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 2.8
EMI Filters for Electrical Conductors. All of the electrical
service conductors which penetrate the electromagnetic shielding surface, such
as power, signal, telephone, control and alarm must be provided with
electrical filters. Typical insertion loss requirements in dB as a function
of frequency to be specified for these filters are shown in Figure 10. These
filters are best located at the conductor penetration locations, either inside
or out, depending usually on available access or space. If they must be
mounted remotely from the shielding surface, then runs of continuous metal
conduit (threaded ferrous with welded joints preferred) must be provided from
the filter to the shielding penetration. For TEMPEST installation
requirements, the conduit runs are best located inside of the shielded
enclosure. If the filter must be mounted outside of the TEMPEST enclosure in
a remote location, then the conduit run from the filter to the shielding must
be within a controlled access area. Any required suspension hangers or
conduit clamps must provide electrical isolation of the conduit from the
building structure ground. The required electrical isolation section on the
conduit connection to the filter must be on the input or source side of the
filter. The preferred method of penetration of the filter connection in the
shielding surface is by means of continuously welded metal piping as seen in
Figures 3 and 4. The use of radio frequency gasket materials to make this
penetration should be specifically forbidden in the specifications because of
the temporary nature of such gasketed connections.
2.8.1
EMI Filters for Electrical Power. Power filter performance
requirements are typically stated in terms of dB insertion loss as a function
of frequency as seen in Figure 10, as measured in accordance with
MIL-STD-220A, Method of Insertion-Loss Measurement. Power filters are
generally of the passive type, consisting of resonant combinations of
inductors and capacitors. They are available with both inductor and capacitor
inputs. Those with capacitor inputs usually include a low inductance type
feedthrough capacitor, followed by capacitor-inductor pi networks. The
contribution of the inductors is critical in the low frequency end of the stop
band (from 14 to a few hundred kilohertz). When inductor cores saturate as a
result of load currents, the insertion loss curve tends to shift to a higher
frequency, increasing the cutoff frequency (the frequency where the insertion
loss has reached 3.01 dB) and the range of pass-band frequencies. At
frequencies above a few hundred kilohertz, the turn-to-turn capacitance of the
inductor winding tends to limit its contribution and the capacitor performance
is more critical. The series inductance provided by the connecting leads of
the capacitors tends to limit their higher frequency performance (a few MHz
and higher), and here the feedthrough capacitors contribution become critical.
Feedthrough capacitors of the size usually employed in power filter
applications typically have a self parallel resonance with winding inductance
ranging from a few hundred kilohertz to a few megahertz. Some manufacturers
overcome this resonance defect by staggering sizes of feedthrough capacitors,
or by adding low inductance capacitors in parallel. When not corrected for,
this resonance results in a sharp dip in the stop-band insertion loss curve at
the resonant frequency to values below the typically required 100 dB. These
resonant defects are not usually identified in MIL-STD-220A test results
because tests are run typically at a few discrete frequencies covering the
required range of performance. The defects are normally identified when tests
are monitored or supervised by knowledgeable government representatives
looking specifically for them. The insertion loss performance of the power
filter above 20 MHz is measured under no-load conditions in a matched 50 ohm
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