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 MIL-HDBK-232A
5.4.7 Fiber optics. The use of FOC has been touted as the ultimate bonded
medium for transmission of classified information because of its inherent
property of neither radiating nor absorbing energy (RFI, EMI). While FOC is
electrically immune, it does require physical protection. If FOC is used as
a plain text medium between controlled access areas (CAAs) and crossing
uncontrolled access areas (UAAs), the FOC must be installed in a PDS (see
5.7.3).
5.5
Filter and isolator requirements and installation.
5.5.1 Filters. Filters are used to pass signals or currents at certain
frequencies to the load, while shunting unwanted frequencies either to ground
or back to the source (see figure 43). Filters may be either passive or
active. Passive filters are most commonly used for RFI/EMI control. Four
types of passive filters may be used: lowpass, highpass, bandpass, or
bandstop (see figure 44). Passive filters use an inductive impedance and a
capacitive impedance to achieve the purpose. One impedance is in series with
the load, while the other impedance is in parallel with the load. The
parallel impedance shunts the undesired frequency. Filters are constructed
in three network configurations: L or half-section, T, and Pi (see figure
45). The L network is the simplest, with the impedances connected as single
components. The T network splits the series impedance in half, with half
before and half after the shunt impedance. The Pi network splits the shunt
impedance in half, with half before and half after the series impedance. The
networks may be stacked to satisfy particular applications. Voice frequency
(VF) and power filters can be purchased off the shelf, while filters for
other applications are custom-made.
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