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 MIL-HDBK-419A
e.
For rf and high speed digital paths, use double-sided board with microstrip transmission lines
properly matched to the terminal impedances.
f.
Effectively ground large unetched portions of boards and utilize the grounded portions as shields.
Consider carefully the positions of transformers and inductors on adjacent boards to assure that
g.
undesired magnetic coupling does not occur between circuits.
h.
Assign circuit functions on boards following the principle of physically separating the most sensitive
networks from the high level or transient producing networks.
Arrange or shield magnetic components to avoid interacting stray fields.
i.
Orient the winding axes of adjacent transformers at 90 with respect to each other to minimize
j.
coupling due to the concentration of leakage flux along the winding axis and, hence, to minimize the required
shielding.
Exercise care in placing shields close to circuits in which the circuit Q is a critical factor because
k.
losses in the shield can lower the circuit Q.
Ground any shields on printed circuit boards directly to the main chassis independently of any
l.
grounds located on the board.
m. The shields on printed circuit boards must never be used as a circuit return conductor since current
flowing in the surface of the shield can result in radiated rf energy.
Use modularized construction wherever possible; in particular, place power line input filters in
n.
shielded modules.
Extreme high and low level stages should be isolated in separate compartments.
o.
Circumferentially bond power line filter cases to the chassis. If the surfaces are aluminum, the
p.
surfaces should be iridited, never anodized or primed. Mounting ears or studs must exhibit firm and positive
contact over the entire area of the mounting surface.
Most common lower frequency interference is the so-called "hum" from power conductors and is
q.
predominately magnetic. The shielding of a circuit from power frequency interference may require the use of a
high permeability magnetic shield completely surrounding the sensitive circuit.
Provide for effective electric and magnetic field shielding of the power supplies, battery chargers,
r.
dc to ac inverters, and dc to dc converters.
High voltage power supplies should be adequately shielded and carefully isolated from highly
s.
sensitive circuits.
3-26
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