance testing. I'm aware that reflections can cause resonances and drastically influence readings. What kind of testing could I do to characterize the room (aside from simple experience)? The simple answer would be this: don't even try. You're much better off using the company parking lot to do pre-compliance radiated emissions testing. For conducted emissions/immunity, and to a certain extent radiated immunity, a shielded room is great. But not for RE. The reason for this is the reflections/resonances you get from the walls and the ceiling. You can get higher than 20 dB ripples on your measurements in an untreated (no absorber materials on walls/ceiling) shielded room. And these ripples are not very repeatable, they will change considerably with position (eg, moving your antenna or EUT less than an inch may result in field variations of much more than 10 dB). Because of these huge variations, testing cannot help you characterize your room and take these reflections into account in your emissions measurements. If you absolutely need to use a shielded room, try lining it with absorber materials. Even a few absorbers are better than none at all. Or try using another type of pre-compliance device like a GTEM or something similar. Another alternative would be to turn the shielded room into a mode-stir chamber. By rotating the properly designed mode stirrer, you will even out the variations which will result in fairly usable, repeatable numbers. The size of the room determines the usable frequency range. Regards, -Robert Robert Bonsen Principal Consultant Orion Scientific email: rbon...@orionscientific.com URL: http://www.orionscientific.com phone: (512) 347 7393; FAX: (512) 328 9240 - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
RE: Characterizing a screen room
Here's a caution on conducted testing in a screen room, too: While testing a power supply at a former employer, I noted it was failing in our lab, and passing in another, with the same load board and using the same type of LISN. The failure was due to our test setup; radiated noise from the load was coupled to the unshielded line cord plugged into the LISN, with maxima where the chamber was resonant. For a more realistic test, we remounted our load boards inside chassis of the type we would be using, and our results thereafter agreed with the outside lab's. The vendor was very helpful, and we DID get a quiet power supply -- but it was embarrassing! Cortland == Original Message Follows Date: 28-Apr-99 05:24:05 MsgID: 1067-117034 ToID: 72146,373 From: WOODS, RICHARD INTERNET:wo...@sensormatic.com Subj: RE: Characterizing a screen room Chrg: $0.00 Imp: Norm Sens: StdReceipt: NoParts: 1 A screen room will be useful for conducted emissions but not radiated. Go with the parking lot for radiated. It's cheaper and will yield better results. -- From: plaw...@west.net [SMTP:plaw...@west.net] Sent: Tuesday, April 27, 1999 5:00 PM To: emc-p...@majordomo.ieee.org Subject: Re: Characterizing a screen room On Tue, 27 Apr 1999 14:58:30 -0400, WOODS, RICHARD wo...@sensormatic.com wrote: You cannot perform a characterization that will mean anything. The room will have standing waves that will be strongly dependant upon the size and placement of the unit under test, the placement of the antenna and the frequency. That was the plan - record frequencies where the room is unreliable, so we don't spend time looking at that data. Real tests would be done at an OATS. My boss is interested in adding a screen room, but I'm worried that resonances will render the room worthless. In light of that, do you think I'd be better off developing a 'parking lot procedure', and figure out how to deal with the ambients? The best that you can do is perform a pretest to find the frequencies of interest then move to the OATS for a final test. A screen room can be used for before and after comparison of EMI fixes, as long as the unit under test is not moved. But once you have a fix, you will still have to test on the OATS. Actually, you can perform diagnostic tests in a lab if you set the antenna 1 m away. Just keep other sources a few meters away from the antenna. To do what you want to do, you will need a compact semi-anechoic chamber at a cost of about $140, 000 including the turn table. We just started using one that complies with the NSA test given the constrant that we can't run the antenna up to 4 m. We have found up to 6 dB of variation between the chamber and the OATS. However the variation is small enough that we pretest and fix in the chamber and only move to the OATS once we have confidence that we have at least 6 dB of margin. So far so good, but I don't doubt that some day we will end up out of compliance at the OATS even with 6 dB of margin in the chamber. -- From: plaw...@west.net [SMTP:plaw...@west.net] Sent: Tuesday, April 27, 1999 12:22 PM To: emc-p...@majordomo.ieee.org Subject: Characterizing a screen room My company is planning to purchase a screen room for radiated emissions precompliance testing. I'm aware that reflections can cause resonances and drastically influence readings. What kind of testing could I do to characterize the room (aside from simple experience)? -- Patrick Lawler plaw...@west.net - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). -- Patrick Lawler plaw
RE: Characterizing a screen room
A screen room will be useful for conducted emissions but not radiated. Go with the parking lot for radiated. It's cheaper and will yield better results. -- From: plaw...@west.net [SMTP:plaw...@west.net] Sent: Tuesday, April 27, 1999 5:00 PM To: emc-p...@majordomo.ieee.org Subject: Re: Characterizing a screen room On Tue, 27 Apr 1999 14:58:30 -0400, WOODS, RICHARD wo...@sensormatic.com wrote: You cannot perform a characterization that will mean anything. The room will have standing waves that will be strongly dependant upon the size and placement of the unit under test, the placement of the antenna and the frequency. That was the plan - record frequencies where the room is unreliable, so we don't spend time looking at that data. Real tests would be done at an OATS. My boss is interested in adding a screen room, but I'm worried that resonances will render the room worthless. In light of that, do you think I'd be better off developing a 'parking lot procedure', and figure out how to deal with the ambients? The best that you can do is perform a pretest to find the frequencies of interest then move to the OATS for a final test. A screen room can be used for before and after comparison of EMI fixes, as long as the unit under test is not moved. But once you have a fix, you will still have to test on the OATS. Actually, you can perform diagnostic tests in a lab if you set the antenna 1 m away. Just keep other sources a few meters away from the antenna. To do what you want to do, you will need a compact semi-anechoic chamber at a cost of about $140, 000 including the turn table. We just started using one that complies with the NSA test given the constrant that we can't run the antenna up to 4 m. We have found up to 6 dB of variation between the chamber and the OATS. However the variation is small enough that we pretest and fix in the chamber and only move to the OATS once we have confidence that we have at least 6 dB of margin. So far so good, but I don't doubt that some day we will end up out of compliance at the OATS even with 6 dB of margin in the chamber. -- From: plaw...@west.net [SMTP:plaw...@west.net] Sent: Tuesday, April 27, 1999 12:22 PM To: emc-p...@majordomo.ieee.org Subject: Characterizing a screen room My company is planning to purchase a screen room for radiated emissions precompliance testing. I'm aware that reflections can cause resonances and drastically influence readings. What kind of testing could I do to characterize the room (aside from simple experience)? -- Patrick Lawler plaw...@west.net - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). -- Patrick Lawler plaw...@west.net - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
RE: Characterizing a screen room
You cannot perform a characterization that will mean anything. The room will have standing waves that will be strongly dependant upon the size and placement of the unit under test, the placement of the antenna and the frequency. The best that you can do is perform a pretest to find the frequencies of interest then move to the OATS for a final test. A screen room can be used for before and after comparison of EMI fixes, as long as the unit under test is not moved. But once you have a fix, you will still have to test on the OATS. Actually, you can perform diagnostic tests in a lab if you set the antenna 1 m away. Just keep other sources a few meters away from the antenna. To do what you want to do, you will need a compact semi-anechoic chamber at a cost of about $140, 000 including the turn table. We just started using one that complies with the NSA test given the constrant that we can't run the antenna up to 4 m. We have found up to 6 dB of variation between the chamber and the OATS. However the variation is small enough that we pretest and fix in the chamber and only move to the OATS once we have confidence that we have at least 6 dB of margin. So far so good, but I don't doubt that some day we will end up out of compliance at the OATS even with 6 dB of margin in the chamber. -- From: plaw...@west.net [SMTP:plaw...@west.net] Sent: Tuesday, April 27, 1999 12:22 PM To: emc-p...@majordomo.ieee.org Subject: Characterizing a screen room My company is planning to purchase a screen room for radiated emissions precompliance testing. I'm aware that reflections can cause resonances and drastically influence readings. What kind of testing could I do to characterize the room (aside from simple experience)? -- Patrick Lawler plaw...@west.net - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
Characterizing a screen room
My company is planning to purchase a screen room for radiated emissions precompliance testing. I'm aware that reflections can cause resonances and drastically influence readings. What kind of testing could I do to characterize the room (aside from simple experience)? -- Patrick Lawler plaw...@west.net - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: unsubscribe emc-pstc (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
