I read in !emc-pstc that Ken Javor <[email protected]> wrote (in <bc07af0f.88f0%[email protected]>) about 'CISPR 25 Conducted Emissions (Current Method)' on Thu, 18 Dec 2003:
>It is true that the unit dB Ohm has to be interpreted in context. >However that does not change its utility. Current probe transfer >impedance and shielding transfer impedance both have units of dB Ohm. >What would be a better substitute? Decibels ref 1 volt per amp would be unambiguous and mathematically defensible. Abbreviation dB(V/A). This also has the advantage that the abbreviation immediately explains what the quantity means. If I see a current probe transfer impedance expressed as '20 dB(ohm)', I'm not sure what that means, but if its '20dB(V/A)', then I know immediately that 1 A through it gives 10 V out. Similarly for shield transfer impedance. I can see straight away that 100 mA of shield current gives 1 V from end to end (poor shield or a very long cable!). Formally, the decibel is a power ratio, so P = I^2R applies and thus the only possible legitimate 'dB(ohm)' is 10logR. But that is hardly ever what it means in these EMC standards. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
