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 MIL-HDBK-419A
that the receiving and detection devices do not respond to the noise signal. As another option, the amplitude of
the interference source or the sensitivity of the susceptible circuit can be decreased to reduce the interaction
between the two circuits. Further, the type of modulation used in one or both circuits can be changed to
minimize the interference.
Another technique is the elimination of the interference source. Although this may seem like a trivial solution,
it is a valid alternative in many situations. For example, the source of interference may be a rusty joint which
can be eliminated by proper bonding.
A third alternative is the use of filters. The majority of interfering signals, even if they are free-space coupled
to the signal and power lines, are conductively coupled into the susceptible circuit. The proper application of
filters to both the signal and power lines can reduce this coupling.
6.5 FACILITY AND EQUIPMENT REQUIREMENTS. `The interference rejection principles identified in this
chapter are responsible, in part, for many of the recommendations contained elsewhere in this volume and in
Volume II. For example, intersite or interbuilding common-mode noise voltages in the earth contribute to the
need for a low resistance of 10 ohms to earth at each facility. Even a resistance to earth of as low as 10 ohms
may not, however, alleviate all common mode noise on a data cable connecting two separate locations or
buildings. While a low resistance may help, there will always be potential differences between any two rods in
the ground. The use of shielded, balanced twisted pair for all lower frequency
equipment interfaces
recommended in Volume II, is intended to provide additional common-mode rejection to those unavoidable noise
voltages which exist in any facility. This is not to say that the sources of noise in a facility cannot be
controlled. In fact, much can be done by equalizing the load between the phases of the ac distribution system;
by insuring that the neutral is grounded only at the service disconnecting means as recommended in Volume II;
and by limiting the quantity of leakage current from power line filter capacitors by using the smallest
acceptable value of capacitance or by sharing common filtered lines with several pieces of equipment.
6.6 REFERENCES.
6-1. J. D. Krause, Electromagnetics, McGraw-Hill Book Company, Inc, New York, (1953).
6-2.
F. E. Terman, Radio Engineers' Handbook, McGraw-Hill Book Company, Inc, New York, (1943).
6-3. L. V. Blake, Antennas, John Wiley and Sons, Inc, New York NY, (1966).
6-4. E. C. Jordan, Electromagnetic Waves and Radiating Systems, Prentice-Hall, Inc, Englewood Cliffs NJ,
(1950).
6-25/6-26
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