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 MIL-HDBK-419A
3.4 SHIELDING GUIDELINES. Even during the circuit design phase of the equipment, shielding requirements
should be considered. In this way, electromagnetic compatibility can be achieved during design thus alleviating
much of the post-construction retrofit sometimes required for successful operation. A properly designed
circuit can be compromised in the transition from a breadboard design to a packaged end item unless
appropriate control measures are taken. Thus it is important that components be carefully selected with due
regard to their sensitivity characteristics and shielding requirements. Then careful attention must be directed
to the installation of these components to either take advantage of their inherent shielding properties or to
overcome their shortcomings. In this way, coupling between circuits internal to the equipment and between
internal circuits and external emitters or receptors can be controlled. The documentation and design
requirements for the control of the electromagnetic emission and susceptibility characteristics of military C-E
equipments are provided in MIL-STD-461.
3.4.1 Parts Selection.
In circuits where stray coupling may be detrimental, use shielded inductors.
a.
Wherever possible, use shielded relays and electrically ground the shield.
b.
The input transformer for isolation amplifiers should have electrically shielded primary windings.
c.
d.
Power transformers for susceptible circuits should have an electrostatic shield between the primary
and secondary windings. This shield and the transformer case should be grounded to the equipment chassis.
e.
Use shielded hookup wire for high level leads inside the chassis to prevent interference signals from
coupling to other internal leads which extend through the chassis.
Select connector types which will provide sufficient pins for individual shield terminations.
f.
g.
Select connectors which will be able to withstand environmental conditions without degradation of
the shielding characteristics of the connector.
3.4.2 Layout and Construction.
a.
Do not place low level signal paths adjacent to high level signal paths or unfiltered power supply
conductors.
b.
Avoid the use of long parallel conductor runs.
c.
Where long parallel runs cannot be avoided, e.g., on "Mother boards," arrange conductor functions
so that they successively progress from the low level, most sensitive leads to the highest level leads. The
filtered dc power leads and low rate control functions (potentiometer leads, reference voltages, etc.) may run
down the middle.
d.
Ensure that excessive conductor parallelism does not occur between adjacent boards.
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