Biomedical Engineering Reference
In-Depth Information
connected to perfectly tuned dipole antennas. However, more severe losses can be
assumed for less perfect situations, such as when the length of the signal line under
consideration is much shorter than one-fourth wavelength of the of
ending spectral
component for which the threat analysis is conducted. Assume for this example that
the line of interest presents an impedance of 100
ff
Ω
at the frequency of interest and
that the coupling between the o
ff
ending source and the signal line under analysis is
10 dB below ideal.
4.
Calculate the coe
cient to be used for the o
ff
ending signal at the victim circuit. In
the example 136(dB)
14(dB)
10(dB) yields 112 dB
µ
V across the signal line's
.
5.
Convert this level back to linear units: 112 dB
load impedance of 100
Ω
µ
V
400 mV. This will be the voltage
induced by the of
ending source at the frequency of interest on the signal line under
analysis. Note that if the load impedance increases, so will the induced voltage. For
example, for a 100-k
ff
load, the induced voltage will be as high as 2 V.
6.
Compare the induced voltage levels against typical circuit threshold values at all sus-
ceptible frequencies. For example, if 26 MHz is within the bandwidth of the pro-
cessing circuit connected to the line under analysis, and since the threshold value for
this example was chosen to be 5 mV, the protection level required for a load imped-
ance of 100
Ω
Ω
would be 20 log(400 mV/5 mV)
38 dB. For a 100-k
Ω
load imped-
ance, the protection need would increase to 52 dB.
With this approximation in hand, it is possible to select shielding, grounding, and
fi
filtering
components that will a
ff
ord a combined protection that surpasses the estimate by a certain
safety margin.
Shields Up!
Shielding and grounding (re
ection and conduction) are the primary methods of guarding
against EMI entry and exit to and from a circuit. Chances are that you will not build your
own enclosure. Rather, you will probably use an o
fl
-the-shelf case or hire an enclosure
manufacturer to supply you with custom-made enclosures. In either case, look at the enclo-
sure's data sheets for EMC speci
ff
cations. The authors' preference is to use enclosures
which have a conductive cage that is contained completely inside a plastic enclosure with-
out any exposed metallic parts.
If a conductive enclosure is chosen, ensure that the conductive surface is as electrically
continuous as possible. For a split enclosure, ensure as good an electrical contact as pos-
sible between the parts. Openings in the case that are required for display windows, cool-
ing slots, and so on, must be kept as small as possible. If the size of the opening is larger
than
fi
ending EMI component, use transparent grilles to close the RF gap.
Finally, ensure that unshielded lines that carry o
20
of a potential of
1
ff
ending signals do not pass directly
through a shielded enclosure. Use shielded cables for high-sensitivity inputs.
EMI grounding requires di
ff
ff
erent, sometimes con
fl
icting considerations from those
used to protect low-frequency low-level signal lines. The
erence is the issue of sin-
gle-point versus distributed grounding. Single-point grounding of circuits is a common
practice in the design of low-noise electronic circuits because it eliminates ground loops.
This assumption is valid only up to a few megahertz. At higher frequencies in the radio
spectrum, line inductances and parasitic capacitances become signi
fi
first di
ff
fi
cant elements, voiding
the e
ectiveness of single-point grounding. For example, for the 300-MHz components of
an ESD event, a 0.25-cm length of wire or PCB track acts as a one-fourth wavelength
antenna, providing maximum voltage at the ungrounded end. As such, any cable that is
longer than
ff
10
to
1
20
of of
1
ff
ending spectral components should be grounded at both ends. If
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