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 MIL-HDBK-232A
5.5.1.1 Lowpass filters. Lowpass filters pass signals or currents below a
specified cutoff frequency and attenuate frequencies above the cutoff. In
lowpass filters, the series impedance is inductive, while the shunt impedance
is capacitive. Lowpass filters are most commonly used as power-line filters
and VF filters.
5.5.1.1.1 Power-line filters. Power-line filters are used to remove
transient signals and RFI inducted on the lines from other sources, and to
attenuate frequencies in the power system originating in the on-off cycling
of motors. Power-line filters, if required, should be installed in the
individual equipment rather than filtering the source of the power at the
first service disconnect. This bulk filtering at the first service
disconnect has distinct disadvantages in facilities, because the filter
chosen by the designer must be selected based upon the maximum load on that
service. Since the calculated load may be quite high, and the installed load
may vary across the range of the ampacity of the filter, it is difficult to
construct a filter that adequately attenuates the undesired frequencies.
Further, the larger the ampacity of the filter, the more difficult is its
physical construction. Filtering within the equipment results in reduced
physical size and tailoring of the filter components to a relatively constant
load. If RED equipment is used that is not TEMPEST approved, filtering may
be needed. The preferred method is to install filters at the equipment,
rather than bulk filter all such equipment (see figure 46). In no case
should bulk filtering and individual filtering be used on the same equipment,
since double filtering results in a composite filter of different
characteristics which results in increased power consumption and reduced
depict the effects of double filtering in a multiphase power system. The
line side photos are the critical bus output of a rotating UPS. The load
side photos are of one load center out of fourteen. The wave deformation at
90 degrees and 270 degrees is a synchronizing signal added by the UPS control
devices using power-line filters within each equipment (see figure 48).
5.5.1.1.2 Voice frequency (VF) filters. VF filters, having a range of 300
to 4000 Hz, are used in facilities to filter audio signals for telephone
lines and modems. These filters are used to prevent RFI from upsetting
equipment and to contain RFI generated in a facility so that it is not
carried outside the facility. VF filtering is normally accomplished at the
point of egress from the LEA. The use of lowpass filters is satisfactory for
voice band lines, but may not be satisfactory for quasi-analog modem lines.
Modems using amplitude modulation or phase-shift keying should use bandpass
filters with a center frequency equal to the carrier frequency of the modem.
The rationale is that in the event a potentially compromising signal should
be coupled into a line at VF, the lowpass filter would not attenuate that
signal.
5.5.1.2 Highpass filters. Highpass filters are characterized by a high
impedance below a cutoff frequency, passing all signals above that frequency.
These filters may be used to remove signals induced by power lines from
signal lines. While this filter is not normally used for TEMPEST purposes,
the facility designer may find it necessary to use it for EMI purposes.
5.5.1.3 Bandpass filters. Bandpass filters are characterized by a low
impedance at a specified band of frequencies with a high impedance at
frequencies below and above that band. Such filters are typically used in
equipment that use frequency-division multiplexing, such as voice frequency
telegraph terminals, to breakout individual channels.
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