Biomedical Engineering Reference
In-Depth Information
causes the
flow of a current
I
in a rectangular loop enclosing an area
S
. The source imped-
ance is
Z
source
, and the impedance of the load is
Z
load
, resulting in an overall equivalent
impedance of
Z
circuit
fl
Z
source
Z
load
.
In the near
fi
field, the electric- and magnetic-
fi
field, vector magnitudes are given by
V
π
S
3
0
D
E
(V/m)
4
where
Z
circuit
7.9
D
(m)
f
(MHz), or
0.63
Sl
D
f
(
2
MHz)
E
(V/m)
where
Z
circuit
7.9
D
(m)
f
(MHz), and
IS
D
3
H
(A/m)
4
π
In the far
fi
field, the electric- and magnetic-
fi
field, vector magnitudes are given by
0.013
V
0
S
[
f
(MHz)]
2
DZ
circuit
E
(V/m)
Z
0
H
and
10
6
IS
[
f
(MHz)]
2
D
35
H
(A/m)
The second lesson of controlling radiated emission leaps out from these equations-keep the
area enclosed by loops carrying strong time-varying currents to the minimum possible.
Similarly, traces carrying high voltages should be kept as short as possible and be properly
terminated.
Besides directing our attention to the parameters a
ecting radiated emissions, these
equations are very useful when designing for compliance with EMI requirements. As
exempli
ff
eld ballpark estimates of EMI can be obtained
from known circuit parameters for a large number of common circuit topologies.
fi
ed by Figure 4.7, near- and far-
fi
PROBING E- AND H-FIELDS IN THE NEAR FIELD
The main reason why EMI standards establish that testing should be performed in the far
fi
field is that as demonstrated above, a constant impedance in the far
fi
field causes the ratio of
E to H components to remain constant regardless of how the
field was generated. This
means that measurements can be reproduced with reliability and standardized methods of
testing can be de
fi
ned with ease. From the past equations, however, it would seem possible
to establish a quantitative correlation that would allow far-
fi
fi
eld estimates from near-
fi
eld
measurements. Unfortunately, in practice, this is not the case. Near-
eld measurements are
extremely dependent on the exact geometry of the source, the position of the near-
fi
eld
probe, and the interaction between the probe and the source to accomplish the exact meas-
urements necessary for calculating the behavior of the radiation in the far-
fi
eld region.
Although not applicable to predict the outcome of compliance tests, near-
fi
eld meas-
urements can nevertheless be very useful to the designer in locating potential sources of
radiated emissions. Here, near-
fi
fi
eld qualitative measurements with simple instruments
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