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).
