Hi Ian, I had the unfortunate opportunity to attempt in-situ measurements of a large telecom system about 12 years ago.
We found that getting useful measurements was very, very difficult. As Bob notes, power-cycling the EUT helps to differentiate the noise source but that may not always be possible if the customer has already commissioned the equipment. You'll find that the noise ambients are very high and confound measurements. Getting a space where you can take even 3m measurements in a 360 degree circle and be able to vary the antenna height 1-4m will also be extremely improbable. There are few, if any, rooms that have that much free space around an EUT. Also the cabling can cause issues since it will most likely extend way beyond where the EUT is (contrary to how testing is done at an OFS, etc.). This may cause you to exceed the limit due to near-field effects. On the other hand, the near-field effects may cause you to have unreasonably good results - you just don't know. There is a lot of associated failure risk that it may be wise to try to avoid (see 2nd to last paragraph). Don't forget that you will also have to take conducted measurements and that any other device on that power feed circuit, or even on the entire distribution, may cause you to fail. We had much less conducted emissions margin that we wanted during our testing. The logistics and cost associated with this testing was also significantly higher than a normal EMC test. Getting a spectrum analyzer, antenna, antenna mast, etc. reserved for all the testing took a long time and caused other programs to suffer. This alone could be a show-stopper for some companies. If this unit is a one-of-a-kind, than I believe you only need this one test. However if you are trying to provide a compliance document for future production, the FCC required testing three units. This obviously extends the time, cost, and PITA. My best advice is to document all the issues involved with these measurements and ask for another unit that you can take to an approved test facility. On the positive side, we had a lot of fun doing the measurements since we got to see various installations and real-world deployment. :) ...Marko From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Wednesday, January 02, 2008 5:03 AM To: [email protected] Subject: Fw: Radiated emission testing "in the real world" In general the point is to be able to differentiate the ambient signals >from the energy coming from the EUT. This is generally done by turning the EUT off to measure ambients and turning it on to determine noise from the EUT. Section 10.8 of CISPR 22 has some pointers on measurements made in the presence of ambients and you can probably find articles out on the web as well. For in-situ measurements I would not worry about the 6 dB (unless it precisely coincides with an EUT signal). Bob Heller 3M EMC Laboratory, 76-1-01 St. Paul, MN 55107-1208 Tel: 651- 778-6336 Fax: 651-778-6252 ======================================================================= "Gordon,Ian" <Ian.Gordon@edwar dsvacuum.com> To Sent by: [email protected] [email protected] cc Subject 01/02/2008 05:40 Radiated emission testing "in the AM real world" All Happy New year. Clause 6.1 of CISPR 11 requires that ambient noise levels to be 6dB below the specified limit. However for testing at a customer site i.e. not on an OATS or in a chamber the level of ambient emissions may be higher than this due to neighbouring factories or other sources. Can anybody suggest how to achieve these measurements? Ian Gordon *********************************************************************** The information contained in this email and any attachments may be confidential and is provided solely for the use of the intended recipient(s). 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