------------------------ From: Muriel Bittencourt de Liz <[email protected]> Subject: Doubt on Measuring with Spectrum Analyser Date: Tue, 22 Sep 1998 17:35:05 -0300 To: Lista de EMC da IEEE <[email protected]>
> 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 > > ================================================== Muriel: This procedure is not a good practice for two reasons. First, your ambient is not constant. Signals appear and disappear randomly; signals that remain present often exhibit a slow amplitude modulation. Signals which have multipath components will also exhibit amplitude changes. Consider the logic of your approach. One simple problem condition would be a signal which happened to be stronger during your "equipment on" sample period, then decreased during your "equipment off" sample period. Your technique would label this a valid EUT emission. Another problem condition would be a signal which is FM'ing or frequency hopping. This would really confuse your data acquisition. Second, Corollary 5 of Murphy's Law states that "Ambient signals are always in your way." Let's say your EUT has a 20 MHz clock, actually running at 20.02 MHz. Here in the USA, the clock's fifth harmonic will always be obscured by a 50 KW FM broadcast station on 100.1 MHz. It can be very tough to get an accurate reading of an EUT emission in the presence of a strong ambient signal. The solution here is to find another open field site, or to wait for a time when the signal is not present. Maybe it will be easier for you to find a relatively quiet open field site in Brasil. Here in the USA, the need for a quiet site has forced many labs to locate an open field test facility far away from industrial regions. This leads to some high-tech jobs in very bucolic areas :-) , or some very long commuting! :-( Ed -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: [email protected] Date: 09/22/1998 Time: 15:28:57 -------------------------- --------- 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).
