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 MIL-HDBK-419A
6.4.1.2 Spatial Separation. Inductive or capacitive coupling can be reduced by increasing the physical
distance between signal circuits. As can be seen from Equation 6-6 and Equations 6-11 and 6-16, increasing the
separation between the interfering circuit and the susceptible circuit exponentially decreases the voltage
coupled into the susceptible circuit.
6.4.1.3 Reduction of Circuit Loop Area. Reducing the loop area of either the interference source circuit or
the susceptible circuit will decrease the inductive coupling between the circuits. Equation 6-6 shows that the
inductively coupled voltage can be minimized by reducing the length
or the width (r2 - rl) of the susceptible
circuit. This width can be minimized by running the signal return adjacent to the signal conductor and, hence,
reducing the loop area of the susceptible circuit. A preferable approach is to twist the signal conductor with its
return. The use of twisted wires reduces the inductively coupled voltages since the voltage induced in each
small twist area is approximately equal and opposite to the voltage induced in the adjacent twist area.
6.4.1.4 Shielding. Another effective means for the reduction of coupling is the use of shields around the
circuits and around interconnecting lines. Principles of shielding are presented in Chapter 8.
6.4.1.5 Balanced Lines.
In situations where signal circuits must be grounded at both the source and the load, and hence, establish
conductive coupling paths, the use of balanced signal lines and circuits is an effective means of minimizing the
conductively coupled interference. In a balanced circuit, the two signal conductors are symmetrical with
respect to ground. At equivalent points on the two conductors the desired signal is opposite in polarity and
equal in amplitude relative to ground. A common-mode voltage will be in phase and will exhibit equal
amplitudes on each conductor and will tend to cancel at the load. The amount of cancellation depends upon the
degree to which the two signal lines are balanced relative to ground.
If the signal lines are perfectly balanced, the cancellation would be complete and the coupled interference
voltage at the load will be zero. If the source and load are not normally or cannot be operated in a balanced
mode, balanced-to-unbalanced transformers or other coupling devices should be used at both the source and load
ends of the signal line.
6.4.2 Alternate Methods.
Several alternate methods exist for minimizing interference besides the reduction of coupling. The first
technique consists of actual circuit modification. For example, the signal frequency of either the interfering
source or the susceptible circuit can be changed such that the signals do not interfere with one another.
Similarly, the desired signal can be transposed to another frequency range or to a type of signal not affected by
the noise. An example of the former is the conversion of the desired signal to VHF/UHF or microwave while an
example of the latter is the use of acoustic coupling and electro-optical transmission. Through the use of one
of these techniques, the frequency of transmission over that portion of the path susceptible to pickup is such
6-24
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