Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TWISTED PAIRS COMMUNICATIONS CABLE
BACKGROUND OF THE INVENTION
This invention relates to a cable made of
twisted wire pairs. More particularly, this invention
relates to a twisted pair communications cable designed
for use in high speed data communications applications.
A twisted pair cable includes at least one
pair of insulated conductors twisted about each other
to form a two conductor group. When more than one
twisted pair group is bunched or cabled together, it is
referred to as a multi-pair cable. In certain
communications applications using a multi-pair cable,
such as in high speed data transmission for example,
problems are encountered if the signal transmitted in
one twisted pair arrives at its destination at a
different time than the signal in another twisted pair
in the cable. This phenomenon, known as "phase delay
skew", results from differences in the propagation of
the signal along different pairs in the cable and may
cause transmission errors.
SUMMARY OF THE INVENTION
The present invention recognizes that there
are various factors which may contribute to differences
in signal propagation along different twisted pairs in
a communications cable. These factors may include, for
example, the degree of twist or "lay length" of the
pair, the cable geometry, the chemical composition of
the insulating material of each pair, the thickness of
the insulation, thickness of each, layer of insulation
in multi-layer insulation systems, or the density of
the insulating material. In the cable manufacturing
process, one or more of these factors may produce
differences in physical properties among the various
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twisted pairs of the cable. For example, each twisted
pair may have a different twist or lay length from the
other pairs of the cable, or different pairs may use
different insulating materials, or different
thicknesses or densities of insulation.
In accordance with the present invention,
these differences in physical properties among the
respective pairs are recognized, and those factors
affecting signal propagation are taken into account and
correlated so that the signal propagation
characteristics of the respective pairs of the cable is
matched.
The signal propagation characteristics of a
twisted pair can be determined from standard test
methods using readily available instrumentation, such
as a network analyzer. Phase delay is a measure of the
amount of time a simple sinusoidal signal is delayed
when propagating through the length of a pair. Skew is
defined as the difference in the phase delay value of
two different twisted pairs in a cable. In a cable
containing more than two twisted pairs, the skew value
is represented by the maximum difference in phase delay
between any two pairs of the cable.
According to the present invention, the phase
delay characteristic of a twisted pair in the cable is
matched with the phase delay characteristic of at least
one other twisted pair of the cable to substantially
eliminate skew. The term "matched" as used herein, is
intended to encompass differences in phase delay of
from 0 to 10 nanoseconds per 100 meters of length.
This matching of the phase delay characteristic is
achieved by correlating a physical property of the
insulating material of one or more of the twisted pairs
with those factors in the other twisted pairs which
produce differences in phase delay. Thus, for example,
the chemical composition, thickness or density of the
insulating material of one twisted pair may be
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controllably altered relative to another twisted pair
so as to bring about a matching of the phase delay.
According to one preferred aspect of the present
invention, this may be done by controlled foaming of
the insulating material of one or more of the pairs, as
for example, by controlled addition of a chemical or
gas blowing agent to the insulating material polymer as
it is applied to the electrical conductor.
By way of example, a communications cable in
accordance with the present invention may have a cable
jacket and a first twisted pair of wires disposed
within the jacket. Each wire of the first twisted pair
of wires has a conductor surrounded by an insulating
material. A second twisted pair of wires is also
disposed within the jacket. The second twisted pair of
wires has a twist lay length different from that of the
first twisted pair. Each wire of the second twisted
pair has a conductor surrounded by an insulating
material. The insulating material of the first twisted
pair differs in a physical property from the insulating
material of the second twisted pair, and the physical
property of the insulating material of this second
twisted pair is correlated with the physical property
of the insulating material of the first twisted pair
and with the difference in twist lay length of the
first and second twisted pairs so that the phase delay
of each twisted pair is matched.
Also in accordance with the present invention
there is provided a communications cable having a cable
jacket and a first pair of twisted wires disposed
within the jacket. Each wire of the twisted pair has a
conductor surrounded by a first insulating material.
Second, third and fourth twisted pairs of wires are
also disposed within the jacket. Each wire of the
second, third and fourth twisted pairs has a conductor
surrounded by an insulating material. The second,
third and fourth twisted pairs each have a twist lay
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length different from that of the first twisted pair. The insulating material
of
each of the second, third and fourth twisted pairs differs in a physical
property
from the insulating materials of each of the other twisted pairs in the cable.
The physical property from the insulating material of each pair is correlated
with the physical properties of the insulating material of all the other
twisted
pairs and with the difference in twist lay length of all the other twisted
pairs so
that the phase delay of each of the first, second, third and fourth twisted
pairs
is matched.
Cables in accordance with the present invention may be engineered to
meet the stringent electrical specifications of high speed data
communications, such as Category 5 cables, and also to meet the stringent
fire and smoke specifications required for use in plenums. For plenum
cables, one or more of the conductors may have its insulating material made
of fluorinated polymer selected from the group consisting of fluorinated
ethylene-propylene, ethylene-trifluoroethylene and ethylene-
chlorotrifluoroethylene. Other pairs in the cable may use a polyolefin
insulating material, preferably an unmodified polyethylene.
In accordance with an aspect of the invention, a communications cable
comprising
a cable jacket;
a first pair of twisted wires disposed within said jacket, each wire
thereof having a conductor surrounded by a first insulating material; and
a second twisted pair of wires disposed within said jacket and having a
twist lay length different from that of said first twisted pair, each wire of
said
second twisted pair having a conductor surrounded by an insulating material;
the insulating material of said first twisted pair differing in a physical
property from the insulating material of said second twisted pair, the
physical
property of the insulating material of said second pair being correlated with
said physical property of the insulating material of said first twisted pair
and
with difference in twist lay length of said first and second twisted pairs so
that
the phase delay of said first and second twisted pairs is matched.
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In accordance with another aspect of the invention, a communications
cable comprising
a cable jacket;
a first pair of twisted wires disposed within said jacket, each wire
thereof having a conductor surrounded by a first insulating material; and
second, third and fourth twisted pairs of wires disposed within said
jacket, each wire of said second, third and fourth twisted pairs having a
conductor surrounded by an insulating material, and said second, third and
fourth twisted pairs each having a twist lay length different from that of
said
first twisted pair;
the insulating material of each of said second, third and fourth twisted
pairs differing in a physical property from the insulating materials of each
of
the other twisted pairs in the cable, the physical property of the insulating
material of each said pair being correlated with said physical properties of
the
insulating material of all other said twisted pairs and with difference in
twist
lay length of all other said twisted pairs so that the phase delay of said
first,
second, third and fourth twisted pairs is matched.
In accordance to a further aspect of the invention, a communications
cable comprising
a cable jacket;
a first pair of twisted wires disposed within said jacket, each wire
thereof having a conductor surrounded by a first insulating material formed of
an unmodified polyethylene; and
second, third and fourth twisted pairs of wires disposed within said
jacket, each wire of said second, third and fourth twisted pairs having a
conductor surrounded by an insulating material formed of a fluorinated
polymer, and said second, third and fourth twisted pairs each having a twist
lay length different from that of said first twisted pair;
the insulating material of said first twisted pair differing in a physical
property from the insulating materials of said second, third and fourth
twisted
pairs, the physical property of the insulating material of said first twisted
pair
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being correlated with the physical properties of the insulating material of
said
second, third and fourth twisted pairs and with difference in twist lay length
of
said twisted pairs so that the phase delay of said first twisted pair relative
to
said second, third and fourth twisted pairs is no more than 10 nanoseconds
per 100 meters of length.
In accordance with yet a further aspect of the invention, a
communications cable comprising
a cable jacket;
a first and second pairs of twisted wires disposed within said jacket,
each wire thereof having a conductor surrounded by a first insulating material
formed of an unmodified polyethylene; and
third and fourth twisted pairs of wires disposed with said jacket, each
wire of said third and fourth twisted pairs having a conductor surrounded by
an insulating material formed of a fluorinated polymer, each of said third and
fourth twisted pairs having a twist lay length different from that of said
first and
second twisted pairs;
the insulating material of said first and second twisted pair differing in a
physical property from the insulating materials of said third and fourth
twisted
pairs, with said physical property of the insulating material of said first
and
second twisted pairs being correlated with the physical properties of the
insulating material of said third and fourth twisted pairs and with the
differences in twist lay length of said twisted pairs so that the maximum
phase
delay of said first and second twisted pairs relative to said third and fourth
twisted pairs is no more than 10 nanoseconds per 100 meters of length.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the present invention will become
apparent from the detailed description of the invention which follows, and
from the accompanying drawings, in which:
Figure 1 is a perspective view of a cable according to one embodiment
of this invention, wherein the cable has two twisted conductor pairs;
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Figure 2 is a cross-sectional view of the cable of Figure 1 taken along
line 2-2 of Figure 1;
Figure 3 is a cross-sectional view of a cable similar to that of Figure 2,
L...a ...L...-..:.. ...... ...... 1..... ..
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solid insulating material and the other pair has a
foamed insulating material;
Figure 4 is a cross-sectional view of a cable
similar to that of Figure 2, but wherein one pair has a
solid insulating material and the other pair has a
foamed insulating material with a skin surrounding the
foam layer;
Figure 5 is a perspective view of a cable
according to another embodiment of this invention,.
wherein the cable has four twisted pairs; and
Figure 6 is cross-sectional view of a cable
taken along the line 6-6 of Figure 5, illustrating four
twisted pairs of wires, wherein two pairs have a solid
insulating material, one pair has a foamed insulating
material and one pair has a foam-skin insulating
material.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to Figure 1, there is shown a
communications cable designated generally by 10 having
two pairs of twisted wires disposed within a cable
jacket 17. A first twisted pair of wires 11 is
comprised of conductors 12 each surrounded by a first
insulating material 13. The second twisted pair of
wires 14 comprises conductors 12 surrounded by a second
insulating material 16. The second twisted pair 14 has
a lower degree of twist, i.e. a longer twist lay
length, than that of the first twisted pair 11. The
insulating material of the first twisted pair 11
differs in a physical property from the insulating
material of the second twisted pair 14. The physical
property of the insulating material of the second pair
14 is correlated with the physical property of the
insulating material of the first twisted pair 11 and
with the difference in twist lay length of the first
and second twisted pairs so that the phase delay of the
first and second twisted pairs is matched. The
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physical properties of the insulting material which may
be controllably altered to match the phase delay of the
twisted pairs may include one or more of the following:
chemical composition of the polymeric material forming
the insulating material, chemical additives in the
insulating material composition, thickness of the
insulating material, or density of the insulating
material.
A difference in the twist lay length of the
twisted pairs 11 and 14 will result in differences in
the distance that the signals must travel in the
respective pairs over a given length of cable, and will
thus contribute to a difference in phase delay or
"skew". However, the phase delay can be matched by
appropriately controlling the physical properties of
the insulating materials of the two pairs. For
example, the amount of skew contributed by the
difference in twist lay length can be determined
empirically or by calculation, and can be compensated
for by selecting insulating material for pair 11 with
an appropriately lower dielectric constant than the
dielectric constant of the insulating material used in
pair 14. The dielectric constant of an insulating
material affects the velocity at which the signals
propagate along the insulated conductor. The
dielectric constant (K) of an insulating material may
be determined by standard test methods, such as ASTM
test method D-150.
The conductors 12 may be a wire of any of the
well-known electrically conductive materials used in
wire and cable applications, such as copper, aluminum,
copper-clad steel or plated copper. The wire is
preferably 18 to 26 AWG gauge. The insulating
materials 13, 16 may be an extruded polymer layer,
which may be formed as a solid or a foam. Any of the
polymers conventional used in wire and cable
manufacture can be employed. Where the cable is to be
CA 02206022 1997-OS-23
used in applications requiring good flame resistance
and low smoke generation, it may be desirable to use a
fluorinated polymer as the insulating material for one
or more of the pairs. Members of this group include
fluorinated ethylene-propylene (FEP), ethylene-
trifluoroethylene (ETFE) or ethylene-
chlorotrifluoroethylene (ECTFE). A preferred
fluorinated polymer is fluorinated ethylene-propylene.
Another particularly suitable fluorinated copolymer is
HALAR~ fluorinated copolymer available from Allied
Chemical Corp. Other pairs in the cable may use a
polyolefin insulating material, such as polyethylene or
polypropylene. The polyolefin is preferably
unmodified, i.e., does not include a flame retardant.
Preferably, the polyethylene may be selected from the
group consisting of low density polyethylene, medium
density polyethylene, high density polyethylene,
ethylene-propylene rubber and linear low density
polyethylene. One preferred polyethylene is available
from Union Carbide and is designated 3485. In one
aspect of this invention a polyolefin and a fluorinated
polymer may be mixed to form one of the insulating
materials. Preferably, the polyolefin in the mixture
is polyethylene and is present in an amount from 3.0%
to 50% by weight of the total mixture. The remainder of
the mixture is preferably ethylene-
chlorotrifluoroethylene (ECTFE). Especially preferred
for communications cable is an amount of polyolefin
from about 10% by weight to about 30% by weight of the
total mixture.
The assembly of multi-pairs of twisted wires
and shield, if present, is referred to as a cable core.
A suitable jacket 17 is applied over the cable core.
The jacket may be a polymer of fluoropolymer,
polyvinylchloride, or a polyvinylchloride alloy
suitable for communications cable use.
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Referring now to Figure 3, there is shown
another embodiment of a communications cable made
according to this invention designated generally by 20
having two pairs of twisted wires. A first pair of
twisted wires 21 is comprised of conductors 22 each
surrounded by a first insulating material 23. The
other pair of twisted wires 24 comprises conductors 22
and are surrounded by a foamed insulating material 25.
The two pairs of twisted wires may be enclosed in an
insulating jacket 27 to form the multi-paired cable 20.
The insulating material 23, 25 may be
selected from the polyolefins and fluorinated polymers
discussed above. When the insulating composition is to
be foamed, a blowing agent is added during processing.
The blowing agent may be any suitable physical or
chemical blowing agent which promotes foam formation of
the composition. Physical blowing agents, such as
volatile liquids or gases are typically added during
the extrusion process, while the mixture is in the
molten state. Chemical blowing agents are typically
blended with the insulating material during the
compounding process. Chemical blowing agents include,
but are not limited to, hydrozodicarboxylates, 5-phenyl
tetrazole, diesters of azodiformic acid and carbazides.
Especially preferred is azodicarbonamide. The chemical
blowing agents may be self-nucleating. The blowing
agent is added to the insulating material in an
effective amount sufficient to cause cells to form
within the mixture. There can be from about 0.05% to
about 1.0% by weight of chemical blowing agent present
in the mixture. Preferably, there is from about 0.1%
to about 0.5% by weight. Gas blowing agents which may
be selected include, but are not limited to, nitrogen,
carbon dioxide, chlorodifluoromethane (F22) or any gas
mixture that is soluble in the molten mixture.
By appropriately controlling the foaming
conditions, including the extrusion temperature and the
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type and amount of blowing agent, the density of the
foamed insulating material can be controlled. Since
the dielectric constant of the insulating material is a
function of the composition of the insulation material
and its density, the dielectric constant of the
insulating material is lowered as the degree of foaming
or expansion is increased.
To ensure that a uniform, small diameter cell
structure is present in foam structures, a nucleating
agent may be provided. The nucleating agent also
provides sites for the formation of cells generated by
the blowing agent. Thus, an effective amount of
nucleating agent is an amount sufficient to ensure
proper cell formation. There can be up to 10 parts by
weight of nucleating agent, preferably from about 0.1
to 3 parts of nucleating agent per 100 parts of
mixture. The nucleating agent may be selected from a
group of known nucleators including, but not limited
to, boron nitride, polytetrafluoroethylene, talc,
calcium carbonate, barium carbonate, zinc carbonate,
lead carbonate, and oxides of lead, magnesium, calcium,
barium and zinc.
Other additives may be used to enhance the
flame retardancy, material compatibility and processing
of the mixture. Useful flame retardants include, by
way of example, antimony oxide. Compatibilizers
include KRATON~ rubber and ethylene-propylene rubber,
for example. The insulating composition may also
optionally contain suitable additives, such as
pigments, antioxidants, thermal stabilizers, acid
acceptors and processing aids. When, as is preferred,
the composition is electrically insulating, any
conductive fillers which are present should be used in
small amounts which do not render the composition
conductive.
Turning now to Figure 4, there is shown
another embodiment of the present invention, a cable 30
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having a two twisted pairs 31, 33 generally similar to
those shown in Figure 3. Twisted pair 31 has a
conductor 12 and an insulating layer 32 of an unfoamed
FEP polymer. The second pair of twisted wires 33 has
conductors 12 and a foamed polyethylene insulating
material 34 surrounding the conductors. The insulating
material is surrounded by an outer skin layer 35 of
unfoamed polymer in contact with insulating material
34. The skin layer is a layer of an abrasion resistant
and/or flame-resistant polymer, preferably, unfoamed
polyethylene at a thickness of 0.002". One preferred
polyethylene for the skin layer is a mineral filled
polyethylene designated 0241 from Alpha-Gary. Another
preferred insulated wire is a combination of unmodified
polyethylene foam with an unmodified high density
polyethylene skin. One preferred polyethylene for the
skin layer is Union Carbide 3364. As another example
the insulating layer may be a foamed fluorinated
ethylene-propylene (FEP) and a skin layer of unfoamed
fluorinated ethylene-propylene (FEP).
Another embodiment of the communications
cable is shown in Figures 5-6 wherein there is provided
a communications cable 40 having four pairs of twisted
wires 41, 42, 43, and 44. Each wire of the pairs of
twisted wires is comprised of conductors 12 each
surrounded by an insulating material. The insulating
materials may be selected from those described above.
The insulating material 45, 46, 47 and 48 may be and
preferably is different from at least one of the other
pairs. For example, the insulating material 45 for
twisted pair 41 may be a solid polymer, while the
insulating material 46 for twisted pair 42 may be a
different polymer and the insulating materials 47, 48
for pairs 43, 44 may be a foamed polymer. The foamed
pairs may be the same insulating material foamed to a
different degree. One or more of the pairs may have an
insulating material of a foam-skin construction as
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previously described. The pairs of twisted wires may
be enclosed in an insulating jacket 49 to form the
multi-paired cable 40.
The wires forming the foamed twisted pairs
are made by covering the individual conductors with a
layer of insulating material. The insulating material
is prepared by compounding the insulating material
polymer with an effective amount of a chemical blowing
agent until they are blended. The blended insulating
material is heated in an extruder to a predetermined
temperature above the melt point of the insulating
material under sufficient pressure to cause foam when
the mixture is released. The predetermined temperature
of the material will have a dielectric constant
correlated with that of a first insulating material.
The melt processible insulating material may be applied
to a metal conductor wire via an extrusion process.
When the material leaves the extrusion chamber the
pressure is released forming a foamed insulating
conductive element.
The insulation should be about 5 to about 80
mils in thickness, preferably 8 mils to 50 mils for
either solid or foam insulation. When the insulation
is foamed, the void content of the foam may range from
10% to 60%. A void content of 30% to 45% is
preferable. The foamed insulating material has a lower
dielectric constant than the solid material.
Several examples are set forth below to
illustrate the nature of the invention and the manner
in which it is carried out. However, the invention
should not be considered as being limited to the
details thereof.
Example 1
For this example four pairs of twisted wires
were constructed. Two pairs of twisted wires were
constructed using a solid FEP insulation. The other
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two pairs were constructed of a polyethylene foam made
by extruding a pre-compounded polyethylene containing
0.1% by weight of azodicarbonamide as the blowing agent
and heating at about 350°F. prior to extruding at a
thickness of 5 mil. The extrusion also included a
polyethylene skin having a thickness of 2 mil. The
capacitance and diameter of the polyethylene foam pairs
were controlled to match the phase delay of the FEP
pairs. The foamed pairs had a greater amount of twist
than the solid pairs.
The signal propagation of each pair was
tested and a comparison of the phase delay from the
different pairs of twisted wires was made. The
comparison showed the measured curve fit phase delay
skew was 5.1 nanoseconds per 100 meters between the
solid pairs and the foamed pairs. This was the worst
case from the four pairs.
In contrast, an all FEP insulated cable
having the same amount of twist as the foamed pairs
measured 11.3 nanoseconds per 100 meters. An all
polyethylene cable of the same construction measured
8.9 nanoseconds per 100 meters. The matched dielectric
sample had better phase delay skew performance than the
cable manufactured from common, solid dielectric
materials.
The phase delay measurements were made using
the following apparatus:
Hewlett Packard 8753D Network Analyzer
S-parameter test set
(2) F to BNC connectors
(2) North Hills 0322BF Baluns
HP-IB interface board with interface cable
A sample of a 4 pair category 5 cable was prepared by
cutting a 100 meter length, removing 1.5 inches of the
cable jacket at each end, and removing 0.25 inches of
insulation on all conductors for attachment to the
Network Analyzer. The Network Analyzer was setup in
accordance with ASTM D-4566 39.3 as follows:
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Set unit for forward transmission
Format - Delay
Number of measurement points - 401
Sweeptime - is
IF bandwidth - 1000 kHz
Linear Scale - 0.3 to 100 MHz
Reference - 500 ns
The baluns were connected to the Network Analyzer via F
to BNC connectors and a BNC connectorized patch cable.
Each pair was tested separately. Data gathered for
each pair includes minimum, maximum, average and
standard deviation per ASTM D-4566 40.2.
Example 2
For this example, four pairs of twisted wires
were constructed. All four pairs of twisted wires were
insulated with a foamed polyethylene, similar to that
used in Example 1, extruded at a thickness of 5~ mils.
The extrusion also included a solid ECTFE skin having a
thickness of 2 mils. The capacitance and outer
diameter of the insulated wires were controlled to
compensate for the different twist lengths for each
pair. The wires used in the shortest pair twist has a
lower capacitance than those in the longer twist
length. All wires had the same diameter over the
insulation, but different capacitances.
The signal propagation for each pair was
tested per the method illustrated in Example 1. The
test showed a curve fit phase delay skew of 3.7
nanoseconds per 100 meters.
While the invention has been described and
illustrated herein by references to various specific
materials, procedures and examples, it is understood
that the invention is not restricted to the particular
materials, combinations of materials, and procedures
selected for that purpose. Numerous variations. of such
details can be employed, as will be appreciated by
those skilled in the art.
