ELECTRICAL SIGNAL CABLE

Abstract: The invention relates to an electrical signal cable (10) for transmission of electromagnetic waves comprising at least two electrical conductors (1, 2, 3, 4, 4a, 4b). At least one electrical conductor is a signal conductor (4a, 7, 130, 1030, 1130, 1330, 1630) and at least one other electrical conductor is a ground conductor (4b, 8, 30, 50, 70, 90, 130, 1050, 1060, 1070, 1150, 1270, 1650, 1750). The capacitance of said signal cable (10) is less than 48 pF/m and the electrical signal conductor has a packaging density of between 5 and 100 electrical conductors per mm<2>.

Description: E l e c t ri c a l S i g n a l C a b l e Field of the Invention The invention relates to an electrical signal cable. Prior Art Electrical signal lines are known, for example, from European Patent Application EP-A-0 735 544 (Cartier et al.) assigned to Hewlett-Packard Company. This patent application describes an ultrasound system with a transducer cable for providing an electrical connection between a transducer and a display processor. The first embodiment of the transducer cable in this application has a plurality of stripline or sub-cable assemblies, which are surrounded by a single metal braid as an outer overall shield. The third embodiment of the transducer cable in this application uses three layers of extruded ribbon assemblies separated from each other by shield conductors comprising thin strips of bare copper. Each sub-cable assembly contains an integral electrical shielding which separates some of the electrical conductors within one sub-cable assembly from other electrical conductors, within the same sub-cable assembly . There is, however, no shielding on the outside of each sub-cableassembly provided for separating two sub-cable assemblies from each other. The stack of sub-cable assemblies are extruded with a jacket to form a desired length of the transducer cable. US-A-4 847 443 (Basconi) assigned to the Amphenol Corporation teaches another example of an electrical signal line cable formed from a plurality of generally flat electrical signal line segments stacked together in an interlocking relationship. Each electrical signal line segment of this prior art cable contains at least one signal conductor surrounded on either side by ground conductors. The plurality of ground conductors effectively form a ground plane which inhibit the cross-talk between the adjacent signal conductors. The insulating materials in which the conductors are disposed is extruded over the individual signal conductors. European Patent EP-B-0 605 600 (Springer et al.) assigned to the Minnesota Mining and Manufacturing Company teaches a ribbon cable and a lamination method for manufacturing the same. The ribbon cable manufactured comprises a plurality of evenly spaced flexible conductors surrounded by an insulator which is a microporous polypropylene. US Patent US-A-4 847 443 (Crawley et al.) assigned to W.L.Gore & Associates teaches a multi- conductor flat ribbon cable having a plurality of electrical conductors disposed within an insulator consisting of expanded polytetrafluoroethylene (ePTFE). PCT patent application WO- A-91/09406 (Ritchie et al) teaches an electrical wiring composed of elongated electrically conductive metal foil strips laminated between opposing layers of insulating films by means of adhesive securing the foil strips between the laminating films. German patent application DE-A-24 24 442 assigned to Siemens teaches a cable assembly which comprises a plurality of flat cables laminated between insulating films. PCT patent application WO-A-80/00389 (Clarke) assigned to Square D company of Palatine, Illinois, teaches an input/output data cable for use with programmable controllers. The cable has a ground conductor, a logic level voltage conductor and a number of signal tracks. The conductors are disposed on two or three layers of flexible plastics material in specified ways to give high immunity to interference and low inductive losses. The layers are glued together to form a laminate structure. W.L.Gore & Associates, Inc., in Phoenix, Arizona, sell a round cable under the part number 02-07605 which comprises 132 miniature co-axial cables enclosed within a braided shield of tin-plated copper and a jacket tube of PVC. Electrical signal cables of the described art are used in a variety of applications, for example in medical devices, computers, test equipment or in telecommunications devices. One need that has emerged in the past few years in medical device technology, particularly in ultrasound diagnostic machines, is the requirement to increase the quality or sensitivity of the image obtained. This is particularly important in ultrasound diagnostic machines which have the ability to image in three dimensions or which use colour doppler techniques to aid the physician in diagnosis of defects. An improved sensitivity requires fast rise times and low losses of the analogue data signals travelling along the electrical signal cables. This can be solved by providing a plurality of small diameter electrical conductors within the electrical signal cable. However, small diameter electrical conductors used in prior art electrical signal cables have the disadvantage that their flexlife properties, i.e. the length of time in which they can be flexed backwards and forwards without breakage and thus the lifetime of the cable, is limited. In applications such as medical device technology this is a severe limitation since the electrical signal cable assemblies are used to connect probe heads to machines. The probe heads are held by an operator and moved over, on and through the patient's body. The electrical signal cable assemblies are accordingly regularly flexed. Furthermore the trend towards smaller electronic devices, such as portable computers or portable medical devices, means that the size of the cable assemblies used becomes critical. There therefore exists a need for reducing the size of the electrical signal cable assemblies so that they can be used in such applications. A further need that has arisen because of the desire to have portable electronic devices is the reduction in the size and hence the weight of the eletrical signal cable assemblies. Reduction of the size of the cable used in a medical device reduces the strain on the muscles suffered by the operator of the medical device holding the probe and can allow him or her to operate for longer periods of time. A reduction in the weight of the cable assemblies used in portable computers similarly allows their users to carry them for a longer period of time without causing back strain and other related problems. Summary of the Invention It is an object of the invention to provide an improved electrical signal cable assembly. It is furthermore an object of the invention to reduce the space required for the electrical signal cable assembly. It is furthermore an object of the invention to reduce the weight of the electrical signal cable whilst maintaining high electrical performance. It is a further object of the invention to provide an electrical signal cable assembly in which the cross talk between the individual signal conductors within the assembly is sufficiently low that it is suitable in highly sensitive applications such as for ultrasound probes. These and other objects of the invention are solved by providing an electrical signal cable for transmission of electromagnetic waves comprising at least two electrical conductors spaced apart by an insulator. At least one of the electrical conductors is a signal conductor and at least another one of the other electrical conductors is a ground conductor. The capacitance per length, wire diameter and number of cables per unit area of said signal cable is less than approximately 60 pF/mm/m. The use of such low values allows the use of the electrical signal cable of the invention in the applications mentioned above without significant loss in electrical signal performance whilst maintaining a small signal cable size and good flex life properties.The rise times and losses of the signal along the cable because of the low capacitance are not significant and can, if necessary, be compensated by electronics in the signal analysing unit to which the cable is connected. The flex life properties are good because an adequate compromise can be made between the diameter of the electrical conductors and the overall capacitance of the cable. These values are particularly advantageous for signal cableassemblies used in ultrasound devices. In order to improve the image resolution, the crystal in the probe head is divided into increasingly smaller areas, each of which produces a separate data signal which has to pass along the signal cable assembly. The small portions have small capacitances and thus need to be connected to signal conductors within a signal cable assembly similarly having small capacitances otherwise the impedance mismatch between the crystal and the signal cable assembly will cause decreased sensitivity. In one embodiment the electrical signal cable is build up as a flat cable with coplanarly arranged electrical conductors, wherein for example every second or third electrical conductor is a ground conductor. The electrical conductors are arranged parallel to each other. The ground conductor can be paired with a corresponding signal conductor to form a twisted pair. Alternatively the two electrical conductors are arranged as ribbon pair or the two electrical conductors can be arranged such that the ground conductor is in the form of a ground plane coaxially arranged about the signal conductor. A most simple architecture is achieved by providing the signal conductor with an insulating coating. Since all ground conductor have the same electrical potential it is not necessary to distinguish them. The insulating coating allows different colours to be used thus permitting the identification of equal signal conductors on opposed ends of the electrical signal cable. In a preferred embodiment at least the electrical signal conductor has a diameter of 0.25 mm or smaller. Most preferably conductors having a diameter of AWG 30 (0.25 mm diameter) or smaller diameter are used , especially of diameter AWG 32 (0.203 mm diameter), AWG 34 (0.16 mm diameter), AWG 36 (0.127 mm diameter), AWG 38 (0.1 mm diameter), AWG 40 (0.08 mm diameter), AWG 42 (0.07 mm diameter) or AWG 44 (0.054 mm diameter) diameter. Depending on the actual application of the electrical signal cable the number of electrical conductors, especially signal conductors and ground conductors in the electrical signal cable, is chosen to be 64 or more, more particularly 128, 200, 256, 512 or 1024 conductors. Additionally reserve conductors may be incorporated into the electrical signal cable. Good and stable electrical properties of the electrical signal cable are achieved by arranging the signal conductors in one layer to form a signal conductor layer and the ground conductors in a further different layer to form a ground conductor layer of the electrical signal cable. In a preferred embodiment the signal conductor layer and/or the ground conductor layer is braided or surfed around a central axis. In another preferred embodiment at least one signal conductor layer and at least one ground conductor layer are stacked on each other. In an embodiment the signal conductor layer is cylindrically arranged around a central axis. The signal conductor layer comprises a plurality of flat cables and each of said flat cables has a plurality of coplanar electrical signal conductors encased within and separated at a pitch distance (a) from each other by a flat cable insulator. The cylindrical arrangement of the flat cables around the central axis of the assembly allows the assembly to bend easily in multiple directions and thus any sensor device, such as an ultrasound probe, can be easily put into a position by an operator. The cylindrical arrangement of the flat cables furthermore ensures a high flex life since any stresses within the electrical signal cable are distributed over the whole of the electrical signal cable rather than being concentrated in certain longitudinal planes. In one development of the invention, the flat cables are cylindrically braided about the central axis and in a further embodiment of the invention the flat cables are cylindrically surfed or wrapped about the central axis. In order to ensure that the signal conductors within the electrical signal cable are shielded from interfering magnetic fields, an outer shield is disposed about the signal conductor layer. A plurality of the signal conductor layers can be cylindrically arranged around a central axis and in such a case they are preferably separated from each other by the ground conductor in the form of a separating cylindrical shield. The use of a plurality of flat cables within the electrical signal cable, each of which contains a limited number of signal conductors, has the advantage that flexlife and handling of the electrical signal cable is improved since each of the flat cables has a degree of freedom to move within the assembly. The separating cylindrical shield is used to shield the signal conductor layers from one another such that the stray electromagnetic fields created by signals in the individual signal conductors of one signal conductor layer do not interfere with the signals in the individual signal conductors of a further signal conductor layer. Furthermore the separating cylindrical shield as the at least one ground conductor serves as a reference impedance potential. A tubular spacer can be disposed within said at least one signal conductor layer and serves as a filler or stabiliser about which the flat cables are cylindrically arranged. The tubular spacer is constructed from a solid material, from a stranded material or is in the form of a hollow tube. In the latter case the interior of the tubular spacer can carry fluids or further electric leads, for example for control signals or power. Preferably an inner cylindrical shield is disposed between the tubular spacer and the at least one signal conductor layer to provide further protection against interfering electromagnetic fields when the interior of the tubular spacer carries further electric leads and also acts as a reference ground potential. In another improvement of the invention an outer cylindrical shield is disposed between an outer one of said at least one signal conductor layer and the outer ground shield and is separated from the outer ground shield by a first insulation layer. A second insulation layer is further disposed between said outer ground shield and a jacket. These shields, insulation layers and outer jacket protect the interior of the electrical signal cable from mechanical damage and also from external interfering magnetic fields. The flat cables in the electrical signal cable are constructed from an upper insulator attached to a lower insulator which are, in the preferred embodiment of the invention, laminated to each other. In another embodiment of the invention, the upper insulator is adhered to the lower insulator by an adhesive which is selected from the group of thermoplastic adhesives comprising polyester or polyurethane. Furthermore a coating of adhesion promoters made from fluorinated copolymers, such as fluorinated ethylene/propylene and perfluoroalkoxy, from epoxy resin adhesives, from amino resin adhesives, from phenolic resin adhesives or from silicone adhesives can be used to bond the layers together. The insulator is formed from the group of insulating materials consisting of polyester, perfluoralkoxy, fluoroethylene-propylene, polyolefins including polyethylene and polypropylene, polymethylpentene, polytetrafluoroethylene or expanded polytetrafluorethylene. Preferably a fluoropolymer is used and most preferably the upper insulator and the lower insulator are formed from expanded polytetrafluorethylene (ePTFE). Expanded PTFE has a very low dielectric constant and is light in weight. This ensures that the electrical properties of the assembly are extremely good and also that the electrical signal cable constructed using an ePTFE dielectric material is light in weight which allows for easy handling of the cable. Furthermore the flexlife properties of cables made from ePTFE are known to be good and thus the assembly constructed using this dielectric material also has good 6 flexlife properties. The assembly could also be constructed using an extruded polymer or a foamed polymer. In another preferred embodiment of the invention a plurality of signal conductor layers are stacked on each other, each signal conductor layer comprising a plurality of coplanar electrical signal conductors encased within an insulator and being separated from each other by a pitch distance, The pitch distance is between 0,1 mm and 10 mm and the characteristic impedance of the electrical signal cable is in the range of 50 Ω to 200 Ω. In a further development of the invention, a shielding strip or ground plane is situated between at least two of said signal conductor layers to electromagnetically shield the signal conductors in one signal conductor layer from the signals being carried on the signal conductors in another one of the signal conductor layers. Using the shielding strip, the cross-talk between the signal conductors in two adjacent signal conductor layers is reduced to more than acceptable levels. The shielding strip can be attached to the insulators by lamination bonding. In a further improvement of the invention, first shielding means surrounding said signal conductor layers are provided in electrical contact with at least one end of the said shielding strips. The ends of the shielding strips are thus mechanically protected from damage and can also not act as antennas. Furthermore, an insulating layer can be provided which surrounds said first shielding means and then second shielding means are provided surrounding said insulating layer. The second shielding means shield the signal conductors within the signal conductor layers from stray electromagnetic fields outside the electrical signal cable. A cable jacket is then placed over the second shielding means surrounding said signal conductor layers to protect the complete electrical signal cable assembly from mechanical damage. In one embodiment of the invention, at least one spacer is disposed within the cable jacket for shaping the electrical signal line, i.e. for holding the signal conductor layers in place within the cable jacket. The signal conductor layers can be arranged substantially in parallel planes to each other in which case two crescent-shaped spacers are provided. The signal conductor layers can also be arranged helically around the spacer in which case the spacer is cylindrical in shape. Description of the Drawings Fig. shows one embodiment of an electrical signal cable of the invention in cross Section. Figs, la- Id illustrate different embodiments of signal conductors and ground conductors used in the electrical signal cable of the invention. Fig. 2 shows another preferred embodiment of an electrical signal cable in cross section. Fig- 3 shows an electrical signal cable of the invention in perspective view. Fig. 4 shows another embodiment of the electrical signal cable assembly of the invention. Fig. 5 shows a cross section of a flat cable used in the inventive electrical signal cable. Fig. 6 shows a method of manufacturing the plurality of signal conductor layers for the inventive electrical signal cable. Fig. 7 shows a sintering device used in the manufacture of the signal conductor layers. Fig. 8 shows a diagram of an apparatus for testing the flexlife of the electrical signal cable. Fig- 9 shows a further embodiment of the invention. Fig. 10 shows a further embodiment of the invention. Fig. 1 1 shows a further embodiment of the invention. Fig. 12 shows the electrical signal cable according to a another preferred embodiment of the invention. Fig. 13 shows a method for the manufacture of the electrical signal cable of Fig. 12. Fig. 14 shows the electrical signal cable according to another embodiment of the invention. Fig. 15 shows the electrical signal cable according to another embodiment of the invention. Fig. 16 shows a further example of a signal conductor layers suitable for use in the invention. Fig. 17 shows a further embodiment of the electrical signal cable of the invention. Detailed Description of the Invention A first preferred embodiment of an electrical signal cable 10 of the invention shown in Fig. 1 includes a plurality of electrical conductors of type 1 to 4 which are arranged in coaxial planes about a central axis 9 of the electrical signal cable 10 and which are surrounded by a jacket 5 which is for example made from expanded polytetrafluoroethylene (PTFE). Each electrical conductor 1 to 4 is a signal conductor or a ground conductor or an assembly of at least one signal conductor and at lest one ground conductor. It is important to note that the term "conductor" used herein is not limited to round wires but should also include any geometric type of wire, such as for example plain conductors, flat conductors, shield conductors, net shaped conductors, braided shield conductors, foil type conductors, hollow conductors or braided conductors in different cross section geometries, such as for example edged, triangle, quarter type and so forth. The electrical conductors depcited in Figs, la to Id show different examples of assemblies including one signal conductor and one ground conductor. Electrical signal conductor 1 includes one signal conductor and one ground conductor arranged as twisted pair (Fig. la). Electrical signal conductor 2 includes one signal conductor and one ground conductor arranged as ribbon pair (Fig. lb). Electrical signal conductor 3 includes one signal conductor and one ground conductor in a coaxial arrangement without an outer jacket (Fig. lc). Electrical signal conductor 4 includes one signal conductor and one ground conductor, wherein the signal conductor 4a includes an insulating coating 4c whereas the ground conductor 4b is a plain uninsulated wire, called a drainwire (Fig. Id). In another embodiment shown in Fig. 2 round electrical conductors 7 are signal conductors with an insulating coating 7c similar to signal conductors 4a of the embodiment according to Fig. 1. Instead of ground conductors 4b there is provided at least one shield conductor 8 which is circularly shaped in cross section. This circular ground conductor 8 shields the inner signal conductors from the outer signal conductors. If necessary further circular ground conductors 8 can be provided for further separating groups of signal conductors 7. The capacitance per unit length of the electrical signal cables 10 shown in Fig. 1 and 2 is less than 48 pF/m. In the following with reference to Figs. 3 to 1 1 there are described several further embodiments of an electrical signal cable 10 of the invention, wherein at least one signal conductor layer is cylindrically arranged around a central axis, and one signal conductor layer comprises a plurality of flat cables. Each of said flat cables has a plurality of coplanar electrical signal conductors encased within and separated at a pitch distance (a) from each other by a flat cable insulator. The signal conductors and at least one ground conductor are arranged in different layers, wherein the signal conductors are arranged in several flat cables. The signal conductor layer is also called subcable assembly. Figs. 3 and 4 show two embodiments of the cylindrical electrical signal cable assembly 10 of the invention having a central axis 15. The term cylindrical used in this context does not imply that the assembly be geometrically exactly cylindrical. Rather an assembly 10 whose dimensions are substantially cylindrical is also covered. Such a substantially cylindrical assembly 10 could be formed when outside forces exert pressure on the surface of the assembly 10 and "squash" one side of the assembly 10 to form an assembly 10 with an oval cross-section. In all figures, like numerals are used to denote like elements. The assembly 10 comprises an optional tubular spacer 20 forming the central core of the assembly 10. The tubular spacer 20, if present, is surrounded by a cylindrically arranged inner cylindrical shield 30 onto which is disposed a first signal conductor layer 40. The structure of the signal conductor layer 40 will be described later. The tubular spacer 20 is made from permeable ePTFE, PTFE, polyamide, polyurethane, persion or any other suitable material. The tubular spacer 20 may be solid or have a hollow interior to carry cooling fluids, electrical control lines, electrical power lines, gases etc. The tubular spacer 20 may further be made from a braided or stranded material. In the embodiment of the invention depicted in Figs. 3 and 4, the first signal conductor layer 40 is disposed within a second signal conductor layer 60 and is separated from the second signal conductor layer 60 by a separating ground conductor in the form of a cylindrical shield 50. The second signal conductor layer 60 has the same structure as the first signal conductor layer 40. It is possible to conceive of an embodiment of the invention in which no further signal conductor layers are present. It is also possible to conceive of an embodiment in which further signal conductor layer are disposed about the second signal conductor layer 60 and separated from the second signal conductor layer 60 by further separating cylindrical shields. An outer cylindrical shield 70 is disposed about the outermost one of the signal conductor layers 40, 60. In the embodiment of Figs. 3 and 4 the outer cylindrical shield 70 is disposed about the second signal conductor layer 60. A first insulating layer 80 is arranged about the outer cylindrical shield 70 and on this is disposed an outer shield 90. The outer shield 90 is grounded and shields the signal conductor layers 40, 60 within the assembly 10 from interfering electromagnetic fields. A second insulating layer 100 is disposed about the outer ground shield 90 and the electrical signal cable 10 is then placed within a jacket 110. The first insulating layer 80 and the second insulating layer 100 are made, for example, from PTFE, ePTFE, FEP or polyester. Preferably the first insulating layer 80 and the second insulting layer 100 are made from sintered ePTFE tape and is wrapped about the electrical signal cable 10 using known wire- wrapping techniques. The ePTFE tape used is preferably manufactured in accordance with the techniques taught in US-A-3 953 556, US-A-4 187 390 or US-A-4443 657. The inner cylindrical shield 30 and the separating cylindrical shield 50 are braid, foil, surfed, woven or wire shields made from a metal or a metallised polymer, such as copper, aluminium, tin-plated copper, 10 silver-plated copper, nickel-plated copper, alloys or a metallised fabric such as aluminised polyester. Later embodiments of the invention will be described in which no inner cylindrical shield 30 or separating cylindrical shield 50 are present. The outer ground shield 90 is a braid, foil or wire shield made from a metal or a metallised polymer, such as copper, aluminium, tin-plated copper, silver-plated copper, nickel-plated copper, alloys or aluminised polyester. In one embodiment of the invention, the outer ground shield 90 is made from a copper braid with a braiding angel of about 35°. In some applications, the outer ground shield 90 can be omitted. The jacket 110 is made from silicone or polyolefins such as polyethylene, polypropylene or polyethylpentene; fluorinated polymers such as fluorinated ethylene/propylene (FEP); fluorinated polymers such as fluorinated ethylene/propylene (FEP); fluorinated alkoxypolymer such as perfluoro(alkoxy)alkylanes, e.g. a co-polymer of TFE and perfluorproplyvinyl ether (PFA); polyurethane (PU), polyvinylchloride (PVC), silicone, polytetralfluoroethylene (PTFE) or expanded PTFE. In one embodiment of the invention the jacket 1 10 was made from PVC. In a further embodiment of the invention the jacket 110 is made from ePTFE reinforced with silicone. As can be seen by close inspection of Fig. 3 the first signal conductor layer 40 and the second signal conductor layer 60 are made from a plurality of flat cables 45 which are braided together in a first embodiment of the invention. Braiding techniques are known in the art and suitable machines are available from Ratera in Manresa, Spain, SPIRKA Maschinenbau GmbH in Alfeld (Leine), Germany, Magnatech International, Inc., in Sinking Spring, USA, and Steeger GmbH & Co., Wuppertal, Germany. In the embodiment of the invention depicted in Fig. 4, the first signal conductor layer 40 and the second signal conductor layer 60 are formed respectively from two layers 42a and 42b and 62a and 62b respectively. Each of the two layers 42a, 42b, 62a, 62b is formed of one or a plurality of flat cables 45 which are wrapped or surfed in opposite directions around the electrical signal cable 10. Surfing the flat cables 45 in opposite directions has the advantage that cross-talk between the individual signal conductors 130 in the different flat cables 45 is reduced. A first one of the layers 42a, 62a is wrapped in a first direction. A second one of the layers 42b, 62b is wrapped in a second direction as shown in the Fig. Wrapping techniques are known in the art and suitable machines are available from Ridgeway & Co., Leicester, UK; Roblon, Denmark; Innocable, France or Stolberger, Germany. 11 Fig. 5 shows a cross-sectional view of the flat cables 45 used in the embodiments of Figs. 3 and 4.. The flat cable 45 is made up of a plurality of individual signal conductors 130 arranged in a parallel plane and surrounded by an upper insulating layer 120a and a lower insulting layer 120b. In this example sixteen individual signal conductors 130 are shown spaced at a pitch distance i of 0.35 mm. However, this is not limiting of the invention and different pitch distances a of number of individual signal conductors 130 could be used. The upper insulating layer 120a and the lower insulating layer 120b are laminated together as will be explained later. Whilst the flat cable 45 is shown in this embodiment as being laminated, it would be possible to use extrusion techniques to extrude the individual signal conductors 130 for example within a polyurethane, an FEP-based or a polyester layer. Alternatively foamed or solid polyethylene could be used. Furthermore the upper insulating layer 120a and the lower insulating layer 120b could be adhered together using a thermoplastic adhesive such as a polyester adhesive or a polyurethane adhesive. The individual signal conductors 130 can be made from any conducting material such as copper, nickel-plated copper, tin-plated copper, silver-plated copper, tin-plated alloys, silver-plated alloys or copper alloys. The individual signal conductors 130 may be lacquered. Preferably the individual signal conductors used in the invention are made from round alloy wire. It would also be possible to use flat conductors. The number of individual signal conductors 130 depicted in Fig. 5 is not intended to limiting of the invention. The axes of the individual signal conductors 130 are separated by a first pitch distance a which is in the range of 0,1 to 10 mm. The upper insulating layer 120a and the lower insulating layer 120b can be made of any insulating dielectric material such as polyethylene, polyester, perfluoralkoxy, fluoroethylene-propylene, polypropylene, polymethylpentene, polytetrafluoroethylene or expanded polytetrafluorethylene. Preferably expanded polytetrafluoroethylene such as that described in US-A-3 953 556, US-A-4 187 390 or US-A-4443 657 is used. Manufacture of the flat cable 45 is illustrated in Fig. 6 for the embodiment in which the upper insulating layer 120a and the lower insulating layer 120b are made from expanded PTFE. This method is essentially the same as that taught in US-A-3082292 (Gore). A plurality of individual signal conductors 130, an upper insulator 120a located above the plurality of individual signal conductors 130, and a lower insulator 120b located below the plurality of individual signal conductors 130 were communally passed between two heated contra-rotating pressure rollers 400a and 400b at a lamination temperature sufficient to achieve bonding between the lower insulator 120b and the upper insulator 120a, e.g. between 327°C and 410 °C. The flat cable 45 was thereby formed. For this purpose, the upper pressure roller 400a is provided with a number of upper peripheral grooves 410a each separated by an upper peripheral rib 420a which are lined up at a distance from one another along the circumference of the pressure rollers 400a. Similarly, the lower pressure rollers 400b is provided with a number of lower 12 peripheral grooves 410b each separated by a lower peripheral rib 420b which are lined up at a distance from one another along the circumference of the pressure roller 400b. Each upper peripheral groove 410a of the upper pressure roller 400a together with the adjacent upper peripheral ribs 420a lines up with one of the lower peripheral grooves 410b with the adjacent lower peripheral ribs 420b of the lower pressure roller 400b to form a passageway channel for one of the electrical signal conductors 130. The distances (a) between the two pressure rollers 400a, 400b and the peripheral grooves 410a, 410b are designed in terms of their dimensions in such a way that a single conductor 410 and the upper insulator 420a and the lower insulator 420b pass continuously between a pair consisting of one of the upper peripheral grooves 410a and one of the lower peripheral grooves 410b. The upper peripheral ribs 420a and the lower peripheral ribs 420b have such a small separation from one other that the upper insulator 420a and the lower insulator 420b are firmly pressed together at these positions to form an intermediate zone 440 in the flat cable 45. In order to improve their adhesion of the upper insulator 120a to the lower insulator 120b to the individual signal conductors 130 and with each other within the flat cable 45, the flat cable 45 was led through a sintering device in which the flat cable 45 is heated such that one achieves intimate joining in the intermediate zones 140 of the flat cable 45. If using an upper insulator 120a and a lower insulator 120b made of PTFE, use is made of a sintering temperature in the range from 327° to 410°C. An example of an embodiment of a sintering device in the form of a sintering oven 150 comprising a salt bath is illustrated in a schematic and simplified form in Figure 7. In this example flat cable 45 is continually passed through the sintering oven 150. Flex-life measurements are made using an apparatus as shown in Fig. 8. A one meter long sample of the electrical signal cable 10 to be tested is attached to a movable attachment 520 and hung with a weight 540 of 500g. The movable attachment 520 could swing a 30 cm long first end 525 of the electrical signal cable assembly 10 through an angle of ±90° as shown by the arrow in Fig. 8. The bending radius was 50 mm. A cycle counter 530 is used to count the number of cycles through which the first end 525 of the electrical signal cable 10 was swung, i.e. from the zero, upright position to +90°, back to the zero position, to the -90° position and then back to the zero position. Stops 510a, 510b prevent the other end of the electrical signal cable 10 from being swung. A measurement of the ohmic resistance of the individual signal conductors 130 within the electrical signal cable 10 was carried out by connecting sixteen individual signal conductors 130 in one flat cable 45 in parallel with each other. Eight flat cables 45 were connected in series with each other. The total ohmic resistance of the electrical signal cable assembly 10 comprising eight flat cables 45 each with sixteen individual signal conductors 130 was measured at the beginning of the measurement cycles and then the number 13 of measurement cycle carried out until the ohmic resistance had increased by 10%. A maximum of 180 000 cycles was, however, carried out and then by plotting the results obtained to the end of the measurement, the number of cycles required to increase the ohmic resistance by 10% was calculated by plotting a curve on the graph. The measurements are carried out at a temperature of 22±2°C. Further details of the test are found in German National Standard DIN 0472 603 test F. In the foregoing, the layers 40 and 60 and the flat cable 45 are described as including only electrical signal conductors. However, it is also possible, that such a layer 40, 60 or such a flat cable 45 also includes electrical ground conductors which separate one ore more electrical signal conductors from each other. This is for example possible by just connecting some of the electrical signal conductors of one layer 40, 60 or one flat cable 45 to a ground potential. Such an arrangement of electrical signal and ground conductors in one layer 40, 60 or one flat cable 45 includes for example alternating electrical signal conductors (S) and electrical ground conductors (G), termed a GS-arrangement. In other embodiments arrangements such as GSGGSGGSG.... or GSSGGSSG.... or ...GSGSG... are realised. The flat cable 45 with these different alternating arrangements of electrical ground conductors (G) and electrical signal conductors (S) can be used to construct an electrical signal cable 10 of the invention. In the following with reference to Figs. 12 to 19 there are explained other preferred embodiments of an electrical signal cable of the invention wherein a plurality of signal conductor layers 1020, 1120, 1220, 1320, 1620, 1720 are stacked on each other. Each signal conductor layer 1020, 1120, 1220, 1320, 1620, 1720 comprises a plurality of coplanar electrical signal conductors 1030, 1130, 1230, 1330, 1630, 1730 encased within an insulator 1040a, 1040b and being separated from each other by a first pitch distance a. Each stack can additionally include ground conductors in which case the planar ground conductors in the form of layers 1050, 1150, 1250, 1650, 1750 between each stack can be partly or completely omitted. Fig. 12 shows another embodiment of the invention. It shows an electrical signal cable 1010 comprising a plurality of subcable assemblies or signal conductor layers 1020. In the embodiment of Fig. 12 eight signal conductor layers 1020 are shown. However, this is merely illustrative of the invention and not intended to be limiting. Each signal conductor layer 1020 comprises a plurality of individual signal conductors 1030 arranged in a parallel plane and surrounded by an upper insulating layer 1040a and a lower insulating layer 1040b. The upper insulating layer 1040a and the lower insulating layer 1040b are laminated together 14 using the techniques of Figs. 6 and 7 and explained earlier. The individual signal conductors 1030 can be made from any conducting material as described earlier. The number of individual signal conductors 1030 depicted in Fig. 12 is not intended to limiting of the invention. The axes of the individual signal conductors 1030 are separated by a first pitch distance j i which is in the range of 0,1 to 1 mm. The upper insulating layer 1040a and the lower insulating layer 1040b can be made of any insulating dielectric material as described earlier. The signal conductor layers 1020 are separated from each other by a shielding strip 1050. The shielding strip 1050 is made for example from a metal foil, metal braid, conductive tape or a metallised textile. The following metals can be used: copper, tin, silver, aluminium or alloys thereof. In one embodiment of the invention the shielding strip 1050 was made from copper-coated polyamide fabric of the Kassel type supplied by the Statex company in Hamburg, Germany, and had a thickness of approximately 0,1 mm and a width of around 9 mm. The signal conductor layers 1020 were arranged in a planar manner, one above another, to form a bundle of signal conductor layers 1020 using the apparatus 1 100 shown diagramtically in Fig. 13. Fig. 13 shows a plurality of first spools 1 102 onto which is rolled a first strip 1103 forming the signal conductor layers 1020 and a plurality of second spools 1 104 onto which is rolled a second strip 1105 forming the shielding strip 1050. A plurality of first (subcable assembly or signal conductor layer) strips 1 103, separated from each other by a second (shielding) strip 1 105 is rolled respectively off the plurality of first spools 1102 and the plurality of second spools 1 104 and joined together at position 1 106 to form a bundle 1107. The thus created bundle 1107 of signal conductor layers 1020 was slid into a tube which forms a first shielding means 1060. The first shielding means 1060 may be made of a metal foil, such as a foil made from copper, aluminium or silver, or from metallised textile. In one embodiment of the invention the first shielding means was made from Kassel-type copper-coated polyamide fabric supplied by the Statex company in Hamburg, Germany, and had a thickness of approximately 0,1 mm and a width of around 9 mm. Crescent-shaped Spacers 1090 were positioned between the plurality of subcable assemblies 1020 and the first shielding means 1060 in order to maintain a substantially tubular shape. The spacers 1090 are made from permeable ePTFE, PTFE, polyamide, perlon or any other insulating material. 15 Shielding strip ends 1055 project beyond an edge 1025 of the subcable assemblies 1020 and are bent downwardly or upwardly such that each shielding strip end 1055 touches another one of the shielding strip ends 1055. At least one of the shielding strip ends 1055 is in electrical contact with the first shielding means 1060. In Fig. 12, the shielding strip ends 1057 of the outermost ones of the plurality of signal conductor layers 1020 and the signal conductor layers 1020 immediately adjacent to the outermost ones of the signal conductor layers 1020 is shown as being in electrical contact with the first shielding means 1060. Each of the shielding strips 1050 and the first shielding means 1060 are therefore held at the same potential. It would, of course, be possible to hold the shielding strips 1050 and the first shielding means 1060 at a different potential. In this latter case the shielding strip ends 1055 would not electrically contact with the shielding means 1060. An insulating layer 1065 was then wrapped around the first shielding means 1060 using known wire wrapping techniques. The insulating layer 1065 may be made, for example, from PTFE, FEP, ePTFE or polyester. Preferably the insulating layer 1065 is made from sintered ePTFE tape as described above. A second shielding means 1070 surrounds the first shielding means 1060. The second shielded means 1070 is a braid, foil or wire shield made from a metal or a metallised polymer, such as copper, aluminium, tin-plated copper, silver-plated copper, nickel-plated copper or aluminised polyester. In one embodiment of the invention the second shielding means 1070 is made from a copper braid with a braiding angle of about 35°. A jacket 1080 is placed over the second shielding means 1070. The jacket 1080 is made from the materials described earlier. Another embodiment of the invention is shown in Fig. 14. In this figure, the same reference numerals are used to denote components of the electrical signal cable 1 1 10 having the same function as the components of the electrical signal cable 1010 of Fig. 12 except that the numerals are increased by 100. In this embodiment of the invention a tubular spacer 1 190 is used in the core of the electrical signal cable 1 110 and the signal conductor layers 1120 are wrapped in a helical manner with an axis through the core 1200 of the electrical signal line 1110. The materials used for the construction of this embodiment of the electrical signal cable 1110 are the same as those used above. This embodiment of the electrical signal cable 1110 has the advantage that it is substantially more flexible than the foregoing embodiment 1010. 16 A further embodiment of the electrical signal cable 1210 is shown in Fig. 15. Again the same reference numerals are used to denote components of the electrical signal cable 1210 having the same function as the components of the electrical signal 10 line 10 of Fig. 12 or the electrical signal cable 1 1 10 of Fig. 13 except that the numerals are increased by a further 100. In this embodiment of the electrical signal line, the plurality of signal conductor layers 1220 are twisted before being placed within the first shielding means 1270 thus obtaining a substantially more flexible electrical signal cable 1210. Again the same materials are used for the construction of this electrical signal cable 1210 as are described in the embodiment of Fig. 12. Fig. 16 shows a further example of a signal conductor layer 1620 which comprises a plurality of individual signal electrical signal conductors 1630 arranged in a parallel plane and surrounded by an upper insulating layer 1640a and a lower insulating layer 1640b. The signal conductor layer 1620 further included an upper shielding means 1650a and a lower shielding means 1650b attached to the outer surfaces of the upper insulting layer 1640a and the lower insulating layer 1640b, respectively. The upper shielding means 1650a and the lower shielding means 1650b can be made, for example, from copper or aluminium foil, perforated copper foil or metallised polyamide. In the preferred embodiment they are made from copper foil. The upper shielding means 1650a and the lower shielding means 1650b are joined to each other at ends 1660a and 1660b as shown in Fig. 16. A jacket 1680 made from ePTFE was attached to the upper shielding means 1650a and the lower shielding means 1650b. The jacket 1680 could also be made from PFA, FEP or PTFE. Manufacture of the embodiment of the signal conductor layer 1620 depicted in Fig. 16 is carried out in a similar manner as the signal conductor layer 1320 described above and depicted in Fig.7. In addition to the upper insulator 1340a and the lower insulator 1340b being passed through the contra-rotating pressure rollwer 1400a and 1400b at a lamination temperature, the material to form the upper shielding means 1650a, the lower shielding means 1650b and the jacket 1680 are additionally passed through contra-rotating pressure rollers at a temperature sufficient to ensure that the upper shielding means 1650a and the lower shielding means 1650b are laminated to the upper insulating 1640a and the lower insulator 1640b and to each other at the ends 1660a, 1660b. Use of the laminated upper shielding means 1650a and the lower shielding means 1650b allows the construction of an electrical signal cable 1010 with a plurality of subcable assemblies orsignal conductor layer 1620 without a shielding strip 1050 placed between the signal conductor layer 1620. 17 Examples Example 1 This was constructed as shown in Fig. 3. The tubular spacer 20 was made of a polyurethane tube and had an outer diameter of 4.0 mm. The inner cylindrical shield 30 (outer diameter 4.4 mm), the separating cylindrical shield 50 (outer diameter 6.0 mm) and the outer cylindrical shield 70 (outer diameter 7.6 mm) were made of a copper-plated polyamide fabric supplied by the Statex company of Bremen, Germany under the trade name KASSEL. The first signal conductor layer 40 (outer diameter 5.6 mm) and the second signal conductor layer 60 (outer diameter 7.2 mm) were both made from four flat cables 45 each containing sixteen individual signal conductors 130 of AWG 4007 made from PD 135 alloy obtainable from Fhelps Dodge in Irvine, California, at a pitch distance of 0.35 mm laminated between ePTFE with a dielectric constant of 1.3. These were braided at a braiding angle of 20-22°. The first insulation 80 (outer diameter 7.7 mm) and the second insulation 100 (outer diameter 8.2 mm) were made of an ePTFE GORE-TEX binder available from W.L.Gore & Associates. The outer shield 90 was made from braided bare alloy wire constructed with a braiding angle of approx. 40° using 36 bobbins with eight ends at 10 picks/inch (2.54 cm). The jacket 110 was made from PVC and had an outer diameter of 10.0 mm. Flexlife tests carried out on the electrical signal cable array showed that the ohmic resistance increased by 10% between only 6000 and 60 000 cycles. This was thought to be due to the extremely sharp braiding angle of the subcable assemblies 40 and 60. Example 2 This was constructed as shown in Fig. 4. The tubular spacer 20 was made of a ePTFE joint sealant filler (JSF 50) obtainable from W. L. Gore & Associates, Putzbrunn, Germany, having an outer diameter of 4.0 mm. The inner cylindrical shield 30 (outer diameter 4.4 mm), the separating cylindrical shield 50 (outer diameter 6.0 mm) and the outer cylindrical shield 70 (outer diameter 7.6 mm) were made of a copper-plated polyamide fabric supplied under the trade name KASSEL by the Statex company of Bremen, Germany. The first signal conductor layer 40 (outer diameter 5.6 mm) and the second signal conductor layer 60 (outer diameter 7.2 mm) were both made from two flat cables 45 each containing 32 individual signal conductors 130 of AWG 4207 (0.07 mm) made from PD135 alloy at a pitch distance of 0.35 mm laminated between ePTFE with a dielectric constant of 1.3. The first layers 42a, 62a of the first and second signal conductor layer 40, 60 were formed from one of the flat cables 45. The second layers 42b, 62b of the first and second signal conductor layer 40, 18 60 were formed from the other one of the flat cables 45. These were wrapped at a wrapping angle of 40°. The first insulation 80 (outer diameter 7.7 mm) and the second insulation 100 (outer diameter 8.2 mm) were made of an ePTFE tape binder. The outer shield 90 (outer diameter 8.1 mm) was made from braided bare alloy wire with a braiding angle of 40°. The jacket 110 was made from silicone- reinforced ePTFE obtainable from W. L. Gore & Associates, Phoenix, Arizona, under the name SILKORE and had an outer diameter of 10.5 mm. Flexlife tests carried out on the electrical signal cable array showed that the ohmic resistance increased by 10%) after between 25 000 and 1 10 000 cycles. The capacitance between one of the individual signal conductors 130 and the outer shield 90 was 57 pF/m. Example 3 This is constructed as shown in Fig. 9. This example has a first signal conductor layer 40 which is identical to the first signal conductor layer of Example 2 and having an outer diameter of 5.6 mm. The example has, however, two second signal conductor layer 60' and 60" each containing flat cables 45 with forty-eight individual signal conductors 130 of AWG 4207 (0.07 mm) laminated in the same manner as the flat cables 45 of example 2 and separated by a further cylindrical separating shield 50'. The outer diameter of the first one of the second signal conductor layer 60' has an outer diameter of 7.4 mm and the second one of the second signal conductor layer 60" has an outer diameter of 9.0 mm. The further cylindrical separating shield 50' has an outer diameter of 7.8 mm. As a result of the incorporation of the two second signal conductor layer 60' and 60", the outer diameter of some of the outer layers of the electrical signal cable 10 changes as follows. The outer cylindrical shield 70 has an outer diameter of 9.4 mm, the first insulation 80 had an outer diameter of 9.5 mm and the second insulation 100 had outer diameter of 9.9 mm whilst the outer shield's 90 outer diameter was 9.8. Finally the jacket 110 had an outer diameter of 11.4 mm. In this example an electrical signal cable 10 with 256 individual signal conductors 130 is formed. Example 4 This was constructed as shown in Fig. 10. The tubular spacer 20 was made of an ePTFE joint sealant filler (JSF50) obtainable from W. L. Gore & Associates with an outer diameter of 4.0 mm. In contradistinction to Example 1, there was no inner cylindrical shield 30, separating cylindrical shield 19 50 and outer cylindrical shield 70. A first signal conductor layer 40' and two second signal conductor layer 60' and 60" were both made from four flat cables 45 each containing sixteen individual signal conductors 130 made from PD 135 alloy of AWG 4207 at a pitch distance of 0.35 mm laminated between ePTFE with a dielectric constant of 1.3. These were braided at a braiding angle of approx. 40°. The signal conductor layer 40', 60' and 60" had outer diameter of 5.2 mm, 6.4 mm and 7.6 mm respectively. The outer shield 90 had an outer diameter of 8.0 mm and was made from braided PD 135 alloy wire of AWG 4001 obtainable from Fhelps Dodge in Irvine, California, and was braided using 36 bobbins with eight ends at 10 picks per inch (2.54 cm) with a braiding angle of approx. 35°. The jacket 1 10 was made from silicone-reinforced ePTFE. With a wall thickness of approx. 1mm and had an outer diameter of 10.5 mm. The subcable assemblies 40', 60' and 60" together with the outer shield formed a core which was pulled into the jacket 1 10. Grounding between individual signal conductors 130 in this example is achieved by using every second individual signal conductor 130 as a ground conductor 130. Flexlife tests carried out on the electrical signal cable array showed that the ohmic resistance increased by substantially less than 10% after 180 000 cycles at which point the flexlife test was stopped Example 5 This example is identical with that of Example 1 except that the flat cables 45 made from individual signal conductors of AWG 4207 (0.07 mm diameter) made from PD135 alloy were braided at an angle of 20° and the tubular spacer 20 was made of polyurethane Example 6 This example is identical with that of Example 4 except that the flat cables 45 were braided at an angle of 20° and individual signal conductors 130 of AWG 4007 (0.08 mm diameter) made from PD135 alloy were used. Flexlife tests carried out on the electrical signal cable array 10 showed that the ohmic resistance increased by substantially less than 10% after 240 000 cycles at which point the flexlife test was stopped Example 7 20 This example was identical with that of Example 4 except that the flat cables 45 were surfed or wrapped at an angle of 20° instead of being braided and individual signal conductors 130 of AWG 4007 (0.08 mm diameter) made from PD135 alloy were used. A polyurethane tube was used as the tubular spacer 20. Flexlife tests carried out on the electrical signal cable array 10 showed that the ohmic resistance increased by substantially less than 10% after 240 000 cycles at which point the flexlife test was stopped Example 8 This example was identical with that of Example 2 except that individual signal conductors 130 of AWG 4007 (0.08 mm diameter) made from PD135 alloy were used and a polyurethane tube was used as the tubular spacer 20. Flexlife tests carried out on the electrical signal cable array showed that the ohmic resistance increased by substantially less than 10% after 180 000 cycles at which point the flexlife test was stopped. Example 9 This example is illustrated in Fig. 1 1. The tubular spacer 20 was made of an ePTFE joint sealant filler (JSF 50) obtainable from W. L. Gore & Associates. The first signal conductor layer 40 was constructed from four flat cables 45, denoted LI, L2, L3 and L4, each containing sixteen individual signal conductors 130 of AWG 4007 (0.08 mm diameter) made from PD 135 alloy at a pitch distance of 0.35 mm laminated between ePTFE with a dielectric constant of 1.3. The outer cylindrical shield 70 was made from the KASSEL foil obtainable from the Statex company of Bremen, Germany. The first insulating layer 80 was made from an ePTFE tape binder. Example 10 This example was identical with that of Example 2 except that a polyurethane tube was used as the tubular spacer 20 and the jacket was made of PVC. Flexlife tests carried out on the electrical signal cable array showed that the ohmic resistance increased by substantially less than 10% after 180 000 cycles at which point the flexlife test was stopped. 21 Example 11 This example was identical with that of Example 2 except that the individual signal conductors were made of wire of AWG 4007 and the jacket was made of PVC. Flexlife tests carried out on the electrical signal cable array showed that the ohmic resistance increased by substantially less than 10% after 180 000 cycles at which point the flexlife test was stopped. Example 12 The construction of this example is depicted in Fig. 17 in which the same reference numerals are used to denote the same feature as those in Fig. 12 except that the numerals are increased by 700. The individual signal conductors 1730 were made from AWG 4001 (0.08 mm diameter) silver-plated copper wire and embedded within an upper insulating layer 1740a and a lower insulating layer 1740b of ePTFE GORE- TEX® tapes made in the Putzbrunn, Germany, plant of W. L. Gore & Associates. Each signal conductor layer 1720 contained sixteen of the individual signal conductors 1730. The pitch distancea between the individual signal conductors was 0.35 mm. Four signal conductor layer 1720 were bundled together on top of each other with no shielding strip 1750 between them to form a signal conductor layer bundle 1725. A pair of signal conductor layer bundles 1725 were then placed together with a shielding strip 1750 made of Kassel copper-coated polyamide fabric supplied by the Statex company. The pair of signal conductor layer bundles 1725 were slipped inside a tube forming the first shielding means 1760 and made of Kassel copper-coated polyamide fabric. One of a shielding strip end 1755 was placed in electrical contact with the first shielding means 1765. An insulating layer 1765 of ePTFE GORE-TEX® insulating tape was subsequently wrapped around the first shielding means 1760. The second shielding means 1770 was made of tin-coated copper braid and a jacket 1780 made from polyvinyl chloride was then slipped over the insulating layer 1765. An electrical signal line cable assembly 1710 containing eight signal conductor layer 1720 and 128 individual signal conductors 1730 was thus obtained. Example 13 This example was constructed according to the embodiment of the invention as depicted in Fig. 12. The individual signal conductors 1030 were made from AWG 4001 silver-plated copper wire and embedded within an upper insulating layer 1040a and a lower insulating layer 1040b of GORE-TEX® tapes made 22 in the Putzbrunn, Germany, plant of W. L. Gore & Associates. Each signal conductor layer 1020 contained sixteen of the individual signal conductors 1030. The pitch distancea between the individual signal conductors was 0.35 mm. Eight signal conductor layer 1020 were bundled together on top of each other with a shielding strip 1050 strip made of Kassel copper-coated polyamide fabric supplied by the Statex company between each of the signal conductor layer 1020. The shielding strip ends 1055 were placed in electrical contact with the first shielding means 1065. The eight signal conductor layers 1020 were slipped inside a tube made of Kassel-type copper-coated polyamide fabric forming the first shielding means 1060 and an insulating layer 1065 of GORE-TEX® insulating tape was wrapped around the Kassel fabric. The second shielding means 1070 was made of tin-plated copper braid and a jacket 1080 made from polyvinyl chloride was then slipped over the insulating layer. An electrical signal line cable 1010 containing 8 layers and 128 individual signal conductors was thus obtained. Example 14 This was manufactured in the same manner and using the same materials as example 13 except that AWG 4207 (0.07 mm diameter) PD135 alloy wire was used. Comparative Example 15 As a comparison a conventional flat cable comprising a bundle of 132 miniature co-axial cables was used. The conductors were made of AWG 4207 (0.07 mm diameter) silver-plated alloy wire, the insulator of ePTFE and the outer conductor of silver-plated copper. A jacket of a fluoropolymer was extruded over the outer conductor. A shield of tin-plated copper was braided over the bundle of 132 miniature co-axial cables and a jacket tube of PVC was extruded over the braided shield. This electrical signal line assembly is commercially available from W. L. Gore & Associates under the part number 02- 07605. Examples 16A and 16B A flat cable 45 according to Fig. 16 was constructed from an upper insulator 1340a of ePTFE and a lower insulator 1340b of ePTFE having a thickness of approximately 1 mm. The conductors 1320 are spaced 0.35 mm apart and have a GSG-configuration, i.e. every signal conductor is surrounded by two ground conductors. The flat cable 45 was made in one example (Example 16A) with copper conductors 23 1630 of AWG 4007 (0.08 mm diameter) and in a further example (Example 16B) with copper conductors of AWG 4207 (0.07 mm diameter) to have an impedance of 80 Ohm. Examples 17A and 17B A flat cable 45 according to Fig. 16 was constructed using the same construction as in example 16 that the conductors have a GS-configuration, i.e. every second signal conductor is a ground conductor. The flat cable 45 was made with either AWG 4007 (0.08 mm diameter) (Example 17A) and AWG 4207 (0.07 mm diameter) (Example 17B) to have an impedance of 80 Ohm. Comparative Example 18 In the prior art standard miniature co-axial cabfes are known with conductors made of AWG 46 (0.0399 mm diameter) silver-plated alloy wire. The conductors havefluor thermoplastic insulation which has a dielectric constant of 2.1 A shield of 0.0254 mm thickness surrounds a given number of the conductors and a thermoplastic jacket, also of 0.0254 mm thickness is extruded about the shield. Further Examples using different dielectric materials Further examples are carried out using the structures taught above but using different dielectric materials with different constants. These require different thickness of the upper inulsation layer 120a and the lower insulation layer 120b when the pitch distance is 0.35 mm. Table 1 illustrates the case when an impedance of the electrical signal cable 10 is designed to be 80 Ω. Table 2 illustrates the case when an impedance of the electrical signal cable 10 is designed to be 50 Ω. Table 1 24 ctric constant Dielectric Material AWG size of a Thickness of one of the conductor 130 insulation layers 120a, 120b [mm] 1.4 eeePPPTTTFFFEEE,,, fffoooaaammmeeeddd PPPEEE,,, 44 4222000777 0.1 ffooaammeedd FFEEPP//PPFFAA 2.1 F FFEEEPPP,,, PPPFFFAAA,,, PPPEEE 4 44222 0,16 3.5 P PPooolllyyyeeesssttteeerrr 4 44222 0,3 1.4 eeePPPTTTFFFEEE,,, fffoooaaammmeeeddd PPPEEE,,, 4 44000000777 0.122 foamed FEP/PFA 2.1 FFEEPP,, PPFFAA,, PPEE 4400 0,193 3.5 PPoollyyeesstteerr 4400 0,363 1.4 eePPTTFFEE,, ffooaammeedd PPEE,, 33880077 0.1255 ffooaammeedd FFEEPP//PPFFAA 2.1 FFEEPP,, PPFFAA,, PPEE 3388 0,24 3.5 PPoollyyeesstteerr 3388 0,44 1.4 eePPTTFFEE,, ffooaammeedd PPEE,, 44440077 0.076 ffooaammeedd FFEEPP//PPFFAA 2.1 FFEEPP,, PPFFAA,, PPEE 4444 0,116 3.5 PPoollyyeesstteerr 4444 0,22 25 Table 2 Dielectric constant Dielectric Material AWG size of a Thickness of one of the conductor 130 insulation layers 120a, 120b [mm] 1.4 ePTFE, foamed PE, 4207 0.05 foamed FEP/PFA 2.1 FEP, PFA, PE 42 0.066 3.5 Polyester 42 0,11 1.4 ePTFE, foamed PE, 4007 0.053 foamed FEP/PFA 2.1 FEP, PFA, PE 40 0.078 3.5 Polyester 40 0,128 1.4 ePTFE, foamed PE, 3807 0.07 foamed FEP/PFA 2.1 FEP, PFA, PE 38 0.1 3.5 Polyester 38 0,165 1.4 ePTFE, foamed PE, 4407 0.033 foamed FEP/PFA 2.1 FEP, PFA, PE 44 0.048 3.5 Polyester 44 0,08 Cross-Talk (dB/m) Cross-talk measurements were also carried out and are illustrated in the following table. In this table S indicated an individual signal conductors 130 carrying a signal, G an individual signal conductor 130 serving as a ground conductor in the same flat cable 45. x indicates an individual signal conductor 130 separating the individual signal conductors 130 on which the measurements are made. The last six entries in the table illustrate the cross-talk measurements made between individual signal conductors 130 in different layers of flat cables 45. The first three of these entries show the cross-talk between individual signal conductors 130 of different flat cable layers (L1-L2; L1-L3; L1-L4) in which separating cylindrical shields were at ground potential. The last three of these entries show the crosstalk between individual conductors 130 of different flat cable layers (L1-L2; L1-L3; L1-L4) in which some of the individual signal conductors 130 served as ground conductors. In the case of Examples 1, 2, 5, 8 and 9 the separating cylindrical shields were also at ground potential. 26 Table 3 Measurement Points Example Number 1 2 4 5 8 9 S-S -20.3 -20.4 -19.9 -20.5 -16.8 SxS -24.5 -19.2 SxxS -26.4 -21.6 SxxxS -26.8 -23.1 GSSG -16.9 -22.2 -19.7 GSGS -38.6 -32 -35.4 SS: L1-L2 -51.4 -53.5 -46.6 -51.8 -41.2 SS: L1-L3 -48.8 -45.6 -51.1 -43.6 -36.1 SS: L1-L4 -49.5 -52 GS: L1-L2 -50.1 -42.2 -52.7 -53.6 -48 GS: L1-L3 -51.5 -42 -42.7 GS: L1-L4 -43 -50. Table 4 shows a comparison of the electrical and mechanical properties of the electrical signal line manufactured according to this invention in comparison to the cables of the comparative example, an electrical signal line available from W.L.Gore & Associates. 27 Table 4 Example 12 Example 13 Example 14 Comparative Example 15 (02-07605) Electrical Properties O vo Impedance ~ 100 Ω ~ 75 Ω ~ 100 Ω 78 Ω i O Attenuation @ 5 MHz l ,20 dB/Assy. 0,73 dB/Assy. 0,7 dB/Assy. 1,09 dB/Assy. otΛ\ Attenuation ® 10 MHz l,67 dB/Assy. 0,89 dB/Assy. 1,17 dB/Assy. Velocity of Signal Propagation compared to velocity of light. 81,1 % 81,0 % 81,0 % 76,6 % X-talk: Signal/Signal 17,4 dB 20,3 dB 36 dB Subcable 1 /subcable2 30,1 dB 35,4 dB 36 dB Subcable 1 /subcable3 39,4 dB 42,3 dB Subcable 1 /subcable4 47,6 dB 44,4 dB Bundle/Bundle (senses) 50,5 dB Mechanical Properties co Weight 225 g 237 g ca. 300 g Length 2,5 m 2,5 m 2,5 m o H VO VO © t*> 00 The signal/signal value is the cross-talk between any two adjacent electrical signal conductors 1030 in the same signal conductor layer 1020. For example 12, the value for subcable l/subcable2 is the cross talk between two corresponding electrical signal conductors 1730 in two adjacent signal conductor layers 1720 in the same signal conductor layer bundle 1725, i.e. with no shielding strip 1750 between the two adjacent signal conductor layers 1720. The value for subcable l/subcable3 is the cross talk between two corresponding electrical signal conductors 1730 in two signal conductor layer 1720 separated by one signal conductor layer 1730 in the same signal conductor layer bundle 1725. Similarly, the value for subcable l/subcable4 is the cross talk between two corresponding electrical signal conductors 1730 in two signal conductor layer 1720 separated by two signal conductor layer 1720 in the same signal conductor layer bundle 1725, i.e. the first and last signal conductor layer 1720 in one of the signal conductor layer bundles 1725. The value for the bundle/bundle crosstalk of example 12 is obtained by measuring the cross talk between two corresponding electrical signal conductors 1730 in the signal conductor layer 1720 immediately adjacent to the shielding strip 1750, i.e. the first signal conductor layer 1720 in one of the signal conductor layer bundles 1725 and the last signal conductor layer 1720 in the other of the signal conductor layer bundles 1725. The cross talk values for examples 13 and 14 are measured in the same manner except, of course, that there is always at least one shielding strip 1050 between the two electrical signal conductors 1030 in the different signal conductor layer 1020. There is no value given for the bundle/bundle cross talk since the signal conductor layer 1020 of examples 13 and 14 are not bundled. It can be seen from this table that the electrical signal lines manufactured according to this invention have a much better velocity of signal propagation compared to the comparative example 15. By suitable choice of electrical signal conductors the cross-talk can be reduced to a value which is at least comparable to that in the comparative example. Indeed in practice it is known that any value greater than 20 dB is acceptable. For the same length of line, the inventive electrical signal lines are substantially lighter, i.e. for 132 lines a weight saving of up to 25% is achievable. Capacitance Table 6 shows measurements of a capacitance per length of the examples 16 and 17, whereby in example 16 the capacitance was measured between one signal conductor and two adjacent ground conductors. In example 17 the capacitance was measured between one signal conductor and one adjacent ground conductor. 29 Table 6 AWG 4007 AWG 4207 Example 16 34.8 pF/m 28.6 pF/m Example 17 27.2 pF/m 23.15 pF/m Further capacitance measurements were carried out using the cable 10 of example 9. The capacitance was measured between the signal conductors 130 and the outer cylindrical shield 70. It was found to be 39 pF/m. The best theoretical capacitance which can be achieved with signal conductors of AWG 40 for a construction in which the pitch distance is 0.35 mm and both the upper and lower insulation layers 1340a, 1340b are 0.1 mm thick is 30 pF/m for the stacked electrical signal cable 1010 without ground planes 1055 (Fig. 12) and 38 pF/m for the wrapped electrical signal cable 1 1 10 without ground planes 1 150 (Fig. 14). Both of these cables have ten signal conductors 1030 per mm 2 . Thus the capacitance per number of conductors is 3.0 pF/m and 3.8 pF/m respectively. Finally normalising this with respect to the diameter of the AWG 40 signal conductors 1030, i.e. 0.08 mm., gives a value of the capacitance per number of conductors per unit area of conductor of 37.5 pF.mm/m and 47.5 pF.mm/m respectively. These values can be compared with the prior art cables of Example 18. Table 7 shows the best results for the capacitance which can be achieved for the prior art cables. The last column illustrates the capacitance per number of conductors within shield per cross-sectional area of conductor. Table 7 No. of Co-Ax Diameter of Thickness of Capacitance Capacitance/ Capacitance / conductors Co-Ax cable insulation (pF/m) No. of No. of cables / within shield (mm) (mm) conductors/mm 2 Cross section 7 0.376 0.117 34 4.86 121.8 11 0.282 0.070 41 3.73 93.5 25 0.188 0.023 60.1 2.4 60.15 30 It will be seen that much better results are achieved for the electrical signal cables of the current invention compared to prior art cables. 31

Claims: C L A I M S 1. Electrical signal cable (10; 1010; 1110; 1210; 1710) having a capacitance and for transmission of electromagnetic waves and comprising: at least two electrical conductors (1, 2, 3, 4, 4a, 4b, 7, 8, 30, 50, 70, 90, 130, 1030, 1050, 1060, 1070, 1 150, 1 130, 1270, 1330, 1630, 1650, 1750) spaced apart by an insulator (120a, 120b), and having a diameter wherein at least one of the at least two electrical conductors is a signal conductor (4a, 7, 1030, 1 130, 1330, 1630) and at least one of the at least two electrical conductors is a ground conductor (4b, 8, 30, 50, 70, 90, 130, 1050, 1060, 1070, 1 150, 1270, 1650,1750), and wherein the capacitance per unit length, packing density and diameter of said signal cable is less than 60pF.mm/m. 2. Electrical signal cable (10; 1010; 1 110; 1210; 1710) according to claim 1, wherein the capacitance per unit length, packaging density and area of the diameter of said signal cable (10) is less than 50 pF.mm/m. 3. Electrical signal cable (10) according to claim 1, wherein at least a first plurality of the electrical conductors (1030) are arranged coplanarly as a first flat cable and at least a second plurality of the electrical conductors (1030) are arranged coplanarly as a second flat cable and wherein said first flat cable and said second flat cable are stacked on each other. 4. Electrical signal cable (10) according to claim 3 further comprising a ground plane arranged between said first flat cable and said second flat cable. 5. Electrical signal cable (10) according to claim 1, wherein every second or third electrical conductor is a ground conductor. 6. Electrical signal cable (10) according to claim 1, wherein at least two of the electrical conductors (130,1630), are arranged parallel to each other. 32 7. Electrical signal cable (10) according to claim 1, wherein each ground conductor (4a, 7, 1030, 1 130, 1330, 1630) is arranged together with a corresponding signal conductor (4b, 8, 30, 50, 70, 90, 130, 1050, 1060, 1070, 1150, 1270, 1650, 1750) as twisted pair (1). 8. Electrical signal cable (10) according to claim 1, wherein two of the electrical conductors at least (4b, 30, 50, 70, 90, 130, 1030, 1050, 1060, 1070, 1150, 1130, 1270, 1330, 1630, 1650, 1750) are arranged as ribbon pair (2). 9. Electrical signal cable (10) according to claim 1, wherein the ground conductor (30, 50, 70) is arranged as a cylindrical ground plane and the signal conductor is situated therein. 10. Electrical signal cable (10) according to claim 1, wherein at least one signal conductor (4a) is coated with an insulating material. 1 1. Electrical signal cable (10) according to claim 1, wherein the diameter of at least one of the at least two electrical conductors (1, 2, 3, 4, 4a, 4b, 7, 8, 30, 50, 70, 90, 130, 1030, 1050, 1060, 1070, 1 150, 1 130, 1270, 1330, 1630, 1650, 1750) is less than 0.25 mm. 12. Electrical signal cable (10) according to claim 1, wherein the number of the at least two electrical conductors (1, 2, 3, 4, 4a, 4b, 7, 8, 30, 50, 70, 90, 130, 1030, 1050, 1060, 1070, 1 150, 1 130, 1270, 1330, 1630, 1650, 1750) is between 64 and 1024. 13. Electrical signal cable (10) according to claim 1, wherein the signal conductors are arranged in at least one signal conductor layer and the at least one ground conductor is arranged in at least one ground conductor layer distinct from the signal conductor layer (10). 14. Electrical signal cable (10) according to claim 13, wherein the signal conductor layer and/or the ground conductor layer is braided around a central axis of the electrical signal cable (10). 15. Electrical signal cable (10) according to claim 13, wherein the signal conductor layer and/or the ground conductor layer is surfed around a central axis of the electrical signal cable (10). 16. Electrical signal cable (10) according to claim 13, wherein at least one signal conductor layer (1020, 1120, 1220, 1320, 1620, 1720) and at least one ground conductor layer (1050, 1150, 1250, 1650, 1750) are stacked on each other. 33 17. Electrical signal (10) cable according to claim 13, wherein at least one of the signal conductor layers (40,60) is cylindrically arranged around a central axis (15), said at least one of the signal conductor layers (40,60) comprises a plurality of flat cables (45), each of said flat cables (45) having a plurality of coplanar electrical signal conductors encased within and separated at a pitch distance (a) from each other by the insulator (120). 18. Electrical signal cable (10) according to claim 17, wherein said flat cables (45) are cylindrically braided about the central axis (15). 19. Electrical signal cable (10) according to claim 17, wherein said flat cables (45) are cylindrically surfed about the central axis (15). 20. Electrical signal cable (10) according to claim 17, wherein an outer shield (90) is disposed about said at least one subcable assembly (40, 60). 21. Electrical signal cable ( 10) according to claim 17, wherein a plurality of said at least one signal conductor layer (40, 60) are cylindrically arranged around a central axis (15). 22. Electrical signal cable (10) according to claim 21, wherein said plurality of said at least one subcable assembly (40, 60) are separated from each other by a separating cylindrical shield (50, 70). 23. Electrical signal cable (10) according to claim 17, wherein a tubular spacer (20) is disposed within said at least one subcable assembly (40, 60). 24. Electrical signal cable (10) according to claim 23, wherein said tubular spacer (20) is constructed from a solid material. 25. Electrical signal cable (10) according to claim 23, wherein said tubular spacer (20) is in the form of a hollow tube. 26. Electrical signal cable (10) according to claim 23, wherein said tubular spacer (20) is made from a stranded material. 27. Electrical signal cable (10) according to claim 23, wherein an inner cylindrical shield (30) is disposed between said tubular spacer (20) and said at least one signal conductor layer (40, 60). 34 28. Electrical signal cable (10) according to claim 17, wherein an inner cylindrical shield (30) is disposed within said at least one signal conductor layer (40, 60). 29. Electrical signal cable (10) according to claim 20, wherein an outer cylindrical shield (70) is disposed between an outer one of said at least one signal conductor layer (40, 60) and said outer shield (90). 30. Electrical signal cable (10) according to claim 29, wherein said outer cylindrical shield (70) is separated from said ground shield (90) by a first insulation layer (80). 31. Electrical signal cable (10) according to claim 17, wherein a second insulation layer (100) is disposed between said outer ground shield (90) and said jacket (1 10). 32. Electrical signal cable (10) according to claim 17, wherein said insulator (120) comprises an upper insulator (120a) attached to a lower insulator (120b). 33. Electrical signal cable (10) according to claim 32, wherein said insulator (120) comprises an upper insulator (120a) laminated to a lower insulator (120b). 34. Electrical signal cable (10) according to claim 32, wherein said insulator (120) comprises an upper insulator (120a) adhered to a lower insulator (120b) by an adhesive. 35. Electrical signal cable (10) according to claim 34, wherein said adhesive is selected from the group of thermoplastic adhesives comprising polyester, polyurethane or fluorinated ethylene/propylene. 36. Electrical signal cable (10) according to claim 32, wherein said insulator (120) comprises an upper insulator (120a) attached to a lower insulator (120b) by means of an adhesion promoter. 37. Electrical signal cable (10) according to claim 36, wherein said adhesion promoter is selected from the group of fluorinated copolymers comprising fluorinated ethylene/propylene and perfluoralkoxy. 38. Electrical signal cable (10) according to claim 1, wherein said insulator (120a, 120b) is formed from the group of insulating materials consisting of perfluoralkoxy, fluoroethylene-propylene, 35 polyester, polyolefin including polyethylene and polypropylene, polymethylpentene, polytetrafluoroethylene or expanded polytetrafluorethylene. 39. Electrical signal cable (10) according to claim 38, wherein said insulator (120a, 120b) is formed from expanded polytetrafluorethylene. 40. Electrical signal cable (10) according to claim 17, wherein said insulator (120a, 120b) comprises an extruded polymer. 41. Electrical signal cable ( 10) according to claim 17, wherein a jacket ( 1 10) is disposed about the outside of said electrical signal cable assembly (10). 42. Electrical signal cable (1010, 1 110, 1210, 1710) according to claim 13, wherein a plurality of signal conductor layers (1020, 1 120, 1220, 1320, 1620, 1720) are stacked on each other, each signal conductor layer (1020, 1 120, 1220, 1320, 1620, 1720) comprising a plurality of coplanar electrical signal conductors (1030, 1 130, 1230, 1330, 1730) encased within the insulator (1040a, 1040b) and being separated from each other by a first pitch distance (a), whereby the first pitch distance (a) is between 0,1 mm and 10 mm and the characteristic impedance of the electrical signal cable (1010, 1110, 1210, 1710) is in the range of 50 Ω to 200 Ω. 43. Electrical signal cable (1010, 11 10, 1210, 1710) according to claim 42, wherein the insulator (1040a, 1340a, 1640a, 1740a, 1040b, 1340b, 1640b, 1740b) comprises an upper insulator (1040a, 1340a, 1640a, 1740a) attached to a lower insulator ( 1040b, 1340b, 11640b, 1740b). 44. Electrical signal cable (1010, 1 110, 1210, 1710) according to claim 42, wherein said upper insulator (1040a, 1340a, 1640a, 1740a) is laminated to said lower insulator (1040b, 1340b, 1640b, 1740b). 45. Electrical signal cable (1010, 1 110, 1210, 1710) according to claim 42, wherein a shielding strip (1050, 1150, 1250, 1650, 1750) is situated between at least two of signal conductor layers (1020, 1120, 1220, 1320, 1720). 46. Electrical signal cable (1010, 1110, 1210, 1710) according to claim 45, wherein said shielding strip (1050, 1150, 1250, 1650, 1750) is laminated to said insulator (1040a, 1340a, 1640a, 1740a, 1040b, 1340b, 1640b, 1740b). 36 47. Electrical signal cable (1010, 1110, 1210, 1710) according to claim 45, further including first shielding means (1060, 1260, 1760) in electrical contact with at least one end (155) of the said shielding strips (1050, 1150, 1250, 1650, 1750). 48. Electrical signal cable (1010, 1110, 1210, 1710) according to claim 47, wherein the first shielding means (1060, 1260, 1760) surrounds said subcable assemblies (1020, 1120, 1220, 1320, 1720). 49. Electrical signal cable (1010, 11 10, 1210, 1710) according to claim 50, wherein an insulating layer (1065) surrounds said first shielding means (1060, 1260, 1760). 50. Electrical signal cable (1010, 1 110, 1210, 1710) according to claim 42, wherein second shielding means (1070, 1170, 1270, 1770) surrounding said signal conductor layers (1020, 1 120, 1220, 1320, 1720). 51. Electrical signal cable (1010, 1 110, 1210, 1710) according to claim 50, wherein the insulating layer (1065, 1 165, 1265, 1765) is disposed between the first shielding means (1060, 10260, 10760) and the second shielding means (1070, 1 170, 1270, 1770). 52. Electrical signal cable (1010, 1 1 10, 1210, 1710) according to claim 42, wherein a cable jacket (1080, 1 180, 1280, 1780) surrounding said signal conductor layers (1020, 1120, 1220, 1320, 1720). 53. Electrical signal cable (1010, 1110, 1210, 1710) according to claim 42, wherein at least one spacer (1090, 1 190, 1290, 1790) disposed within the cable jacket (1080, 1180, 1280, 1780) for shaping the electrical signal cable (1010, 1110, 1210, 1710). 54. Electrical signal cable (1010, 1 110, 1210, 1710) according to claim 42, wherein said signal conductor layers (1020, 1320, 1720) are arranged substantially in parallel planes to each other and have a top side and a bottom side. 55. Electrical signal cable (1010, 1110, 1210, 1710) according to claim 54, further including a cable jacket (1080, 1280, 1780) surrounding said signal conductor layers (1020, 1320, 1720), wherein said at least one spacer (1090, 1290, 1790) comprise a first crescent-shaped spacer (1090, 1290, 1790) disposed between the top side of said signal conductor layers (1020, 1320, 1720) arranged substantially in parallel planes to each other and the cable jacket (1080, 1280, 1780), 37 and a second crescent-shaped spacer (1090, 1290, 1790) disposed between the bottom side of said signal conductor layers (1020, 1320, 1720) arranged substantially in parallel planes to each other and the cable jacket (1080, 1280, 1780). 56. Electrical signal cable (1010, 1110, 1210, 1710) according to claim 53, wherein said signal conductor layers (1120) are arranged helically around the spacer (1 190). 57. Electrical signal cable (1010, 1110, 1210, 1710) according to claim 56, wherein the electrical signal cable (1010, 1 110, 1210, 1710) is cylindrical in shape and has an axis and said spacer (1 190) is cylindrical in shape and has an axis, the axis of said spacer (1 190) being concentric with the axis of the electrical signal cable (1010, 1 110, 1210, 1710) . 58. Electrical signal cable (1010, 1110, 1210, 1710) according to claim 42, wherein said electrical signal cable (1220) is twisted about its own axis. 38

Inventors: HOFFMANN HEIKE, HOFFMANN, HEIKE
Applicants: GORE W L & ASS GMBH, HOFFMANN HEIKE, HOFFMANN, HEIKE, W.L. GORE & ASSOCIATES GMBH
Application year: 1999
Main classification: H01B7
IPC codes: H01B7, H01B11