Product specifications

COMMUNICATIONS
Sharp Electronics Corporation
Shielding
Shields are added to twisted pairs, or multiconductor cables, to help prevent in
ingress (interference) or egress (radiation) of noise. Shields are an inherent part of
coax cable. There are six basic shield configurations:
• Unshielded
o Twisted pairs, especially in data, are often unshielded. Coax, by definition,
cannot be unshielded.
• Serve/spiral shields
o Serve or spiral shields are simply wound around the inner conductors.
• Braid shields
o Conductors woven or braided around a core. Most effective from 1,000 Hz
to 50 MHz.
• French braid shields
o This is a combination of serve and braid. Here two serves are braided
along one axis.
• Foil shields
o These are the simplest, cheapest, and easiest to apply. They are most
effective above 50 MHz
• Combination shields
o Combined foil and braid shields are effective at low and high frequencies.
Unshielded
Unshielded cable is appropriate where no noise is present, such as no cross talk
from adjacent wires. Or it can be used if you don’t care if there is noise, that noise
cannot substantially affect the signal on the cable.
Unshielded cables are especially prevalent in the data world where pairs have very
tight twists, or may use conductors that are bonded together. Such high-performance
pairs are good to excellent at not picking up or radiating noise.
Braid Shields
Braid shields are formed by spinning wires or groups of wires around a core. This
slow and labor-intensive process makes braiding the most expensive single step of
cable manufacturing. Single braid coverage of up to 95% can be realized. Double
braid coverage can be up to 98% coverage. Since braids always have "holes" where
the wires cross, 100% coverage not possible with braid.
Braid shields are most effective at frequencies from 1,000 Hz to 50 MHz. For these
frequencies, the low resistance of a braid gives good coverage. Below 1,000 Hz
there is no standard braid material, which is effective. The wavelengths are so long,
and the low frequency energy so pronounced, that the only effective shielding is solid
steel conduit. And, at 60 Hz, even steel conduit gives 27 dB of noise reduction.
Foil Shields
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