http://emcwizard.com/wp Interesting and useful EMI/EMC/E3/RFI Information Thu, 19 Nov 2009 16:36:00 -0800 http://wordpress.org/?v=2.9.2 hourly 1 http://emcwizard.com/wp/?p=20&cpage=1#comment-84 stu Thu, 19 Nov 2009 16:36:00 +0000 http://emcwizard.com/wp/?p=20#comment-84 Thanks, Kerryn. An unshielded Cat 5 cable can have surprising low emissions if the signals, cables, and terminations are balanced thereby minimizing differential mode to common mode conversion. Contamination of the signal pairs with common mode noise from elsewhere in the circuit or system is still a potential problem, but is manageable with proper layout and design. While this is very often adequate to meet commercial (e.g., FCC Part 15) EMC standards, military emissions and susceptibility requirements are much more stringent and usually require a shield as you describe. When properly implemented, such a shielded cable can provide on the order of 30 to 60 dB of attenuation. The cable shield's surface transfer impedance is one of most useful parameters in predicting the effectiveness of the shield. Just remember that the shield terminations are almost always the weakest link in the design. Thanks, Kerryn.
An unshielded Cat 5 cable can have surprising low emissions if the signals, cables, and terminations are balanced thereby minimizing differential mode to common mode conversion. Contamination of the signal pairs with common mode noise from elsewhere in the circuit or system is still a potential problem, but is manageable with proper layout and design. While this is very often adequate to meet commercial (e.g., FCC Part 15) EMC standards, military emissions and susceptibility requirements are much more stringent and usually require a shield as you describe. When properly implemented, such a shielded cable can provide on the order of 30 to 60 dB of attenuation. The cable shield’s surface transfer impedance is one of most useful parameters in predicting the effectiveness of the shield. Just remember that the shield terminations are almost always the weakest link in the design.

]]> http://emcwizard.com/wp/?p=20&cpage=1#comment-83 Kerryn Thu, 19 Nov 2009 06:12:34 +0000 http://emcwizard.com/wp/?p=20#comment-83 Well explained - question, what happens if the conductor passed through has it's own signal eg. a Cat 5 data cable or a firewire cable? How much can that be eliminated by using bulkhead mount connectors and a shielded cable within the room? Well explained – question, what happens if the conductor passed through has it’s own signal eg. a Cat 5 data cable or a firewire cable? How much can that be eliminated by using bulkhead mount connectors and a shielded cable within the room?

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