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, [email protected] writes: > Subj:Re: EMC-related safety issues > Date:06/01/02 06:56:07 GMT Standard Time > From: [email protected] (Ken Javor) > To: [email protected] (Tom Cokenias), [email protected], > [email protected] > > 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 [email protected] 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 >
