A multi-conductor electrical cable of controlled electrical performance

Abstract

 A multi-conductor electrical cable having controlled impedance and capacitance conductor-to-conductor and conductor-to-shield comprises an inner insulative core (2) around which spaced conductors (1) are spiraled, a dielectric layer (3) surrounding the conductors, a shielding layer (4) surrounding the dielectric layer (3) and an insulating jacket (5) surrounding the shielding layer (4).

Description

[0001] The present invention relates to a multi-conductor cable construction, in particular of miniature size, for transmission of data signals which has been designed to have a controlled electrical performance. More particularly the impedance and capacitance conductor-to-conductor and conductor-to-shield within the cable can be controlled.

[0002] Modern digital data processing and computing systems and other electronic apparatus have become increasingly miniaturized, the apparatus of many kinds are linked together in systems which may work together sometimes at a distance from the source of signal generation, and among which signals of less and less uniformity of shape and spacing are transmitted at higher and higher speeds. Significant problems of clarity and accuracy of data transmission increasingly arise from faulty signal synchronization, recognition, and assembly and impedance matching where transmission cables offer less uniform or less controlled attenuation, impedance , and capacitance cpabilities to the systems.

[0003] According to the present invention there is provided a multi-conductor electrical cable having controlled impedance and capacitance conductor to conductor and conductor to shield, comprising an inner core of an insulative material around which a plurality of evenly-spaced electrical conductors are located, a layer of dielectric material surrounding said conductors and said inner core of insulation, and an outer insulating jacket.

[0004] In particular, the present invention provides a cable which controls impedance and capacitances signal to signal and signal to shield in small gauge wire applications such as, for example, those utilizing AWG 40 to AWG 44 wire. Existing tape wrap technologies require too narrow a slit width at these gauge sizes.

[0005] The invention will now be particularly described, by way of example, with reference to the accompanying drawings in which:-

Figure 1 shows a cross-section of a cable according to the invention, not to scale, with four conductors distributed about a central core;

Figure 2 is a side view of the cable with each layer exposed to view;

Figure 3 is a cross-section of an alternative embodiment of a cable according to the invention wherein the central core has a conductor down its centre.

[0006] In the embodiment of the invention shown in Figure 1, metal conductors 1 are spiraled about a central core 2 of insulation material by a wire serving machine or other means. Although the invention is not confined to any particular size or range of sizes or composition of wire, the preferred range of wire size is AWG 20 - AWG 44. A capstan can be used to pull insulation 2 at the required rate such that wires 1 can be spiraled or served on to it to achieve the desired spiral. Optionally centre core insulation 2 may contain a centre conductor 6, as shown in Figure 3. Insulation core 2 preferably comprises MIL-ENE (Registered trade mark) polyester, polytetrafluoroethylene (PTFE), or porous expanded PTFE (EPTFE), although other polymeric materials of suitable tensile strength and dielectric constant could be used. The most preferred core is porous EPTFE. A closing die on the serving machine may be used in some instances to embed the wires 1 partially into insulation 2 to ensure positional stability of wires 1 under stress when the cable is in use.

[0007] The desired thickness of insulation 3 is now placed over the cable by winding, serving or extrusion. Typical insulative materials would be MIL-ENE (Registered Trade Mark) polyester, full density PTFE, or most preferably, EPTFE. If desired, as for instance in making a coaxial cable, a shielding layer 4 can be braided, served or wound around the insulation layer 3. This can be aluminized foil, copper alloy braid, or served metal foil.

[0008] To protect the cable, a final jacket layer 5 of protective insulation, usually of rugged material to resist damage from the environment, is placed or extruded on to the cable. This jacket is suitably polyvinyl chloride (PVC), fluorinated ethylene-propylene (FEP) polymer, or polyfluoroalkyl vinyl ether (PFA).

[0009] The cable, when containing small gauge conductors 1 (36 to 44 AWG), will provide strain relief to tension placed on the conductors, by distributing the majority of the load to the inner dielectric 2 rather than the conductors themselves since the conductors are spirally wrapped around the inner dielectric material 2. Since the inner dielectric 2 can be made of a dielectric, such as EPTFE whose matrix tensile strength exceeds 7,000 psi (48,230Kpa), which makes it stronger than the conductors 1 and whose elongation is minimal, the vast majority of the load will be distributed to the dielectric material 2 rather than to the conductors 1. This property allows for the use of smaller conductors 2 without external strain relief techniques which add to the bulk of the cable. The smaller overall diameter of the cable and the use of smaller gauge conductors makes for a lighter and more flexible cable, making the cable advantageous in peripheral equipment interconnection applications.

[0010] A feature of the cable is its flexibility which allows the use of a variety of signal transmission applications. Several different gauges of conductor 1 may be used at the same time around core 2 depending on the particular application. A conductor may be placed in the centre of the inner core with each conductor placed on the outer shell of the dielectric equally separated, as shown in Figure 3, from the inner conductor thus giving each conductor a consistent and similar impedance signal (outer conductor 1) to ground (inner conductor 6). Inner conductor 6 may be optionally utilized for a ground or a signal conductor. If an overall shield 4 is employed separated from the conductors 1 by an outer dielectric 3 of EPTFE, PTFE, MIL-ENE (Trade Mark) or other insulating material, as depicted in Figure 2, by controlling the thickness of the insulation between conductors and shield, a consistent and similar impedance may be achieved between each conductor and the overall shield. In addition, by choosing a desirable inner dielectric overall diameter and equally spacing each conductor around the dielectric, similar impedances can be achieved between any conductor and any adjacent conductor.

[0011] Alternate conductors may be ground, not signal conductors, where low cross-talk is desired.

[0012] Many of the electrical properties including impedance, capacitance, velocity of propagation, and time delay can be controlled by choosing a dielectric material 2 suitable for the application and or designating a particular number of revolutions of conductor per centimeter around the inner dielectric 2.

[0013] One benefit of the present invention is that it enhances productivity in the manufacturing of multi-conductor cables since in many cases the signal carrying conductors need not be individually insulated.

[0014] Another preferred feature of the invention is the decrease in overall diameter of a multiple conductor cable having an equal number of conductors of the same size over one fabricated using existing technology. This smaller overall diameter is achieved by embedding each conductor in a dielectric material and by not separately insulating each individual conductor.

[0015] The most preferred EPTFE used in this invention is the porous expanded PTFE disclosed in one or more of U.S. Patents 3,953,566; 3,962,153; 4,096,227; 4,110,392; and 4,187,390.

Claims

1. A multi-conductor electrical cable having controlled impedance and capacitance conductor to conductor and conductor to shield, comprising an inner core of an insulative material around which a plurality of evenly-spaced electrical conductors are located, a layer of dielectric material surrounding said conductors and said inner core of insulation, and an outer insulating jacket.

2. A cable according to claim 1 further comprising an electrically conductive protective shield surrounding said layer of dielectric material beneath said outer insulating jacket.

3. A cable according to claim 1 or claim 2 wherein the inner core of insulation is polyester.

4. A cable according to claim 1 or claim 2 wherein the inner core of insulation is polytetrafluoroethylene.

5. A cable according to claim 4, wherein the polytetrafluoroethylene is porous expanded polytetrafluoroethylene.

6. A cable according to claim 1 wherein said inner core has shallow grooves for receiving said conductors in which said conductors are held by said dielectric material and said shield is of braided metal wire, served metal tape, or metallized plastic tape.

7. A cable according to any preceding claim wherein the inner core of dielectric material has a conductor disposed down its centre.

8. A cable according to claim 7, wherein the centre conductor is optionally a ground or signal conductor.

9. A cable according to claim 8, wherein alternate ones of the conductors spaced about the inner core of insulation are grounded.

Drawing