I abstracted the physical process and did not attach numbers. 80 dBuV/m is orders of magnitude above RE limits. That is 10 mV/m. If you use the worst case assumptions of IEC 1000-4-6 and associate an open circuit drive potential numerically equivalent to the field intensity, but with a 150 Ohm source impedance, then you would have 10 mV driving current from a 150 Ohm source. If you assume the impedance (not transfer impedance) of the shield to be a dead short, then 10 mV will cause 67 uA to flow on the shield. If shield transfer impedance were 100 milliohms (pretty high, especially at frequencies where an op-amp might respond) you would get 6.7 uV induced common mode on the circuit. Assuming no CMR, this is the potential that is added to the intentional signal. That can affect a radio front-end, but not an op-amp.
Once again, unintentional RE from ITE, even at unsuppressed levels, do not cause interference to other non-antenna connected electronics. on 1/6/02 10:28 AM, cherryclo...@aol.com at cherryclo...@aol.com wrote: Ken, can we take it that in the posting below you are agreeing that interference with non radio-receiving circuits from what you meant by "unintentional emitters" is a possibility, albeit a worst-case one? If my reading above is correct, how can you then go on to say that "...unintentional emissions from ITE can only upset a radio receiver tuned to the emission frequency." It seems to me that the words "can only" in the your quote above should be replaced by "are most likely to", which I would agree with. Regards, Keith Armstrong In a message dated 06/01/02 06:56:07 GMT Standard Time, ken.ja...@emccompliance.com writes: Subj:Re: EMC-related safety issues List-Post: emc-pstc@listserv.ieee.org Date:06/01/02 06:56:07 GMT Standard Time From: ken.ja...@emccompliance.com (Ken Javor) To: t...@tncokenias.org (Tom Cokenias), cherryclo...@aol.com, emc-p...@majordomo.ieee.org The analytical portion of this post is, as the author stated, worst case. A cable attached to a susceptible circuit picks up a common-mode potential, which most likely drives a current on a shield if the the circuit is sensitive. Then only the current multiplied by shield transfer impedance actually gets into the victim, assuming no CMR. Which just makes my original point - unintentional emissions from ITE can only upset a radio receiver tuned to the emission frequency. That is why, as another contributor posted, we use EMI receivers and spectrum analyzers with preamps to make OATS measurements. on 1/4/02 12:51 PM, Tom Cokenias at t...@tncokenias.org wrote: > At 8:34 AM -0500 1/4/2002, Keith Armstrong wrote: > >> Does anyone else think that ordinary semiconductors doesn't respond to RF? > > > > I agree that commonly used semiconductors have responses well into > the 100's of MHz. > > How much of a problem this is will depend on the nature and function > of the circuitry using these components. > > The EUT wires, cables, pcb traces etc. act like antennae, on which > the incident field voltages and currents. An antenna factor can be > thought of as ratio of the field strength to the voltage induced on > the terminated cable connected to the antenna. > > In an impedance matched system, > > > AF=9.734/lamda*(G)^0.5, lamda being wavelength in meters, G being > antenna gain over isotropic, > > or in dB > > AF dB = - G dBi -29.7 dB + 20logFMHz > > Assuming G is 1 (isotropic antenna), AF is 1 (= 0 dB) at about 30.8 > MHz, and AF get larger as frequency increases, to a factor of 32.7 > (= 30.3 dB) at 1 GHz . Since AF is field strength divided by > induced voltage, the voltage induced on the trace goes down as > frequency goes up for the same incident field strength. > > An effective receive antenna needs to be on the order of 1/2 > wavelength or so; for 30 MHz this is 15m, for 1000 MHz this is 15 cm. > > So if a victim EUT circuit has a pretty effective receive antenna, > and does not have any filtering and is equally sensitive across the > frequency range under consideration (all taken together, a worst case > scenario for susceptibility), > > (1) A 10 V/m field will theoretically induce a voltage 0.33V to > 10V, depending on frequency > > (2) A 5000 uV/m field (10x the FCC class B limit above 960 MHz) will > theoretically induce a voltage from 152 uV to 5 mV, depending on > frequency. > > (3) A 500 uV/m field will theoretically induce a voltage from 15 uV > to 500 uV depending on frequency. > > These are first order approximations, but they are useful in > determining the level of the potential EMI threat. For instance a > 4-30 mA sensor circuit using high gain operational amps will most > likely be interfered with in scenario (1), there may be some > susceptibility detected in scenario (2), and most likely no problem > encountered with scenario (3). > > A sensitive all - band AM communications receiver will have problems > with all three, a broadcast TV operating in a strong signal area will > probably be OK with scenario 3 but not with 1 or 2. > > I guess what I'm really trying to say with all this is that EMC is a > systems thing, taking into account the nature of the culprit EMI > generator, the nature of the victim EMI receiver, and the path > between them. Then we have the economics of operating different > devices in the same vicinity, the politics of who gets how much of > what kind of protection, etc., etc. All things considered, we should > have jobs for life! > > best regards and a Happy New Year to all. > > Tom Cokenias > > T.N. Cokenias Consulting > P.O. Box 1086 > El Granada CA 94018 > > tel 650 726 1263 > cell 650 302 0887 > fax 650 726 1252
