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
