Little belated response, but I thought it may be worth sending it anyway. Laboratory Report: "Common-mode impedance measurements on cables in the frequency range 10-1000 MHz", report no. EIE 92004, by Mart Coenen from Philips in Eindhoven, the Netherlands, is probably the best reference on this subject.
If you connect a piece of long wire to a network analyzer, you can measure any impedance, from sub-Ohm to above kOhm. However, if you create the condition of cable-resonance, which is what the emission-testing is about when you maximize emission, then it can be shown that statistically the impedance is close to 150 Ohm. The report describes a test results where a cable was set on a long bench over a plane, with an absorption clamp on it. One end of the cable was connected to a network analyzer (NWA). The clamp output was connected to the other NWA port and movedalong the cable to achieve max S21 (= max. emission condition). Then the input impedance of the cable was measured. All that was performed in many different configurations, and the measured impedances analyzed. The results support 150 Ohm as a reasonable "expected" value for the common-mode impedance of the cables. BTW, if you look at the antenna theory, and realize that a cable in resonance is just a simple linear antenna such as monopole, half-wavelength or full- wavelength dipole, then you'll see that its expected impedance at resonance is between about 30 Ohm and 300 Ohm, which again supports the 150 Ohm selection. Neven > > Please forgive my ignorance but can anyone tell me why above are designed to > give load and source impedances of 150 ohms? > > Cheers > > Chris Colgan > Compliance Engineer > TAG McLaren Audio Ltd > The Summit, Latham Road > Huntingdon, Cambs, PE29 6ZU > *Tel: +44 (0)1480 415 627 > *Fax: +44 (0)1480 52159 > * Mailto:[email protected] > * http://www.tagmclaren.com > > > > > ************************************************************** > Please visit us at www.tagmclaren.com > ************************************************************** > > The contents of this E-mail are confidential and for the exclusive > use of the intended recipient. If you receive this E-mail in error, > please delete it from your system immediately and notify us either > by E-mail, telephone or fax. You should not copy, forward or > otherwise disclose the content of the E-mail. > > Important Note: Any typographical, clerical or other error in this > communication is subject to correction without any liability on > the part of TAG McLaren Audio Limited. Any orders placed shall > be subject to acceptance by TAG McLaren Audio Limited on its > standard terms and conditions of sale which shall govern the > contract for the sale and purchase of the products ordered to the > exclusion of any other terms and conditions. > > TAG McLaren Audio Ltd > The Summit, 11 Latham Road > Huntingdon, Cambs, PE29 6ZU > Telephone : 01480 415600 (+44 1480 415600) > Facsimile : 01480 52159 (+44 1480 52159) > > ************************************************************** > Please visit us at www.tagmclaren.com > ************************************************************** > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > 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://ieeepstc.mindcruiser.com/ > Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
