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 > > > > > > > > ------------------------------------------- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
