Low-level circuits are often interconnected
using shielded cables, mainly for protection against
externally induced noise. Aside from the effectiveness
of the shield designs themselves, even more important
is the integrity of the connection to ground. A poor
ground may be worse than none at all.
The Signal Return Path
Used as a signal return path, typical
in coaxial cables, the shield serves a secondary purpose
of screening the center conductor from external fields.
This dual role can exist in non-coaxial cables as well.
The resistance of a braided shield
is typically much lower than that of other conductors.
This may be desirable in the return circuit, but it
is especially important in presenting a low impedance
against external induction. For this reason, terminations
must be carefully made or the shield will be useless
as a noise barrier. In fact, since the shield is a straight
"wire" and its area much larger than the conductor(s)
inside, a massive imbalance of induced current can occur,
causing noise. An unshielded twisted pair would likely
be more immune to noise.
Simple Shielding
As a pure barrier to EMI, a grounded
shield acts as a conductive channel for the conductor(s)
within. "Anchored" at ground potential, which
prevents it from floating wherever the magnetic or electrical
environment may lead it, the shield serves not so much
as a means of walling off noise as to simply divide
an offending electric field and send part of it into
the ground.
It is important to know that no shield
can protect against magnetic fields as effectively as
mere physical space between the noise source and the
affected conductors. As little as 0.5 inches can reduce
magnetic coupling by more than 30 dB, improving to about
70 dB with a 4-inch separation. This is probably the
best remedy for noise originating in high-current wiring.
Ground Loops
 Ground
loops cause a lot more noise than warranted, considering
how easy it is to eliminate them with thoughtful installation.
Ground loop noise is easy to picture: every conductor,
including the airframe itself, has some resistance,
and any current passing through it produces a voltage
drop between its source and its load. See
Figure 1. The ground-loop problem develops where
more than one circuit share the same return conductor,
often a shield intended to be the ground, so the voltage
drop from one current path simply shows up in another,
adding "someone else's noise" to an otherwise-clean
signal.
Further, where several systems are
"daisy-chained" to ground, any one of them
can act as a weak link and reflect its current fluctuations
as noise through them all. Common grounds are best run
in a star configuration.
Frame Ground
The return path should never be a frame
ground. While it may be metal and able to conduct, it
not intended to be an active conductor. The ideal return
for any circuit should be exclusive to that circuit,
though it is not uncommon to share return paths where
the current of every signal is very low. Nevertheless,
good practice suggests no more than five signal conductors
per ground. Using a frame for a signal return or current-carrying
"lo" is poor economy and an open invitation
to problems. Best to reserve the frame for its structural
role; however, connecting it to battery ground at one
point will achieve the effect of a universal electrostatic
shield for the entire aircraft.
An improved all-around approach to
using a ground is to use it only as a ground, confining
signals and power lines to dedicated conductors. A balanced,
shielded twisted-pair audio line would be an example.
Will the Real Ground Please Identify
Itself?
Is there more than one "real"
ground? Yes.
- For a signal path the "real"
ground is the system ground, the true destination
for the return signal. No other reference point is
as good. Good practice for connecting a protection-only
shield calls for terminating it at system ground.
This keeps it from floating with "alien"
signal sources and becoming a source of noise to the
very circuit it is intended to protect. It might be
likened to an extension of the system housing itself.
- What ground do you use between
systems? Fact is, they may necessarily share the ground,
as in the case of a signal between them via coaxial
cable. If this is so, meticulous ground termination
procedures are required at each end.
 Otherwise,
a poor connection may cause serious noise almost anywhere,
and continuity of the return path may be important.
Ideally, signal returns should be isolated from ground
and any shields terminated at one end only. See
Figure 2. In some cases, the insulated outer
shield of triax and quadrax offer the shielding and
the isolation desired. This permits the establishment
of inner shielding as signal ground and return path,
if necessary.
- The "universal" ground,
the airframe, is the shell housing all other systems.
But it is only the shell and is best in its strictly
passive role to all other systems, such as the avionics
box which deserves its own recognition. This is true
also of every other system on board. Each one, from
engine starters to cabin entertainment systems to
TCAS, will perform more effectively and with less
interference using its own ground path.
The value of the "universal"
shield is questionable in aircraft with composite-material
construction, which leads to other concerns about the
effects of HIRF — High Intensity Radiated Fields.
Twisted Pairs
Not the name of a rock group.
Shielding is at its best in blocking
electrostatic noise fields, and is helpful in shunting
some electromagnetic interference, but not
near as effective in eliminating EMI as twisting pairs
of signal conductors. This has been a popular noise-killer
from the early days of telephones.
Every conductor residing in a changing
magnetic field acts like the secondary of a transformer
— producing some current replicating the waveform
of the “primary,” or source of the field.
(Changes in the field may be due to AC or any varying
current in the field source conductor(s) or even physical
motion of a DC conductor, such as vibration.) In fact,
transformers are designed to take advantage of this
fact.
Allowing that every circuit is
a circuit (a two-way path for the movement of electrons,
or the signal), current will flow in both wires: the
pair. External fields happily induce some current in
these conductors, caring not at all whether this will
pollute the signal in the circuit.
Untwisted pairs invariably
position one conductor closer to the field source, and
while they may receive the near the same induction (thus
having a “balanced” noise in both), it is
never actually quite the same. As a result, there will
always be at least some undesirable differential current
induced.
By twisting the signal pair, the conductors
alternate in nearness to the noise field over each twist
cycle, effectively canceling the effect of the polluting
field. In effect, while the noise is present in both
wires, the twist helps assure it will be equal in each
wire, and the result is near-perfect balancing.
The best approach to minimizing unwanted
signal "exchanges" is to simply add space
between cables — but often this is difficult or
impractical. In such cases, triax, twisting, and/or
sensible grounding can help a lot.
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