Chris, some excellent points! My take on this is that a fiber optical cable has a cutoff frequency that is way above the "RF" frequencies we are concerned with. It just won't act as a waveguide for what we consider RF wavelengths. However, I think the optical cable certainly does leak a bit at light wavelengths. It seems like one could perform light emissions and susceptibility testing. Now most of our light wave communications use cables. Sort of like if all RF communications was done in copper. With nothing intentionally radiated, and with the cables operating as very poor antennas, we might have no need for emission and susceptibility testing. On the other hand, the FCC does not regulate radiated optical communications. Maybe it's time to do so. With laser range finders, optical radar, IR remote control, and other primitive devices we are accumulating pollution of this part of the EM spectrum. Reminds me of spark transmitters spewing RF over a wide frequency range.
Shouldn't an optical cable with a metal sheath be treated just like any other cable? Hook it up during EMC testing? And as you point out, where is the transition from RF to light? RF generation methods (such as gyrotrons, seem to peter out at wavelengths of 1000 microns while visible light begins at 0.7 microns. There seems to be a huge no man's land in the EM spectrum. Dave Cuthbert Micron Technology From: Chris Maxwell [mailto:[email protected]] Sent: Friday, January 10, 2003 8:24 AM To: [email protected] Subject: RE: Fiber optic cable testing per EN 55022:1998 ? OK, Enough of this regulatory blah, blah, blah...(although that's what we get paid for). How about a hypothetical question... Typical radiated emissions measure a time varying electric field produced by the acceleration of electrons. When electrons accelerate back and forth at a given frequency; then you get EM (ElectroMagnetic) radiation at that frequency. At these frequencies, we have electron flow in conductors. The electron acceleration in one conductor (say your computer backplane) gives off an EM field which will cause a similar electron flow in another conductor placed some distance away (the measurement antenna). Notice that in this case, we don't have electrons changing energy states, we just have free electrons flowing and accelerating. Fiber optic cables carry light, which is modeled as photons produced by electrons changing energy states. We still can model this with similar wave equations as used for any old EM radiation; but here we have radiation flow in an insulator. We also throw in the concept of "photons" whereby we try to quantize the radiation. Light won't (appreciably) flow at all in a conductor. So, we don't consider fiber optic cables to be susceptable to "EMI"; and we don't consider them to give off "EMI". I think that we all agree that trying to measure the "conducted" or "radiated" emisions from fiber optic cables is not required by any standard. They do "conduct" light; but it is a conduction of photons; not the conduction of free electrons that the standards try to measure. However, I can think of some lower frequencies (lower than light, that is) that use dielectric waveguides similar to fiber optics; yet they produce and are susceptable to EMI. For example, many GPS antennas us dielectric waveguides at the GPS frequency (about 1.5GHz, if I recall correctly) So where is the "crossover point"? Does it have to do with skin depth? Maybe the photoelectric effect? Why don't we talk about photons at 1Ghz? Is it just because we don't have a material with the correct band gap to produce a 1Ghz photon? On the other hand, can free electrons be "conducted" at light frequencies; or isn't there a material with enough of a skin depth at such frequencies? Anybody want to take a stab at enlightening(no pun intended) us all on this one? I guess I'm just too lazy to brush up on my quantum mechanics. It's too bad that Einstein died before we came up with listservers. I have about a million questions for him. He probably would have taken a job as an EMC guy just to pay the bills while he was working on relativity. Sure, its a hypothetical question; but it may provide a deeper understanding of why we don't throw fiber optic cables in the coupling clamp. I can smell the collective cranial smoke from the group already. That's good. Inquizzitively and antagonistically, Chris Maxwell | Design Engineer - Optical Division email [email protected] | dir +1 315 266 5128 | fax +1 315 797 8024 NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA web www.nettest.com | tel +1 315 797 4449 | 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"
