I had a similar problem with my Conducted Emissions test site. The Power Cord of the EUT was actually acting as an antenna picking up local AM Radio stations, and passing them through the LISN to the receiver. My Solution - Shield the Test Site.
Regards, Randy Flinders EMC Test Engineer Emulex Network Systems randall.flind...@emulex.com ---------- From: Bob Cresswell Sent: Wednesday, September 23, 1998 7:44 AM To: Ing. Gert Gremmen; Muriel Bittencourt de Liz; Lista de EMC da IEEE Subject: RE: Doubt on Measuring with Spectrum Analyser Hello Meuiel & Gert There is yet another approach to the problem of common mode currents on cables which are produced by AM, TV, FM... broadcast station. In the past I have made a common mode Balun which is simply 2 or 3 ferrite rings (maybe 2.5 to 3 inch diam.) and route your coax cable around the ferrite, maybe 5 turns. This will limit the interference current while leaving the conducted emissions currents untouched. This has worked well for me in the past. At 12:10 PM 9/23/98 +0200, Ing. Gert Gremmen wrote: >Hello Muriel, > >What you are trying to do is a valid method of pré-compliance testing but: > >make sure tour subtraction is done in the linear domain. If subtracting dB's >you will obviously make huge mistakes. > >FA a 40dBuV signal is present as an FM radio station in you set-up. The EUT >makes interference at 43 dBuV. The difference in uV is only 30% if you >subtract both signals only 3 dBuV will reside. That clearly is a big error. >As no correlation between the signals can be present, the sum of two signals >rarely is the double of both. This means that an interfering environment >source which is less then 6 dB below the requested signal DOES NOT >contribute significally to the output of your measurements. No subtraction >required thus, just take the EUT signal level value. >If you want to estimate the sum of two uncorrelated signals, try the root >from the sum of the mean squares method ( in uVolts, not in dB) > >make sure your subtracted signal consists of continuous signal only. Do not >take a spectrums analyzers sweep, because it might contain intermittent >signals > >The only useful subtraction you can make is subtracting "peaks that should >not be there" and that interfere with the interference plot that you want to >make. >This requires you to measure and identify peaks in a mathematics way, and >then selectively subtracting them form you plot. > >Be especially very careful if you suspect that environment interference is >at the same frequency as EUT signal. They might be different in relation to >smallband wideband character, therefore give different results in your >"peak" "quasi-peak" and average detector and thus invalidating every method >of subtraction you may try. > >Regards, > >Gert Gremmen > >ce-test. > >== Ce-test, Qualified testing == >Consultants in EMC, Electrical safety and Telecommunication >Compliance tests for European standards and ce-marking >Member of NEC/IEC voting committee for EMC. >Our Web presence: http://www.cetest.nl >List of current harmonized standards http://www.cetest.nl/emc-harm.htm >15 great tips for the EMC-designer http://www.cetest.nl/features01.htm > >Support the International Red Cross Million 2000 Lottery: >http://www.pluslotto.com/default.asp?urlref=3300008136443 > > > >-----Original Message----- >From: owner-emc-p...@majordomo.ieee.org >[mailto:owner-emc-p...@majordomo.ieee.org] On Behalf Of Muriel Bittencourt >de Liz >Sent: dinsdag 22 september 1998 22:35 >To: Lista de EMC da IEEE >Subject: Doubt on Measuring with Spectrum Analyser > >Hello all, > >I'm doing my measurements of conducted emissions using a Spectrum >Analyser, but my site isn't shielded, i.e. some interferences ( radio >stations ) appear at the screen of the SA. So, i'm doing this: i record >the signal with the product tested at "off" and when i turn the product >"on", i subtract the signal of "on" minus the signal of "off". this >means that i subtract the "ambient noise" from the noise being generated >by the product itself. > >What i really want to know is: is this procedure correct? am i doing a >nonsense thing? > >Any comments will be welcome. > >thanks > >Muriel > >-- > >================================================== > > Muriel Bittencourt de Liz > INEP - Instituto de Eletronica de Potencia > Universidade Federal de Santa Catarina > Caixa Postal - 5119 > 88.040-970 - Florianopolis - SC - BRASIL > Phone: +55.48.331.9204 - Fax: +55.48.234.5422 > e-mail: mur...@inep.ufsc.br > Homepage: http://www.inep.ufsc.br > >================================================== > >--------- >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, >j...@gwmail.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, >j...@gwmail.monarch.com, ri...@sdd.hp.com, or >roger.volgst...@compaq.com (the list administrators). > > > Best Regards Bob Cresswell Senior Engineer, Intellistor OATS email: bcressw...@criteriontech.com phone: USA (303) 682-6600 Fax: USA (303) 682-6672 --------- 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, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
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