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 MIL-HDBK-232A
5.5.1.8 Active filters. Active filters use operational amplifiers with
externally applied resistances and capacitances to achieve filtering without
the physical bulk and ferromagnetic effects of LC passive filters. Active
filters exhibit high input impedance, low output impedance, and high gain
over the useful bandwidth. The designer may elect to use active filters when
space prohibits the use of bulky passive filters.
5.5.2 Isolators. Isolators differ from filters in that isolators appear as
open circuits beyond the cutoff frequency. Isolators may he as simple as a
relay or as complex as fiber optic systems.
5.5.2.1 Relay isolation. Low-speed signaling may use simple relays to
provide isolation. The signal from the terminal device provides the voltage
that alternately opens and closes the relay. The output contacts may
alternate between open and ground, may provide a path for a voltage to
ground, or may switch between two polarities. While high isolation can be
achieved, the mechanical nature of relays limits speed. The possible arcing
of contacts may produce undesired signals or may distort the intended signal.
Relays may prove effective in he facility design for control signal lines
originating in the RED areas that must be distributed to the BLACK areas.
Specially constructed electronic relays are commercially available that
operate at speeds compatible with 2400 bps transmission, and with 100 megohm
isolation between input and output.
5.5.2.2 Optical isolation. Optical isolators use a light source and a light
detector to transmit a signal across a space. The driver circuits typically
are designed to ignore voltage levels above or below a voltage of interest.
This ability to ignore unwanted voltages provides the blocking of signals
coupled to the desired signal. Optical isolators may be constructed with the
source and the detector aligned and separated by a fixed space, or may use an
FOC to connect the two devices. The, FOC scheme is often found at the point
of egress of the LEA. Isolators are commercially available to handle data
rates to the megabit range. Isolators that use FOC are preferred for
shielded facilities with the FOC passing through the shield through
waveguides-beyond-cutoff. Where a shield does not exist, isolators using
fixed space separation of the source and detector may be used.
5.6 Grounding, bonding, and shielding (GBS). It is essential that
appropriate GBS practices be followed to provide adequate TEMPEST protection.
5.6.1 Grounding. Grounding is the measure taken to provide the electrical
connection to earth through an EESS. The facility ground system consists of
the EESS, the signal reference subsystem, the FPSS, and the lightning
protection subsystem designed using MIL-STD-188-124 and MIL-HDBK-419. Figure
16 depicts a facility grounded for TEMPEST and EMP.
5.6.1.1 Earth electrode subsystem (EESS). The EESS consists of a bare No.
1/0 AWG 7-strand copper wire buried a minimum of 1.5 feet (0.45 m) below the
earth surface and not less than 2 feet (0.6 m) nor more than 6 feet (1.8 m)
from the building drip line. Copper-clad steel ground rods measuring 0.75
inch (19 mm) by 10 feet (3 m) are to be installed not more than 20 feet (6 m)
apart. The rods are bonded to the copper wire by welding or brazing. The
EESS is a closed loop which surrounds the facility. The design objective for
the ground resistance of the EESS should not exceed 10 ohms. For additional
design considerations, see MIL-STD-188-124 and MIL-HDBK-419. Local
electrical codes in overseas areas should also be considered.
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