1188 Chapter 32
Similarly, as shown in Fig. 32-10, shielding can
prevent capacitive coupling to or from signal conduc-
tors in a cable. Solid shields, such as conduit or over-
lapped foil, are said to have 100% coverage. Braided
shields, because of the tiny holes, offer from 70% to
98% coverage. At very high frequencies, where the hole
size becomes significant compared with interference
wavelength, cables with combination foil/braid or
multiple braided shields are sometimes used.
32.3.5 Inductive Coupling and Shielding
When any conductor cuts magnetic lines of force, in
accordance with the law of induction, a voltage is
induced in it. If an alternating current flows in the
conductor, as shown at the left in Fig. 32-11, the
magnetic field also alternates, varying in intensity and
polarity. We can visualize the magnetic field, repre-
sented by the concentric circles, as expanding and
collapsing periodically. Because the conductor at the
right cuts the magnetic lines of force as they move
across it, an ac voltage is induced over its length. This is
the essential principle of a transformer. Therefore,
current flowing in a wire in one circuit can induce a
noise voltage in another wire in a different circuit.
Because the magnetic field is developed only when
current flows in the source circuit, noise coupling from
an ac power circuit, for example, will exist only when
load current actually flows.
If two identical conductors are exposed to identical
ac magnetic fields, they will have identical voltages
induced in them. If they are series connected as shown
in Fig. 32-12, their identical induced voltages tend to
cancel. In theory, there would be zero output if the two
conductors could occupy the same space.
Magnetic fields become weaker rapidly as distance
from the source increases, usually as the square of the
distance. Therefore, cancellation depends critically on
the two conductors being at precisely the same distance
from the magnetic field source. Twisting essentially
places each conductor at the same average distance
from the source. So-called star quad cable uses four
conductors with those opposing each other connected in
parallel at each cable end. The effective magnetic center
for each of these pairs is their center line and the two
sets of pairs now have coincident center lines reducing
the loop area to zero. Star quad cable has approximately
100 times (40 dB) better immunity to power-frequency
magnetic fields than standard twisted pair. The shield of
a coaxial cable also has an average location coincident
with the center conductor. These construction tech-
niques are widely used to reduce susceptibility of
balanced signal cables to magnetic fields. In general, a
smaller physical area inside the loop results in less
magnetic radiation as well as less magnetic induction.Another way to reduce magnetic induction effects is
shown in Fig. 32-13. If two conductors are oriented at a
90° (right) angle, the second doesn’t cut the magnetic
lines produced by the first and will have zero induced
voltage. Therefore, cables crossing at right angles have
minimum coupling and those running parallel have
maximum coupling. The same principles also apply to
circuit board traces and internal wiring of electronic
equipment.
Magnetic circuits are similar to electric circuits.
Magnetic lines of force always follow a closed path or
circuit, from one magnetic pole to the opposite pole,
always following the path of least resistance or highestFigure 32-10. Capacitive noise coupling.
Device A Device BDevice A Device BZoZo
ZiZiNoise sourceNoise sourceCcCcFigure 32-11. Inductive coupling between wires.Figure 32-12. Coupling cancellation in loop.