Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A SEMI-SHIELDED CABLE
Background Of The Invention
1. Field of the Invention
The present invention relates to twisted pair wires. More particularly, it
relates to
twisted pair wires including at least one shield wire intertwined with the
twisted wire
pair.
2. Related Art
High speed data communications cables in current usage include pairs of wire
twisted together forming a balanced transmission line. Such pairs of wire are
referred to
as twisted pairs.
One common type of conventional cable for high-speed data communications
includes multiple twisted pairs. In each pair, the wires are twisted together
in a helical
fashion forming a balanced transmission line. When twisted pairs are placed in
close
proximity, such as in a cable, electrical energy may be transferred from one
pair of the
cable to another. Such energy transfer between pairs is undesirable and is
referred to as
crosstalk. Crosstalk causes interference to the information being transmitted
through the
twisted pair and can reduce the data transmission rate and can cause an
increase in the bit
error rate. The Telecommunications Industry Association (TIA) and Electronics
Industry
Association (EIA) have defined standards for crosstalk in a data
communications cable
including: TIABIA-568-A, published October 24, 1995; TIA/EIA 568-A-1 published
September 25, 1997; and TIA/EIA 568-A-2, published August 14, 1998. The
International Electrotechnical Commission (IEC) has also defined standards for
data
communications cable crosstalk, including ISO/IEC 11801 that is the
international
equivalent to TIA/EIA 568-A. One high performance standard for data
communications
cable is ISO/IEC 11801, Category 5.
Crosstalk is primarily capacitively coupled or inductively coupled energy
passing
between adjacent twisted pairs within a cable. Among the factors that
determine the
amount of energy coupled between the wires in adjacent twisted pairs, the
center-to-
center distance between the wires in the adjacent twisted pairs is very
important. The
center-to-center distance is defined herein to be the distance between the
center of one
wire of a twisted pair to the center of another wire in an adjacent twisted
pair. The
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magnitude of both capacitively coupled and inductively coupled crosstalk is
inversely
proportional to the center-to-center distance between wires. Increasing the
distance
between twisted pairs will thus reduce the level of crosstalk interference.
Another
important factor relating to the level of crosstalk is the distance over which
the wires run
parallel to each other. Twisted pairs that have longer parallel runs will have
higher
levels of crosstalk occurring between them.
A twisted pair can considered to be a chain of loops longitudinally staggered
and
alternating. If a twisted pair within a cable includes the same pattern for
its chain of
loops as does an adjacent pair, the two twisted pairs will become closely
intertwined with
each other and follow the same general path. The chain of loops in a twisted
pair can be
described using two parameters: the twist lay of the cable and the twist
direction. In
twisted pairs, the twist lay is the longitudinal distance between twists of
the wire.
Likewise, the direction of the twist is known as the twist direction. If
adjacent twisted
pairs have the same twist lay and the opposite twist direction, the twisted
pairs will tend
to lie more closely together within a cable than if they have different twist
lays and the
same twist directions. Thus, compared to twisted pairs having different twist
lays and
the same twist directions, adjacent twisted pairs having the same twist lay
and an
opposite twist direction have a reduced center-to-center distance. In
addition, because
the two intertwined twisted pairs follow the same path, the intertwined
twisted pairs can
also have a longer parallel run. Therefore, the level of crosstalk tends to be
higher
between the twisted pairs having the same twist lay and the same twist
direction when
compared to other twisted pairs having different twist lays the same twist
directions. To
avoid the increased level of cross talk associated with other twisted pairs
within a cable,
the twisted pairs within a cable are sometimes given unique twist lays when
compared to
other adjacent twisted pairs within the cable. Thus, the unique twist lay
serves to
decrease the level of crosstalk between the adjacent twisted pairs within the
cable.
Even if each adjacent twisted pairs in cable has a unique twist lay other
problems
may occur. In particular, during use, mechanical stress rnay interlink
adjacent twisted
pairs. Interlinking occurs when two adjacent twisted pairs are pressed
together filling
any interstitial spaces between the wires comprising the twisted pairs.
Interlinking will
cause a decrease in the center-to-center distance between the wires in
adjacent twisted
pairs and can cause a periodic coupling of two or more twisted pairs. This can
lead to an
increase in crosstalk among the wires in adjacent twisted pairs within the
cable.
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Shielded twisted pair cable, although exhibiting better crosstalk isolation,
is more
difficult and time consuming to install and terminate. Shielded conductors are
generally
terminated using special tools, devices and techniques adapted for the job.
Although not
as effective as shielded twisted pair cable, one popular cable type meeting
the above
specifications is unshielded twisted pair (UTP) cable. Because it does not
include shield
conductors, UTP cable is preferred by installers and plant managers as it is
easily
installed and terminated. The requirements for modern state of the art
transmission
systems require UTP cables to meet very stringent requirements. UTP cables are
produced today with a very high degree of balance and impedance regularity. In
order to
achieve this balance and regularity, the manufacturing process of UTP cables
includes
twisters that have internal take-ups and give-ups that allow a back torsion to
be created
on each wire prior to entering the twister. Therefore, UTP cable has very high
impedance regularities due to the randomization of eventual eccentricities in
a twisted
wire pair during manufacturing.
What is needed therefore is a cable exceeding the capabilities of UTP cables
and
able to prevent the interlinking of two or more twisted pairs without the
difficulties and
expenses associated with shielded twisted pair cable.
Summary Of The Invention
The present invention is embodied in a semi-shielded twisted wire pair for
reducing crosstalk within a UTP cable that overcomes the above and other
drawbacks of
conventional UTP cables.
According to one aspect, the semi-shielded cable includes a pair of twisted
insulated conductors having at least one shield wire intertwined with the pair
of insulated
conductors at substantially the same helicoidal pitch as the conductors.
In one embodiment, the pair of insulated conductors forming the twisted pair
have a plurality of interstitial spaces defined between them. In this
embodiment, the
shield wire is intertwined with the twisted pair within the plurality of
interstitial spaces.
In another embodiment, first and second shield wires are intertwined with the
twisted pair within the plurality of interstitial spaces.
In another embodiment, the two shield wires are uninsulated conductors and are
in periodic physical contact and are electrically connected together.
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In another embodiment, the semi-shielded cable includes a pair of insulated
conductors twisted together having a first helicoidal pitch, and two shield
wires being
intertwined with the pair of insulated conductors at substantially the same
helicoidal
pitch. In another embodiment, the two shielded wires are intertwined with the
pair of
insulated conductors being placed substantially diametrically opposed to one
another.
A method is also provided for manufacturing a semi-shielded cable. In one
aspect , the method provides for supplying a pair of individually insulated
conductors
and a shield wire, applying the pair of individually insulated conductors and
the first
shield wire to a die for orienting the pair of individually insulated
conductors in a
predetermined orientation, and twisting the pair of individually insulated
conductors and
the shield wire together at a predetermined twist lay and in a predetermined
twist
direction. In one embodiment during the twisting operation, back torsion is
applied to
the pair of individually insulated conductors. In another embodiment, a second
shield
wire is used, and can be disposed diametrically opposed to the first shield
wire.
In another aspect the method of manufacturing a semi-shielded wire can include
supplying a pair of insulated parallel conductors, wherein the insulation
surrounding
each of the conductors is joined at the centerline between the conductors.
Twisting the
joined, parallel insulated conductors in a helical manner, and providing a
shield wire and
laying up the shield wire into the interstices between the twisted joined
insulated
conductors. In one embodiment a second shield wire may be used and in another
embodiment, the twisting the joined parallel insulated conductors includes
twisting them
together with a predetermined distance between twists.
Brief Description Of The Drawings
In the drawings in which like reference numerals designate like elements,
Fig. 1 is a perspective view of a semi-shielded cable according to one
embodiment of the invention;
Fig. 2 is a transverse cross-sectional view of the semi-shielded cable
according to
the embodiment of Fig. 1; and
Fig. 3 is a flow chart of one embodiment of a process for manufacturing a semi-
shielded cable.
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Detailed Description
Generally, the illustrative embodiment of the present invention includes a
cable
having twisted pairs of individually insulated conductors, hereinafter
referred to as
"twisted pairs," with reduced levels of crosstalk when compared to
conventional twisted
pairs. The reduction of crosstalk is achieved by the use of at least one
shield wire
intertwined with each twisted pair. The at least one shield wire has shielding
properties
that reduce the level of inductive coupling between adjacent pairs. In
addition, the
intertwining of the at least one shield wire with the twisted pair increases
the distance
between the center of adjacent wires in adjacent twisted pairs, reduces the
electric field
coupling.
Fig. 1 is a perspective view of one embodiment of a semi-shielded cable 100. A
pair of insulated conductors 102 and 104 are arranged such that they are
helically twisted
about one another. The two insulated conductors 102 and 104 thus form a
twisted pair
having a twist lay as described above. As shown in Fig. 2, insulated
conductors 102 and
104 include an inner conductor 112 and 116 respectively surrounded by an
insulating
cable jacket 110 and 114 respectively. Referring back to Fig. l, a pair of
shield wires
106 and 108, are intertwined with the twisted pair of insulated conductors
102, 104 at
substantially the same twist lay. In one embodiment of the present invention,
the twisted
pair of insulated conductors, 102 and 104, form interstitial spaces 118
between them.
The shield wires, 106 and 108 are then intertwined within these interstitial
spaces 118.
In one embodiment, the shield wires 106 and 108 are uninsulated conductors
that are in
physical and electrical contact with each other. The shielding effectiveness
depends
upon how closely the helical twist lay of the shield wires 106, 108 is matched
to the
helical twist lay of the twisted pairs of insulated conductors 102, 104. As
will be
explained in more detail below, the closer the helical twist lay is matched,
the greater the
shielding effect will be.
Crosstalk interference within a cable having multiple twisted pairs can be
primarily from two sources. The first source of crosstalk interference can be
from
adjacent twisted pairs within the cable. This is referred to as internal
crosstalk or inner
cable crosstalk. The second source of crosstalk interference is from other
adjacent cables
or other electrical noise sources external to the cable. This is referred to
as external
crosstalk or alien crosstalk.
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In one embodiment, both inner cable and alien crosstalk can be reduced due to
two different mechanisms. First, the shield wires themselves act as a magnetic
shield,
reducing the level of inductively coupled crosstalk. This reduction in
inductively
coupled crosstalk will reduce both inner-cable crosstalk and alien crosstalk.
In addition,
although the transverse view shown in Fig. 2 resembles a quad-cable, the
shield wires are
preferably common mode terminated during use. Preferably, the wire pair 102,
104 is
driven in differential mode, and the shield wires should be connected to the
center tap of
the balanced transformer in the differential driver. If the twisted pair is
driven by a pair
of amplifiers forming a differential amplifier then shield wires can be
electrically
connected at the neutral point between the two amplifiers. This will ensure a
highly
balanced transmission mode and achieve a high shielding effect as well. A
highly
balanced transmission line is desirable in order to reduce common mode
signals. A
highly balanced transmission line driven differentially will provide two sets
of signals to
the differential output amplifier at the end of the transmission line. The
first is the
differential data signal. The second is the common mode signals, that is,
signals having
substantially equivalent interference levels at each wire. These signals are
attenuated by
the common mode rejection ratio of the differential amplifier. The more highly
balanced
the twisted pair, the more equivalent the interference levels on each wire
are. Therefore,
the more highly balanced the twisted pair is, the greater the attenuation of
interference
will be at the output differential amplifier.
The shield wires 106, 108 also will prevent adjacent pairs within the cable
from
interlinking thus increasing the center-to-center distance between the twisted
pairs. As
explained above, interlinking occurs in conventional twisted pair cables when
adjacent
twisted pairs become intertwined within the interstitial spaces between the
insulated
conductors. In one embodiment, the present invention prevents interlinking
between
twisted pairs from occurring because the shield wires 106, 108 are intertwined
within the
interstitial spaces between the insulated conductors 102 and 104. This will
fill in the
interstitial spaces between the insulated conductors 102 and 104, and has the
effect of
increasing the center-to-center distance between the pairs within the cable
because the
cables are prevented from interlinking. This can reduce the level of crosstalk
between
adjacent twisted pairs when compared to a standard cable. In general, the
shield wires of
an individual twisted pair are not in physical or electrical contact. However,
in a cable
containing multiple twisted pairs, each having two shield wires, the shield
wires of each
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twisted pair will be in physical contact at a plurality of locations with
other shield wires,
and therefore are in electrical contact with each other as well. Thus, only
one shield wire
need be connected to an external reference mode.
Fig. 3 is a method for manufacturing a semi-shielded cable according to one
aspect of the present invention. In step 302, a pair of individually insulated
conductors
and are provided and in step 304 are applied to a die to be properly oriented.
In one
embodiment, a back torsion is applied to the pair of properly oriented
insulated
conductors, step 306, in order to provide for a high degree of impedance
regularity
between the pair of insulated conductors. A pair of shield wires is provided,
step 308,
and the pair of insulated conductors and shield wires are twisted together
forming a
semi-shielded cable, step 310. The shield wires themselves preferably do not
need any
kind of back torsioning and therefore may be paid off the regular give-ups.
The tensions
applied to the shielding wires 106, 108 preferably are smaller than the
insulated
conductors 102, 104. Because the shield wires have a substantially lower
tension than
the insulated conductors, the shield wires are substantially filling only the
interstices
between the insulated conductors 102, 104 of the twisted pairs. This ensures
that the
formation of pair 102, 104 and, in particular the longitudinal contact line
between the
pair 102 and 104 is maintained. Preferably, this longitudinal contact line
between the
pair 102, 104 forms substantially a straight line between the two wires. Due
to the lower
tension applied to the shield wires 106, 108 during the twisting stage, the
shield wires
106, 108 fill in the interstices between the pair 102, 104. Thus, shield wires
106, 108
will have a longitudinal contact line with each of the wires of pair 102, 104
that forms
substantially a helix.
In another embodiment, the pair of insulated conductors could also have a web
of
insulation disposed between them, connecting the two wires together prior to
the twisting
process. This can be produced by surrounding the two parallel conductors with
insulation that is joined at the centerline of the conductors. Advantageously
this
arrangement provides for a consistent center-to-center distance. The wires are
then
generally twisted and provide good impedance stability and attenuation
regularity. The
shield wires may be laid up during the twisting of the insulated conductors
and then
added directly to the interstitial spaces between the insulated conductors
without
applying a back torsion to them. In this way, the shield wires are not likely
to split the
twisted pair apart by separating the web of insulation disposed between the
insulated
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conductors. Advantageously, the produces twisted pairs less subject to tension
variations
of the shield wires and insulated conductors.
Having now described an embodiment of the invention, it should be apparent to
those skilled in the art that the foregoing is merely illustrative and not
limiting, having
been presented by way of example only. Numerous modifications and other
embodiments are within the scope of one of ordinary skill in the art. These
and other
modifications are contemplated as falling within the scope of the invention as
defined by
the appended claims and equivalents thereto.
What is claimed is:
