Patent Grant
7534964
1. Field of Invention
The present invention relates to high-speed data communications cables. More particularly, it relates to cables including shaped separators and jackets.
2. Discussion of Related Art
High-speed data communications media include pairs of wire twisted together to form 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 that may be bundled and twisted (cabled) together then covered with a jacket to form the cable.
Modern communication cables must meet electrical performance characteristics required for transmission at high frequencies. When twisted pairs are closely placed, as may be the case in a multi-pair cable, electrical energy may be transferred from one twisted pair to another. Such energy transferred between pairs is referred to as crosstalk and is generally undesirable. Crosstalk causes interference to the information being transmitted through the twisted pair(s) and can reduce the data transmission rate and cause an increase in the bit error rate. The Telecommunications Industry Association and the Electronics Industry Association (TIA/EIA) have developed standards which specify specific categories of performance for cable impedance, attenuation, skew and crosstalk isolation. The International Electrotechnical Commission (IEC) has also defined standards for data communication cable crosstalk, including ISO/IEC 11801. One high-performance standard for 100Ω cable is ISO/IEC 11801, Category 5; another is ISO/IEC 11801 Category 6.
In twisted pairs, the rate of twist is defined as a specified distance between twists along the longitudinal direction, that distance being referred to as the pair lay or twist lay. When adjacent twisted pairs have the same pair lay and/or twist direction, they tend to lie within a cable more closely spaced than when they have different pair lays and/or twist direction. Such close spacing may increase the amount of undesirable crosstalk which occurs between adjacent pairs. Therefore, twisted pairs within a cable are sometimes given unique pair lays so as to reduce the crosstalk between twisted pairs of a cable. Twist direction may also be varied. Along with varying pair lays and twist directions, individual solid metal or woven metal pair shields are sometimes used to electromagnetically isolate pairs from one another.
In some cables, a separator is used to separate one twisted pair from another to improve crosstalk between the pairs and/or to provide added structural stability to the cable. For example, referring to
In building design, many precautions are taken to resist the spread of flame and the generation of and spread of smoke throughout a building in case of an outbreak of fire. Clearly, it is desired to protect against loss of life and also to minimize the costs of a fire due to the destruction of electrical and other equipment. Therefore, wires and cables for in building installations are required to comply with the various flammability requirements of the National Electrical Code (NEC) and/or the Canadian Electrical Code (CEC).
Cables intended for installation in the air handling spaces (i.e. plenums, ducts, etc.) of buildings are specifically required by NEC or CEC to pass the flame test specified by Underwriters Laboratories Inc. (UL), UL-910, or its Canadian Standards Association (CSA) equivalent, the FT6. The UL-910 and the FT6 represent the top of the fire rating hierarchy established by the NEC and CEC respectively. Cables possessing this rating, generically known as “plenum” or “plenum rated”, may be substituted for cables having a lower rating (i.e. CMR, CM, CMX, FT4, FT1 or their equivalents), while lower rated cables may not be used where plenum rated cable is required. Cables conforming to NEC or CEC requirements are characterized as possessing superior resistance to ignitability, greater resistant to contribute to flame spread and generate lower levels of smoke during fires than cables having a lower fire rating. Conventional designs of data grade telecommunications cables for installation in plenum chambers have a low smoke generating jacket material, e.g. of a PVC formulation or a fluoropolymer material, surrounding a core of twisted conductor pairs, each conductor individually insulated with a fluorinated ethylene propylene (FEP) insulation layer. Cable produced as described above satisfies recognized plenum test requirements such as the “peak smoke” and “average smoke” requirements of the Underwriters Laboratories, Inc., UL910 Steiner test and/or Canadian Standards Association CSA-FT6 (Plenum Flame Test) while also achieving desired electrical performance in accordance with EIA/TIA-568A for high frequency signal transmission.
Aspects and embodiments of the invention are directed to cables for data transmission that have constructions that may reduce alien crosstalk and/or may improve data transmission performance of the cable as compared to conventional cables. In one embodiment, a cable comprises a cable core including a plurality of twisted pairs of insulated conductors including a first twisted pair and a second twisted pair, each twisted pair comprising two insulated conductors twisted together in a helical manner, a jacket surrounding the plurality of twisted pairs of insulated conductors, and an element disposed between cable core and the jacket along a length of the cable, the element providing an air gap between the jacket and the cable core. The element may comprise, for example, one or more dielectric helixed splines (made of any of a variety of materials, including, for example, a fluoropolymer) or a conductive rod. The element(s) may be disposed about a circumference of the cable core or may be helically wrapped about the cable core. In one example, the cable core may further comprises a separator disposed among the plurality of twisted pairs of insulated conductors so as to separate at least one the plurality of twisted pairs from others of the plurality of twisted pairs. In one example, the cable core, the jacket and the element are helically twisted together with a cable twist lay that is within a range of about 2 to 6 inches.
In another embodiment, a cable for data transmission comprises a cable core comprising a plurality of twisted pairs of insulated conductors including a first twisted pair and a second twisted pair, each twisted pair comprising two insulated conductors twisted together in a helical manner, a jacket surrounding the plurality of twisted pairs of insulated conductors, and a dielectric helixed spline disposed between the cable core and the jacket along a length of the cable, the dielectric helixed spline providing an air gap between the jacket and the cable core, wherein the dielectric helixed spline comprises a fluoropolymer. In one example, the cable core further comprises a separator disposed among the plurality of twisted pairs of insulated conductors so as to separate the first twisted pair from the second twisted pair.
According to one embodiment, a bundled cable comprises a plurality of individual cables for data transmission. At least one of the individual cables for data transmission comprises a cable core including a plurality of twisted pairs of insulated conductors including a first twisted pair and a second twisted pair, each twisted pair comprising two insulated conductors twisted together in a helical manner, a jacket surrounding the cable core, and an element disposed between the cable core and the jacket along a length of the at least one individual cable, the element being separate from both the jacket and the cable core and providing an air gap between the jacket and the cable core, wherein, for any transverse cross-section taken along a radius of the at least one individual cable, the element contacts the jacket at only one location on an inner circumference of the jacket.
According to another embodiment, a cable for data transmission comprises a plurality of twisted pairs of insulated conductors including a first twisted pair and a second twisted pair, each twisted pair comprising two insulated conductors helically twisted together, a separator disposed among the plurality of twisted pairs of insulated conductors so as to separate at least one the plurality of twisted pairs from others of the plurality of twisted pairs, and a jacket surrounding the plurality of twisted pairs and the separator, wherein the jacket comprises a plurality of inwardly-projecting protrusions that extend away from an inner circumferential surface of the jacket toward the plurality of twisted pairs of insulated conductors.
In one example, the jacket comprises a dual-layer structure including a first jacket layer and a second jacket layer, and wherein the plurality of protrusions extends away from an inner circumferential surface of the first jacket layer. In another example, a conductive shield is disposed between the first jacket layer and the second jacket layer. The first jacket layer may comprise, for example, a first material having a first effective dielectric constant and the second jacket layer comprise, for example, a second material having a second effective dielectric constant; and wherein the first effective dielectric constant is lower than the second effective dielectric constant. In addition, or alternatively, the first jacket layer may comprise a first material having a first dissipation factor and the second jacket layer may comprise a second material having a second dissipation factor; and wherein the first dissipation factor is lower than the second dissipation factor. In one embodiment, the cable may further comprise a dielectric element, for example, a helixed spline, disposed between the first jacket layer and the second jacket layer to create an air gap between the first and second jacket layers. In another example, the first jacket layer may be bonded to the second jacket layer.
In another example, the cable jacket may comprise a dual-layer structure including a first jacket layer and a second jacket layer, and wherein the plurality of protrusions extend away from an inner circumferential surface of the second jacket layer to create an air gap between the first and second jacket layers. The first jacket layer may comprise a first material and the second jacket layer may comprise a second material, wherein at least one of an effective dielectric and a dissipation factor is lower for the first material than for the second material. In one example, the first jacket layer may comprise a foamed material. In one embodiment, the plurality of inwardly-projecting protrusions may include at least a first inwardly-projecting protrusion and a second inwardly-projecting protrusion; and wherein the first inwardly-projecting protrusion has a first height and the second inwardly-projecting protrusion has a second height that is substantially larger than the first height. In another embodiment, the plurality of inwardly-projecting protrusions may include at least a first inwardly-projecting protrusion and a second inwardly-projecting protrusion; and wherein the first inwardly-projecting protrusion has a first width and the second inwardly-projecting protrusion has a second width that is substantially larger than the first width.
Another embodiment of a cable comprises a plurality of twisted pairs of insulated conductors including a first twisted pair and a second twisted pair, each twisted pair comprising two insulated conductors helically twisted together, a helixed spline comprising a plurality of fins extending outwardly from a central connection point to create a plurality of channels, each channel being defined by a pair of fins, the helixed spline having a substantially dielectric body, and the fins each having a base connected to the central connection point and a tip, and a conductive layer disposed on the tips of the fins, wherein the twisted pairs of insulated conductors are disposed at least partially within the plurality of channels. In one example, the cable may further comprise a conductive shield substantially surrounding the plurality of twisted pairs of insulated conductors and the helixed spline; wherein the conductive layer is in contact with the conductive shield at least at some points along a length of the cable.
In the drawings, which are not intended to be drawn to scale, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. The drawings are provided for the purposes of illustration and explanation and are not intended as a definition of the limits of the invention. In the drawings:
Aspects and embodiments of the invention are directed to twisted pair communication cables that may exhibit superior transmission properties through the use of structures which may reduce alien crosstalk, internal crosstalk and signal attenuation in the twisted pairs. Various illustrative embodiments and aspects thereof are described in detail below with reference to the accompanying figures. It is to be appreciated that this invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, elements and features discussed in connection with one embodiment are not intended to be excluded from a similar role in other embodiments. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.
Referring to
As shown in
Embodiments of the above-described separator can be constructed using a number of different materials. While the invention is not limited to the materials now given, the invention may be advantageously practiced using these materials in some circumstances. In one embodiment, particularly for use in shielded cables, the core material may include a conductive material. For example, the core may include a metallic or other conductive coating over a dielectric body. In another example, a filler may be added to the core compound to render the extruded product conductive. The core compound may include any material generally compatible with use in data communications cable applications, including any applicable fire safety standards. Suitable fillers are those compatible with the compound into which they are mixed, including but not limited to powdered ferrite, semiconductive thermoplastic elastomers and carbon black. Conductivity of the core helps to further isolate the twisted pairs from each other and may be particularly useful in cables including a shield layer, as discussed below. In non-plenum applications, the core may be formed of solid or foamed flame retardant polyolefin or similar materials. The core may also be formed of non-flame retardant materials. In plenum applications, the core can be, for example, any one or more of the following compounds: a solid low dielectric constant fluoropolymer, e.g., ethylene chlortrifluoroethylene (E-CTFE), MFA or fluorinated ethylene propylene (FEP) or material in the FEP family, a foamed fluoropolymer, e.g., foamed FEP, or foamed MFA, and polyvinyl chloride (PVC) in either solid, low dielectric constant form or foamed. It is to be appreciated that the term FEP as used herein is intended to refer not only to fluorinated ethylene propylene, but also to all materials in its family. Similarly, it is to be understood that where examples of other materials (e.g., PVC) are given herein, the intent is to include all similar and/or related materials that may be used interchangeably with the example material.
The cable may be completed in any one of several ways, for example, as shown in
As is known in this art, when plural elements are cabled together, an overall twist is imparted to the assembly to improve geometric stability and help prevent separation. In some embodiments of a process of manufacturing the cable of the invention, twisting of the profile of the core along with the individual twisted pairs is controlled. The process may include providing the extruded core to maintain a physical spacing between the twisted pairs and to maintain geometrical stability within the cable. Thus, the process assists in the achievement of and maintenance of high crosstalk isolation by placing a conductive or non-conductive core in the cable to maintain pair spacing.
According to another embodiment, greater cross-talk isolation may achieved in the construction of
As discussed above, the core 101 may have a variety of different profiles and may be conductive or non-conductive. According to one embodiment, the core 101 may further include features that may facilitate removal of the core 101 from the cable. For example, referring to
According to another embodiment, the core 101 may comprise a helixed spline, as illustrated in
Referring to
It is known that the attenuation and propagation velocity of a signal through a twisted pair (or single conductor) is influenced by the dielectric constant of the insulation material of the twisted pair as well as by the dielectric constant of nearby elements, such as a separator or the cable jacket. Generally, the lower the effective dielectric constant of dielectrics in proximity to the transmission media, the better for electrical performance of the cable. The effective dielectric constant of a material depends on the thickness of the material as well as on the inherent characteristics of the material. A helixed spline according to embodiments of the invention may provide improved electrical properties (e.g., effect on signal propagation and attenuation in nearby twisted pairs) because, as mentioned above, it may displace less air than does a conventional solid round separator. In particular, referring to the embodiment illustrated in
In one embodiment, the twist lay of the spline may be varied to fine tune electrical and physical properties of the core and the overall cable. For example, a shorter twist lay may provide a core with greater crush resistance and a longer lay may provide increased material savings and improved electrical properties. The twist lay may be selected depending on the application for which the spline is to be used. The diameter of the spline may also vary depending on the application. For example, where a spline may be used as a filler in the cable to facilitate maintaining the shape of the cable, e.g., to keep it geometrically round, the filler may be appropriately sized and twisted for this application. For example, in a six-pair cable in which the insulated conductors have an outer diameter of about 0.09 inches, the conductors may be cabled around a filler spline having a diameter of about 0.09 inches. In this case, the filler spline may have a twist lay tight enough to allow the spline to act as a solid round filler and may be less than about 1 inch, and in some applications, less than about 0.3 inches. Alternatively, where a spline may be used as a spacer between the cable core and the cable jacket, it may again be appropriately sized, for example, to reduce alien crosstalk, and have a diameter of about 0.04 inches and a twist lay of less than about 0.5 inches. It is to be appreciated that many other sizes and applications are also possible and the invention is not limited to these examples.
As mentioned above, a helixed spline may provide significant material savings compared to a conventional solid round filler. For comparison, assume a solid rod filler having a diameter of 0.08 inches. A helixed spline may be defined as having an X shape, with an outer diameter of 0.08 inches and each segment of the X having a thickness of 0.12 inches. This spline, not accounting for twist loss, would provide a material savings of about 64% compared to the conventional solid round filler of the same diameter.
According to one embodiment, a helixed spline such as described above may be formed by extrusion. For example, the helixed spline may be continuously extruded using a die having a shaped head and a material that can be extruded (e.g., an extrudable polymer). In one example, to form a spline that has a “cross-shape” and is twisted with a certain twist lay, as described above, an extrusion die may be used with a crosshead that can be rotated during extrusion to provide the twist lay. In one example, a die may be used that rotates alternately in a clockwise and anticlockwise direction, such that the spline may be extruded with an “S/Z” configuration, as is known in the art.
A helixed spline according to embodiments of the invention may offer a number of advantages through the relative (i.e., compared to its solid round counterpart) reduction in the amount of material needed to make the separator. For example, the cost of the cable may be reduced because the amount of material is reduced. This may be particularly significant is the core is made from, or includes, expensive materials such as FEP. In addition, reducing the volume of material in the cable may make it easier to meet applicable fire safety standards as well as the requirements for the cable to be plenum-rated. In conventional cables, if the separator is made of a flammable material, a thick jacket may be needed to achieve the required flame performance. Even if the helixed spline is also made from or includes a flammable material, because the volume of material present is reduced, a thinner jacket may be used while still achieving the same or better flame performance. This may further reduce the cost of the cable as the volume of jacket material may also be reduced. The reduction in materials may also reduce the weight of the cable, which may be advantageous in terms of shipping costs and ease of handling.
According to another embodiment, a helixed spline such as described above may be used in as a separation barrier between layers of a multi-layer cable. One embodiment of a multi-layer cable 206 is illustrated in
Referring to
It is to be appreciated that the invention is not limited to the use of helixed splines and that in some embodiments, the helixed spline(s) 212 may be replaced by solid or foamed rods (having a round or other cross-sectional shape) that may perform the same functions described above. In addition, the invention is not limited to the construction illustrated in
As discussed above, a cable according to various embodiments of the invention may include a jacket, having a single layer or multiple layers, that may surround the transmission media and any other internal elements (e.g., a separator, binder or shield) making up the cable. In one embodiment, a cable may have a striated or “fluted” jacket that includes one or more protrusions that extend either inwardly toward a center of the cable from an internal circumference of the jacket or outwardly from an exterior circumference of the jacket. These protrusions may increase the distance between the twisted pairs of one cable and the twisted pairs of another adjacent cable and, in the case of inwardly extending protrusions may increase the distance between the twisted pairs and the cable jacket. As a result, such a jacket may provide numerous advantages such as, for example, reducing alien crosstalk (compared to cables with conventional round, smooth jackets) and/or providing a cable having a lower value of signal attenuation (also compared to a conventional cable with a round, smooth jacket) due to the decreased absorption of the signal by the dielectric cable jacket.
Referring to
According to one embodiment, the inwardly extending protrusions 218 may be formed such that the twisted pairs 103 may be contained within the inner region 160 and are spaced apart from the inner border of the cable jacket 216 by a distance “s,” as shown in
As illustrated in
According to one embodiment, the inwardly extending protrusions 218 may be helically formed along the inner circumferential surface of the jacket 216 such that the jacket is helically striated along the inner circumferential surface. In this embodiment, one or a few helically formed inwardly extending protrusions may provide a barrier along the longitudinal length of the cable that may maintain the twisted pairs 103 within the inner region 160 that is defined by the end(s) 164 of the protrusions(s) 220. It will be appreciated that a shorter “twist lay” of such helical striations may provide more containment of the twisted pairs at the expense of using more dielectric material to form the projection(s), whereas a longer “twist lay” of the striations may reduce the amount of material used, but may allow one or more twisted pairs to occasionally or periodically contact the inner border 162 of the jacket.
According to another embodiment, the cable jacket may be twisted (referred to as “cabled”) with the twisted pairs (and optional other elements such as a separator, shield or binder) with a given cable lay. In this embodiment, even if the one or more inwardly extending protrusions are formed longitudinally along the length of the cable as straight or substantially straight ridges, the cabling procedure will result in the protrusions forming helical ridges along the inside of the cable jacket with a twist lay equal to the cable lay. Thus, as discussed above in reference to helically formed projection(s), the helical ridges formed one or more protrusions may provide a barrier along the longitudinal length of the cable that may contain the twisted pairs within the inner region. Again, depending on the cable lay, it may be possible for one or more twisted pairs to “dip” between the helical ridges and contact the inner circumferential surface of the jacket. Thus, it will be recognized by those skilled in the art that there may be a tradeoff between a tight (or short) cable lay that allow the projection(s) to better contain the twisted pairs within the inner region and the effects of a shorter cable lay on the performance and material and manufacturing costs of the cable.
As discussed above, provision of a rod or spacer, such as a helixed spline described above, wrapped around the transmission media (and separator if present) may achieve the same or a similar result as providing a jacket with internal striations. In either case, the bulk of the jacket may be held away from the cable transmission media, which may be kept more toward a center of the cable. These constructions may therefore serve to reduce alien crosstalk and/or to reduce the effect of the jacket on the data transmission properties and performance of the cable. It is to be appreciated that cables having either or both of an internally striated jacket and a spacer (which may be a helixed spline or a solid or foamed dielectric spacer having a non-helixed construction) are considered part of the invention, as well as the many variations in structure (e.g., size, shape, materials etc.) of the jacket and/or spacer that may be apparent to those skilled in the art.
The cable jacket may include any insulating material that is used in the industry and can be shaped to form the jacket, for example, by extrusion. In one embodiment, the jacket 117 may be constructed of a low dielectric constant thermoplastic material. In some other examples, the jacket may be made from a solid low dielectric constant fluoropolymer or fluorocopolymer such as, for example, ethylene chlortrifluoroethylene (E-CTFE), FEP or FEP family materials, MFA, low smoke PVC, flame retardant polyolefin or other similar materials.
According to some embodiments, the cable jacket may have any shape that can be extruded. For example, referring to
In another example, a jacket may include a plurality of inwardly extending protrusions that are shaped and arranged to maintain transmission media in a predetermined arrangement. Referring to
According to another embodiment, several cables such as those described above may be bundled together to provide a bundled cable. Within the bundled cable may be provided numerous embodiments of the cables described above. For example, the bundled cable may include some shielded and some unshielded cables, some four-pair cables and some having a different number of pairs. In addition, the cables making up the bundled cable may include conductive or non-conductive cores having various profiles, including the helixed spline discussed above. One example of a bundled cable 175 including a plurality of individual cables 117, each having a jacket 220 including one or more inwardly extending protrusions 216, is illustrated in
Referring to
According to another embodiment, a cable may be provided with a jacket having one or more outwardly extending protrusions from an outer circumferential surface of the cable. Such a construction may facilitate reduction of alien crosstalk between twisted pairs of nearby cables, as discussed further below. Referring to
Referring to
It is to be appreciated that, as was the case with the jackets having inwardly extending protrusions discussed above, the outwardly extending protrusions 165 may have various shapes and sizes and the illustrated examples are not intended to be limiting. For example, in some embodiments, a jacket may have a plurality of outwardly extending protrusions from the outer circumferential surface 163 of the jacket that may be evenly, randomly or otherwise spaced about the outer circumferential surface of the jacket. Alternatively, a jacket may only one outwardly extending projection that may extend longitudinally along the length of the jacket. In one example, such a single outwardly extending projection may be helically formed about the jacket. Alternatively, the outwardly extending projection may initially be formed as a substantially straight stripe along the jacket, but cabling of the jacket may result in the projection forming a helical ridge along the outer circumferential surface of the jacket. In addition, the width and depth (or height) of the outwardly extending projection(s), as well as their shape, may be varied as was discussed above in reference to jackets comprising inwardly extending projection(s).
Referring to
According to another embodiment, a cable may comprise a multi-layer jacket. The various jacket layers may comprise the same or different materials and, in some examples, may include inwardly extending or outwardly extending protrusions. One example of a cable including a dual-layer jacket is illustrated in
Numerous embodiments of a cable having a multi-layer jacket construction are contemplated in addition to the example illustrated in
In one example, the cavities may be filled with air and may therefore serve to lower the overall effective dielectric constant of the jacket. In addition, such air pockets may allow provision of a thicker overall jacket without requiring an increase in total jacket material used. This may be a cost effective way in which to improve alien crosstalk by increasing the spacing between transmission media of adjacent cables due to the thicker jackets. Furthermore, in embodiments where the air pockets exist between two jacket layers of dissimilar materials, the air pockets may further distance the bulk of the outer jacket layer from the transmission media. This may allow the second jacket layer material to be selected without concern regarding its effect on the transmission media, as discussed above.
According to another embodiment, a cable may comprise a dual-layer jacket with an element disposed between the first and second jacket layers. This element may be non-conductive or conductive (for example, a drain wire or a conductive shield that) and, optionally, may be wrapped around the inner jacket layer. Referring to
In another embodiment, the element disposed between the first and second jacket layers of a dual-layer jacket may include a dielectric spacer. This spacer may be, for example, a dielectric rod or filler or a helixed spline such as described above. In one example, the dielectric spacer may be helically wrapped around the inner jacket layer, as illustrated in
In one example, any of the jacket layers making up a multi-layer jacket may be bonded to one another using any suitable bonding technology known to those skilled in the art, including but not limited to, using a bonding agent (e.g., an adhesive applied to the surfaces of one or more jacket layers), heat-bonding, etc. In addition, in the embodiments in which an element is placed between the jacket layers, the element may be bonded to either layer it contacts. For example, referring to
As discussed above, a goal of cable designers may be to reduce crosstalk in the twisted pairs of a cable because crosstalk may adversely affect the quality and/or speed of data transmission through the twisted pairs. Various embodiments of cable jackets and other elements (e.g., shields or spacers) discussed herein may serve to reduce alien crosstalk. In addition, various embodiments of separators discussed herein may reduce crosstalk between pairs within a single cable. In some embodiments, particularly where the core 101 may be non-conductive, it may be advantageous to provide additional crosstalk isolation between the twisted pairs 103 by varying the twist lays of each twisted pair 103. For example, referring to
As discussed above, varying the twist lay lengths between the twisted pairs in the cable may help to reduce crosstalk between the twisted pairs. However, the shorter a pair's twist lay length, the longer the “untwisted length” of that pair and thus the greater the signal phase delay added to an electrical signal that propagates through the twisted pair. It is to be understood that the term “untwisted length” herein denotes the electrical length of the twisted pair of conductors when the twisted pair of conductors has no twist lay (i.e., when the twisted pair of conductors is untwisted). Therefore, using different twist lays among the twisted pairs within a cable may cause a variation in the phase delay added to the signals propagating through different ones of the conductors pairs. It is to be appreciated that for this specification the term “skew” is a difference in a phase delay added to the electrical signal for each of the plurality of twisted pairs of the cable. Skew may result from the twisted pairs in a cable having differing twist lays. As discussed above, the TIA/EIA has set specifications that dictate that cables, such as category 5 or category 6 cables, must meet certain skew requirements.
In addition, in order to impedance match a cable to a load (e.g., a network component), the impedance of a cable may be rated with a particular characteristic impedance. For example, many radio frequency (RF) components may have characteristic impedances of 50 or 100 Ohms. Therefore, many high frequency cables may similarly be rated with a characteristic impedance of 50 or 100 Ohms so as to facilitate connecting of different RF loads. The characteristic impedance of the cable may generally be determined based on a composite of the individual nominal impedances of each of the twisted pairs making up the cable. Referring to
The nominal characteristic impedance of each pair may be determined by measuring the input impedance of the twisted pair over a range of frequencies, for example, the range of desired operating frequencies for the cable. A curve fit of each of the measured input impedances, for example, up to 801 measured points, across the operating frequency range of the cable may then be used to determine a “fitted” characteristic impedance of each twisted pair making up the cable, and thus of the cable as a whole. The TIA/EIA specification for characteristic impedance is given in terms of this fitted characteristic impedance. For example, the specification for a category 5 or 6 100 Ohm cable is 100 Ohms, +−15 Ohms for frequencies between 100 and 350 MHz and 100 Ohms +−12 Ohms for frequencies below 100 MHz.
In conventional manufacturing, it is generally considered more beneficial to design and manufacture twisted pairs to achieve as close to the specified characteristic impedance of the cable as possible, generally within plus or minus 2 Ohms. The primary reason for this is to take into account impedance variations that may occur during manufacture of the twisted pairs and the cable. The further away from the specified characteristic impedance a particular twisted pair is, the more likely a momentary deviation from the specified characteristic impedance at any particular frequency due to impedance roughness will exceed limits for both input impedance and return loss of the cable.
As the dielectric constant of an insulation material covering the conductors of a twisted pair decreases, the velocity of propagation of a signal traveling through the twisted pair of conductors increases and the phase delay added to the signal as it travels through the twisted pair decreases. In other words, the velocity of propagation of the signal through the twisted pair of conductors is inversely proportional to the dielectric constant of the insulation material and the added phase delay is proportional to the dielectric constant of the insulation material. For example, for a so-called “faster” insulation, such as fluoroethylenepropylene (FEP), the propagation velocity of a signal through a twisted pair 103 may be approximately 0.69 c (where c is the speed of light in a vacuum). For a “slower” insulation, such as polyethylene, the propagation velocity of a signal through the twisted pair 103 may be approximately 0.66 c.
The effective dielectric constant of the insulation material may also depend, at least in part, on the thickness of the insulating layer. This is because the effective dielectric constant may be a composite of the dielectric constant of the insulating material itself in combination with the surrounding air. Therefore, the propagation velocity of a signal through a twisted pair may also depend on the thickness of the insulation of that twisted pair. However, as discussed above, the characteristic impedance of a twisted pair also depends on the insulation thickness.
Applicant has recognized that by optimizing the insulation diameters relative to the twist lays of each twisted pair in the cable, the skew can be substantially reduced. Although varying the insulation diameters may cause variation in the characteristic impedance values of the twisted pairs, under improved manufacturing processes, impedance roughness over frequency (i.e., variation of the impedance of any one twisted pair over the operating frequency range) can be controlled to be reduced, thus allowing for a design optimized for skew while still meeting the specification for impedance.
According to one embodiment of the invention, a cable may comprise a plurality of twisted pairs of insulated conductors, wherein twisted pairs with longer pair lays have a relatively higher characteristic impedance and larger insulation diameter, while twisted pairs with shorter pair lays have relatively lower characteristic impedance and smaller insulation diameter. In this manner, pair lays and insulation thickness may be controlled so as to reduce the overall skew of the cable. One example of such a cable, using polyethylene insulation is given in Table 1 below.
This concept may be better understood with reference to
According to another embodiment, a four-pair cable was designed, using slower insulation material (e.g., polyethylene) and using the same pair lays as shown in Table 1, where all insulation diameters were set to 0.041 inches. This cable exhibited a skew reduction of about 8 ns/100 meters (relative to the conventional cable described above—this cable was measured to have a worst case skew of approximately 21 ns whereas the conventional, impedance-optimized cable exhibits a skew of approximately 30 ns or higher), yet the individual pair impedances were within 0 to 2.5 ohms of deviation from nominal, leaving plenty of room for further impedance deviation, and therefore skew reduction.
Allowing some deviation in the twisted pair characteristic impedances relative to the nominal impedance value allows for a greater range of insulation diameters. Smaller diameters for a given pair lay results in a lower pair angle and shorter non-twisted pair length. Conversely, larger pair diameters result in a higher pair angles and longer non-twisted pair length. Where a tighter pair lay would normally require an insulation diameter of 0.043″ for 100 ohms, a diameter of 0.041″ would yield a reduced impedance of about 98 ohms. Longer pair lays using the same insulation material would require a lower insulation diameter of about 0.039″ for 100 ohms, and a diameter of 0.041″ would yield about 103 ohms. As shown in
Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. For example, any of the cables described herein may include any number of twisted pairs and any of the jackets, insulations and separators shown herein may comprise any suitable materials. In addition, the separators may be any shape, such as, but not limited to, a cross- or star-shape, or a flat tape etc., and may be positioned within the cable so as to separate one or more of the twisted pairs from one another. Any of the various separator embodiments described herein may be used with any of the jacket constructions described herein. In addition, features or aspects of any jacket embodiments described herein may be applied to any other jacket and/or separator embodiments. Such and other alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.
This application is a continuation of and claims priority under 35 U.S.C. § 120 to U.S. application Ser. No. 11/673,357 entitled “Data Cable with Cross-Twist Cabled Core Profile” filed on Feb. 9, 2007, now U.S. Pat. No. 7,405,360, which is a continuation-in-part of and claims priority under 35 U.S.C. § 120 to U.S. application Ser. No. 11/584,825 entitled “Data Cable with Cross-Twist Cabled Core Profile,” filed on Oct. 23, 2006 and, which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 11/445,448, entitled “Data Cable with Cross-Twist Cabled Core, ” filed on Jun. 1, 2006 and now abandoned, which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 11/197,718 entitled “Data Cable With Cross-Twist Cabled Core Profile,” filed on Aug. 4, 2005, now U.S. Pat. No. 7,135,641, which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 10/705,672 entitled “Data Cable With Cross-Twist Cabled Core Profile,” filed on Nov. 10, 2003, now U.S. Pat. No. 7,154,043 which is a continuation-in-part of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 10/430,365 entitled “Enhanced Data Cable With Cross-Twist Cabled Core Profile,” filed May 5, 2003, now abandoned, which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 09/532,837 entitled “Enhanced Data Cable With Cross-Twist Cabled Core Profile,” filed on Mar. 21, 2000, now U.S. Pat. No. 6,596,944 which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 08/841,440, filed Apr. 22, 1997 entitled “Making Enhanced Data Cable with Cross-Twist Cabled Core Profile” (as amended) now U.S. Pat. No. 6,074,503, each of which is herein incorporated by reference in its entirety.
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