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 coaxial measurement system. For the filter performance at the high end of the
frequency spectrum (1 to 10 GHz) the feedthrough capacitor and filter-can
performance as a waveguide below cutoff attenuator is critical.
Power filters for 400 Hz applications typically require
power factor
correction coils to compensate for the large capacitors contained
in the
filters. When not provided, the current demand by the filter may
be
excessive, and the filters are usually acoustically noisy. These
correction
coils are not normally supplied by the filter manufacturer unless
they are
required in the specifications.
2.8.2
EMI Filters for Signal Circuits. Digital type signal circuits
utilize square-wave type pulses as observed in the time domain. The fast rise
and fall of the square-wave signal leading and trailing edges, tends to make
passive, inductor/capacitor (L/C) filters ring at frequencies close to the
cutoff frequency in their pass band where L/C combinations in the filter
result in a gain rather than a loss in attenuation. The best performance for
digital circuit filters occurs when the filter source and load impedances
match the driving source and load impedances, respectively, and when the
filter pass band is tailored to the baud rate of the signal circuit. TEMPEST
recommended filter attenuation curves are shown in Figure 11, with curve A
applicable up to a baud rate of 1,000 baud pulses per second (B.P.S.). Curve
B is used to 3,000 B.P.S., curve C to 10,000 and curve D to 30,000 B.P.S.
When properly designed, the useable baud rate can approach 1/3 of the filter
cutoff frequency (3.01 dB insertion loss). The curves in Figure 11 show
attenuation in dB as a function of frequency and they are measured differently
than the insertion loss curves of Figure 10, which are measured to
MIL-STD-220A specifications. For filter attenuation as shown in Figure 11,
the filter is inserted in a measurement circuit with source impedance matched
to the filter input impedance, and load impedance matched to the filter output
impedance. A measurement signal Si, induced at the filter input is compared
to the signal So remaining at the filter output, with the ratio 20 log Si/So
providing the attenuation in dB. If the circuit source and load impedances
where the filter is installed, are substantially mismatched to the filter, the
ringing of the filter is increased drastically, and the acceptable stop band
curves of Figure 11 will be altered significantly. As a result of these
critical factors in the signal circuit filter design, it is best to let the
government either furnish the signal filters for contractor installation, or
provide well defined impedance parameters for specifying required filter
attenuation performance with frequency. For audio or tone type signal
circuits, the filter curve as shown in Figure 11, must have a cutoff frequency
which exceeds the highest signal frequency so that the signal distortion does
not occur at the higher signal frequencies.
2.8.3
Electrical Filter Design Specification Requirements. General
electrical filter specification MIL-F-15733E, Filter, Radio Interference,
General Specification for, governs critical design features for all required
electrical filters such as range of operating temperatures, impregnant flash
point, terminal size and strength, dielectric withstand voltage, voltage drop,
insulation resistance, filter sealing means, overload, impregnant, finish,
moisture resistance and filter marking. NAVFAC Guide Specification NFGS 16650,
Radio Frequency Filters for 60 Hertz Power Lines contains recommendations for
the fabrication, testing and installation of EMI filters, and should be
utilized in preparing specifications. It includes a heat rise limit of 20
deg. C,
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