Biomedical Engineering Reference
In-Depth Information
Figure 4.7 Simple differential-mode radiating circuit configurations are created when an ac current flows on a current path that forms a
loop enclosing a certain area S . ( a ) Transient power demands of an IC are supplied by a decoupling capacitor, causing brief, strong currents
that circulate on a loop formed by the supply-bus PCB tracks. ( b ) Fast digital signals driving low-impedance inputs form EMI-radiating loops
when current returns through distant ground paths.
can accurately pinpoint sources of EMI and identify their basic characteristics. In
essence, if a strong E-
fi
eld is detected from a certain circuit section but a relatively weak
H-
eld is sensed at the same point, the culprit can usually be traced to a train of high-
voltage pulses on a long wire, an unterminated line, or a trace driving a high-impedance
load. Conversely, if the H-
fi
eld probe detects little activity, the
source of EMI is most probably a looplike circuit through which strong currents circu-
late. Examples of such situations are PCB tracks carrying strong currents, inductors in
switching power supplies, and eddy currents induced in metal enclosures by strong
fi
eld is strong but the E-
fi
fi
elds
inside the case.
Since the same equations used to describe emission of radiation are applicable to the
reception of emissions, it is apparent that a small loop of wire can act as a near-
fi
eld probe
which is mostly sensitive to H-
elds, on the other hand, would then be detected,
preferably by a short exposed wire. Measurements can then be taken with either a wide-
band ac voltmeter or a spectrum analyzer. Even a simple single-turn wire loop at the end
of a coax cable can be a very e
fi
elds. E-
fi
eld probe. With this arrangement, maximum
output from the probe is recorded when the loop is in immediate proximity and aligned
with a current-carrying wire. This directionality is very useful for pinpointing the exact
source of a suspicious signal.
The diameter of the loop makes a large di
ff
ective H-
fi
ff
erence on H-
fi
eld measurements [Kraz,
1995]. The area enclosed by the loop in
fl
uences the sensitivity of the probe, since it deter-
mines the number of magnetic
flux lines that are intercepted to produce a detectable signal.
A larger loop will obviously develop a larger voltage at the input of the voltmeter or spec-
trum analyzer. On the other hand, larger loops have inherently larger self-inductance and
equivalent capacitance than small loops. As inductance increases, the network formed with
the complex impedance of the measurement setup resonates at lower frequencies, beyond
which the probe cannot be used. Moreover, larger loops make it much more di
fl
cult to iden-
tify the exact source of an interfering signal, because their size does not allow them to pick
up radiations selectively from single lines when a multitude of the latter are clustered close
together. Coils with multiple turns can be used to increase the sensitivity without appreciably
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