Taking Toms' calculations a little further.... Typical thermocouple sensors have output voltages in the range 3 - 50 microvolts per degree C. So to create a 60C error in a thermocouple-based temperature control system (see my recent posting about the RF immunity of a blood sample incubator) all we need is an error voltage in the range 180 microvolts to 3 millivolts.
According to Toms' calculations below: "(3) A 500 uV/m field will theoretically induce a voltage from 15 uV to 500 uV depending on frequency." Now, 500 microvolts per meter = 54dBmicrovolts/meter which is only 7dB above the CISPR 22 10 metre Class A limit and 17dB above the Class B limit, between 230 and 1000MHz. This implies a fully CISPR 22 compliant laptop PC is capable of generating significant errors (10s of degrees C) in certain kinds of thermocouple measuring and control systems even when the laptop is 10 metres away. If the laptop was closer to the thermocouple system than 10 metres the field strength of its RF emissions will obviously increase. But when it is in the laptop's near field we would expect the 10 metre measurements to be meaningless because there are components of near field emissions which fall off with the cube of the distance and are not detected by 10 metre tests. For example, many products seem to have very quite strong frequency magnetic fields nearby, at audio frequencies and lower, caused by variations in the loading on their DC power supplies. For this and several other reasons I can't agree with the idea that emissions from what are known under some US laws as "unintentional radiators" cannot possibly cause interference in circuits which are not intentional radio receivers. It seems to me that many types of transducer systems (maybe even the fluxgate magnetometers used in some types of compasses) can be vulnerable. Regards, Keith Armstrong In a message dated 04/01/02 19:03:15 GMT Standard Time, [email protected] writes: > Subj:Re: EMC-related safety issues > Date:04/01/02 19:03:15 GMT Standard Time > From: [email protected] (Tom Cokenias) > Sender: [email protected] > Reply-to: <A HREF="mailto:[email protected]";>[email protected]</A> (Tom > Cokenias) > To: [email protected], [email protected] > > 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 >
