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:[email protected] Subj: RE: Characterizing a screen room Chrg: $0.00 Imp: Norm Sens: Std Receipt: No Parts: 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: [email protected] [SMTP:[email protected]] Sent: Tuesday, April 27, 1999 5:00 PM To: [email protected] Subject: Re: Characterizing a screen room On Tue, 27 Apr 1999 14:58:30 -0400, "WOODS, RICHARD" <[email protected]> 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: [email protected] [SMTP:[email protected]] > Sent: Tuesday, April 27, 1999 12:22 PM > To: [email protected] > 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 > [email protected] > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). > >--------- >This message is coming from the emc-pstc discussion list. >To cancel your subscription, send mail to [email protected] >with the single line: "unsubscribe emc-pstc" (without the >quotes). For help, send mail to [email protected], >[email protected], [email protected], or >[email protected] (the list administrators). > -- Patrick Lawler [email protected] --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). ====================== End of Original Message ===================== --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
