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: [email protected] >[mailto:[email protected]] 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: [email protected] > Homepage: http://www.inep.ufsc.br > >================================================== > >--------- >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). > > > Best Regards Bob Cresswell Senior Engineer, Intellistor OATS email: [email protected] 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 [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).
