---------------------- Forwarded by Don Borowski/SEL on 01/10/2003 10:37 AM
Don Borowski 01/10/2003 10:35 AM To: "Chris Maxwell" <[email protected]> cc: Subject: RE: Fiber optic cable testing per EN 55022:1998 ? (Document link: Database 'Don Borowski', View '($Sent)') OK, some comments. The radiated and conducted emissions of concern are there due to the possibility of the modulation signal sent to the light source getting coupled onto the metalic coating of the fiber. There are seperate standards covering light leakage that are there for eye protection. We conventionally think about current flow in a pair of wires carrying energy. The energy is indeed guided by the wires, but is carried by the transverse electromagnetic (TEM) field around the wires. The energy is not flowing in the conductors, but in the space around the conductors. Even at DC where there is electron flow through the bulk of a (non-superconductor) material, the energy is transmitted in the electric and magnetic field outside the wires. There is indeed a magnetic field inside the wire. However, the only electric field inside the wire is along the direction of electron flow, and is due to resistance. If you do the Poynting vector E×H, the direction of the vector is radial into the wire. There is energy flow into the wire, which causes the wire to produce heat. This heat production is better known as I²R loss. Light certainly can be guided by metal, just as lower frequencies. Take a cylinder with a nice shiney mirrored surface on the inside, and shine light into it. You will see light come out at the other end, even if there is a bend so that there is not a straight shot through. It is possible to guide light with two conductors in TEM mode, though it is quite lossy. Fiber is nice because of its low loss. There certainly can be light emissions from fiber - a sharp bend can cause it. Conversely, bend a fiber sharply and shine a light on the bend, and the light will go into the fiber. So there is possible emission and susceptibility. But put a jacket over the fiber, or a metalic coating, and this problem is solved for any practical source or victim. A system with a light source and light receiver is certainly still subject to problems with RF emissions and RF susceptibility, even if the fiber itself is not for all practical purposes. The optical components are good enought the there is no significant susceptibility to or emissions of light, assuming nothing is broken. Similarly, a GPS receiver may be susceptible to or radiated emissions that are not at the GPS frequency. There are certainly photons at 1 GHz. However, due to the low frequency (compared to light), an individual photon does not carry much energy - not enough energy to free an electron as can be done by a single light photon on a sensitive surface (and then usually multiplied using an electron multiplier). Due to the difficulty in detecting single photons at 1 GHz, photons are not part of the normal vocabulary when discussing RF. Enough cranial smoke yet? Don Borowski Schweitzer Engineering Labs Pullman, WA "Chris Maxwell" <[email protected]>@majordomo.ieee.org on 01/10/2003 07:23:40 AM Please respond to "Chris Maxwell" <[email protected]> Sent by: [email protected] To: <[email protected]> cc: 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. 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