1. Field of the Invention
This invention relates to shielded electrical cables generally, and in particular to a twin-axial shielded cable for use in high frequency data transmission applications. More specifically, the twin-axial cable of this invention is intended to be utilized in harsh environments such as on the bottom of the ocean where the cable will be subjected to flexing stress due to bending along with the hydrostatic pressure of the water.
2. Description of the Prior Art
Twin-axial cable is generally formed by encasing two electric conductors, separated a predetermined distance, within a dielectric layer. The distance of separation between the two conductors is selected, in part, to prevent degradation of the electrical signal. Signal degradation occurs when there is a change in the electrical signal. The electrical characteristics that represent two forms of degradation are the "attenuation" ratio and a mismatch between the "characteristic impedance of transmission" value and the interface requirements of the mating system. Both of these characteristics can be partially controlled by the distance of separation between the two conductors. If the cable contains other conductive materials such as in the form of a metallic shield, then the distance of separation between each of the conductors and the conductive material would likewise be critical to the electrical characteristics of the cable. Therefore, it is desirable to maintain an optimum distance of separation both between the conductors and between each of the conductors and the shielding. These two separation distances directly affect the signal degradation and overall performance of a shielded twin-axial cable.
A significant problem in connection with the distances of separation between cable elements is in maintaining that separation along the entire length of the cable during manufacture of the cable. Twin conductor cables made in accordance with the prior art require a high degree of skill to ensure a desired distance of separation between the two conductors and further still, between the conductors and the shielding. Although a cable design has specific requirements, meeting those requirements is often heavily dependent on the skill level of the particular manufacturer. Another problem occurs in adjusting the separation distances of a specific design to meet varying electrical requirements. Typically, in the prior art, significant alterations to a cable design must be made to new electrical requirements. These alterations, in turn, result in significant changes in the manufacturing process.
Therefore, it is desirable when addressing conductor separation problems to decrease the skill level requirements of the manufacturer and design a cable that is easily altered to meet specific electrical needs.
In the prior art, the dielectric layer that encases the conductors serves several purposes. First, it is used to physically hold the conductors in their proper geometric configuration. Second, it serves as a protective barrier for the conductors. In certain applications, a cable may be subject to harsh chemical or physical environments, such as extreme temperatures, high pressures and corrosive conditions. Particularly in ocean bottom applications, the extreme high pressure may cause the cable to deform or possibly even collapse when the dielectric layer is not sufficiently strong to withstand the external pressures. The natural consequence of any deformation are changes in distances of separation between the cable elements. In the more extreme case where the conductors come into contact with the environment, both signal degradation and cable failure are inevitable.
A common problem with the dielectric layer arises in its capacity to act as a protective barrier. If the layer is not sufficiently thick around the conductors, cracks that form in the dielectric layer can penetrate to the conductors, exposing them to outside environments. Nevertheless, if the layer is too thick, the cable is less flexible rendering the cable inappropriate for use in many applications. Another problem that stems from encasing the conductors with the dielectric layer is the formation of interstitial spaces. If the encasing process is not properly performed, interstitial spaces often form between the insulated conductors and the dielectric layer. Formation of interstitial spaces adds to the problems associated with the formation of cracks in the dielectric layer by allowing the penetration of undesirable materials into the critical regions of the cable. Further still, in deep water applications, the presence of interstitial spaces among cable elements will increase the likelihood that the cable will collapse or otherwise deform under the extreme external pressures.
FIG. 1 depicts one type of prior art twin axial cable 36 as shown and described in U.S. Pat. No. 5,313,020. This cable consists of two insulated conductors 30, 30', twisted together, and encased in a dielectric layer 31. In forming the cable 36, the first step involves surrounding each conductor with large diameter insulation jacket 32, 32'. The thickness of the jacket is selected to be one half of the desired distance between the two conductors. The insulated conductors should be in contact with each other, not spaced apart as shown in FIG. 1, if the thickness of the jacket is 1/2 the spacing desired. This gap between the insulated conductors is unintentional and process dependent, but typically will be held as small as possible. Clearly, when the design spacing between the conductors is to be set only by the insulation diameter, this process variability will result in variations in the electrical performance of the cable.
Once the insulated conductors are twisted together, the conductors will be separated by at least the combined thickness of the insulation jackets around the conductors. After dielectric layer 31 is formed around the twisted, insulated conductors, metallic shield 33 is braided over the dielectric layer and outside jacket 35 is formed around the braided shield to complete the cable.
There are several disadvantages with the twin-axial cable illustrated in FIG. 1. The insulated conductors may be spaced apart so that the thickness of the dielectric at point "a" is relatively small compared to the thickness at point "b" causing the dielectric to fail at this point when the cable is bent. Further, if the insulated conductors do not remain substantially concentric with the dielectric layer 31 during manufacturing, portions of the insulated conductors in the areas where the layer's thickness is small, i.e. point "a", may not be covered at all by the layer. Depending on the skill of the manufacturer, therefore, these thin sections of dielectric layer provide, at best, only a comparatively thin barrier of protection for the insulated conductors, and at worst, provide no barrier of protection at all.
A second disadvantage with the cable of FIG. 1 also stems from the thick and thin sections of the dielectric layer 31 encasing the insulated conductors 30, 30'. Typically, the dielectric layer is formed around the insulated conductors by extrusion. The extrusion process ideally produces a tubular body of insulation that is substantially concentric with the twisted, insulated conductors. The physical design of cable 36, however, creates difficulties in achieving this goal because of the shrinkage of dielectric layer 31 during the curing and cooling of the layer. Since the wall thickness of the tubular body of dielectric material is not uniform, the shrinkage of sections of the wall of varying thickness is not uniform. Thicker sections such as that of point "b" will tend to shrink more than the thinner section of point "a". The disparity on shrinkage produces an extrusion that is not cylindrical. The non-cylindrical shape of the dielectric layer results in additional problems and expense in making and installing the braided shield 33.
The design of the cable of FIG. 1 may also result in the formation of interstitial spaces. The materials selected for dielectric layer 31 and insulation jackets 32, 32' generally do not have the same chemical base to prevent bonding so that the two layers may be separated without damaging the conductors during the process of terminating the cable. When dielectric layer 31 is extruded around the insulated conductors, the two materials will not bond due to differences in the materials. At the points where a bond does not form between the two materials, interstitial spaces form due to the shrinkage of dielectric 31. In addition, an interstitial space is formed in the gaps between the insulated conductors. These gaps are not filled due to the viscosity limited flow of dielectric 31. Alternatively, when dielectric materials are selected to insure strong bonding between these cable elements, a corresponding loss in flexibility will occur. Therefore, when a flexible cable is required, it is not desirable to bond the conductor insulation or the spacers to the dielectric material. Rather, it is preferable that the conductors not only be separated at a fixed distance from one another and from the other conductive elements of the cable, but that they be allowed to move longitudinally along the length of the cable to relieve stress and insure flexibility.
FIG. 2 depicts a second prior art method of making a twin-axial cable. This cable is FIG. 4 of my '020 patent discussed above. Cable 26 consists of two uninsulated conductors 20, 20', separated by a predetermined distance, and encased in dielectric layer 21. In forming the cable, the first step involves extruding a dielectric layer around the two uninsulated conductors. As the dielectric is extruded around the conductors, the conductors are held substantially parallel to each other at the required distance. Again, braided shield 23 of conductive material is placed around the extrusion and protective jacket 25 is extruded over the braid.
This type of twin-axial configuration has an advantage over the cable of FIG. 1 in that there are no conductor insulation jackets with which the dielectric layer material must bond. Instead, the dielectric layer bonds directly to the conductors and there is no opportunity for interstitial spaces to form around the conductors. The cost of this advantage is the loss in flexibility of the cable. The formation of a direct bond between the dielectric layer and the conductors, as the dielectric material is extruded over the uninsulated conductors, creates a bond between these cable elements over the entire length of the cable. This bonding fixes the conductors in place within the cable and prevents their longitudinal movement within the cable. This is an undesirable condition since the ability of the conductors to move longitudinally within the cable would allow them to relieve some of the tension and compression that is created when the cable is flexed.
A further disadvantage of the twin axial design shown in FIG. 2, is that the conductors are fixed in the same geometric plane for the entire length of the cable. When the cable is flexed, or bent orthogonally to the plane containing the conductors, each conductor has an equal amount of tension and compression placed upon it. However, if the cable is made to flex so that it is bent in the plane of the conductors so that the direction of the bend is coplanar with the two conductors, one of the conductors will be placed in a much higher state of tension than the other conductor. This arrangement of the conductors over the entire length of the cable render it more susceptible to fatigue stress and thus failure.
An additional disadvantage of the prior art twin axial design shown in FIG. 2 is that it is difficult to terminate the cable such as at a junction or instrument. To terminate the cable the solid extruded dielectric layer 21 must be split and removed from each conductor 20 for a sufficient length to allow the conductor to be attached to the instrument. Removal of the extruded solid dielectric layer is difficult and greatly hampers this procedure.
FIG. 3 depicts yet a third type of prior art twin-axial cable that overcomes some of the conductor spacing concerns noted above. Illustrated in FIG. 3 is cable 10, having central core 11 about which a pair of insulated conductors 12, 12' are helically wrapped along with a plurality of spacers 14 used to maintain the insulated conductors in fixed relative positions. A layer 15 of dielectric material encases the core, conductors and spacers. A braid 16 of metallic wire is placed over the dielectric layer 15 and the cable is completed with an outer jacket 17 of dielectric material.
The design of the twin-axial cable in FIG. 3 provides proper spacing of the conductors within the cable and maintains them in fixed relative positions. This cable is the subject of the '020 patent. The means by which this spacing is accomplished in this cable creates unavoidable interstitial spaces. In particular, the use of spacers 14 to insure proper spacing of the conductors creates voids 19 between the conductors, spacers, and core that may not be filled by the extruded dielectric material. The presence of such interstitial spaces may allow the cable to deform and possibly even collapse under the high pressure conditions present in deep water applications. Naturally, any deformation or collapse of the cable structure will change the spacing between the cable elements altering its electrical characteristics.
An additional disadvantage of the twin-axial cable design shown in FIG. 3 is again one of manufacturing. The level of skill required to insure a uniform product over the length of cable 10 is relatively high due to the problems of interstitial spaces and voids in the dielectric layer. Re-configuring the cable design for different electronic specifications would also be an involved process requiring the alteration of multiple cable elements.
Therefore, it is an object and feature of this invention to provide a twin-axial cable that has a substantially solid core to support the conductors, to maintain those conductors in relatively fixed lateral positions, and to reduce the likelihood of interstitial spaces and voids forming between cable elements. Specifically, the cable of the present invention comprises a central core that is relatively rigid and non-compressible, and an outer core of a flexible polymeric material that surrounds the inner core and is provided with diametrically opposed grooves in which insulated conductors are positioned.
It is another object and feature of the present invention to maintain the insulated conductors in laterally fixed, spaced, relationship within the cable. This feature is achieved in the present invention by holding the conductors in engagement with the bottoms of the grooves formed in the outer core by a layer of dielectric tape wrapped around the outer core along with an annular body of uniform thickness of dielectric material extruded around the dielectric tape.
It is a further feature of the present invention to provide a twin axial cable design that allows each insulated conductor to move longitudinally within the cable independent of the other insulated conductor. This feature is achieved by holding the insulated conductors in the grooves in the outer core by the dielectric tape that separates the conductors from an outer annular body of dielectric material to which the conductor insulation might otherwise bond during cable manufacturing. A water-blocking compound may optionally be employed to fill any interstitial spaces between the insulated conductors and the walls of the grooves. If used, this compound should be of such a nature that it allows the conductors to move longitudinally within the grooves.
It is a further feature of the present invention to provide a shielded twin-axial cable in which an annular layer of metallic highly conductive material surrounds the annular body of dielectric material that surrounds the dielectric tape, the conductors and the core, thereby providing a fixed distance of separation between the conductors and the shielding. Because the cable of the present invention is intended for deep water applications, the strength of the dielectric layer and its ability to withstand the high external pressures without deformation are critical. The annular shield is typically metal wire braided about the dielectric layer. A filling compound can be used to fill interstitial spaces that may exist in the shield.
It is a yet another feature of the present invention to provide a twin-axial cable that is durable and less susceptible to fatigue stress due to excessive bending and flexing. This feature is achieved through the selection of cable materials, by enabling the conductors to move longitudinally within the cable and by arranging the conductors within the cable so that an applied stress may be more readily relieved. Each conductor is held in one of two separate diametrically opposed, longitudinally extending helical grooves in the central core. This arrangement of the conductors allows for an improved distribution of the tensile and compressive stresses that are created in the conductors as the cable is flexed.