FR2693588A1 - Twisted pair data bus cable - Google Patents

Abstract

Conductor wires (1) made from any desired electrical conductor, are surrounded by insulation (2). The insulation is sintered, expanded, porous polytetrafluoroethylene tape applied as a helical wrap. Additional layers of insulation (3) are placed over the first insulation in places where it is required that the cable be shielded. The wires are bonded by heating and shielding (4) is then applied. The shielding is cut away from those areas that are to be unshielded and a protective jacket is placed over the shielded areas.

Description

 The present invention relates to electric cables and in particular to a twisted pair data bus cable which has screen areas and non-screen areas, and a method of manufacturing the cable.

 In an electrical device where the signals are transferred from one instrument to another, a known data transfer cabling device can be used as a data bus cabling device.

 These complex devices are dependent on electronic instruments for determining a condition or set of conditions and then transmit the information or signals received relating to the information by electronic wiring. Such a device is found in "instrument flight" devices for aircraft in which the instruments detect different states of the aircraft and develop information for controlling the flight of the aircraft. To transfer the different signals from one instrument to another, a data bus cabling device is used.

 A twisted pair of insulated conductors can be used as a data bus. However, while a few areas along the length of the data bus need not be screened so that the "patching" cables on and from the instruments can be associated with the bus, other areas along the bus shall be provided with a screen to prevent electrical signals from being radiated or subject to external noise. However, the layout of a screen involves a change in the size, weight, flexibility and electrical properties of the twisted pair for data bus. For example, a screen lowers the impedance in this area of the cable and adversely affects the performance of the device.

 The present invention provides a twisted pair data bus cable that is small, lightweight, flexible, and has substantially the same impedance in screened areas as in non-screened areas.

 More specifically, the present invention is a continuous cable consisting of a pair of insulated conductors twisted around each other, which has cable runs surrounded by a metal screen, in which the sections surrounded by a screen metal section, the thickness of the thicker insulation being such that the impedance of the sections surrounded by a metal screen is substantially the same as that of the stretch not surrounded by a metal screen.

The present invention will now be described, by way of example, with reference to the accompanying drawings in which
FIG. 1 is a sectional view of a portion of the cable according to the present invention which comprises a screen and an insulator added,
FIG. 2 is a sectional view of the cable according to the present invention which has no screen or additional insulation,
- Figure 3 is a perspective view of a twisted pair which comprises in selected sections an additional insulator and a screen.

 Referring to the drawings, there is shown in Figures 1 and 2 a section of a cable according to the present invention having a twisted pair of conductors. In Figure 1 is shown in section a portion of the cable which is provided with a screen. Figure 2 shows in section a part without screen. In both figures, the conductor wire 1 is surrounded by an insulator 2 to form an insulated primary wire. As shown in Figure 1, an additional layer or layers of insulation 3 are placed above the insulator 2 in areas of the cable to be provided with a screen. So the cable will have areas of different diameters.

 Then, the two insulated conductors are twisted around each other in the usual manner, taking care to ensure that the sections of each insulated wire that have additional insulation are matched and twisted with each other. This is shown in FIG. 3 in which the section A comprises only the wires 1 and the insulators 2 and in which the section B is larger because of the presence of additional insulator.

 Then, the twisted pair of insulated primary wires is connected by heating until a link is formed. A screen 4 is then placed above the twisted pair connected and is cut in the areas that should not include additional insulation.

 Finally, a protective envelope 5 is placed above the area provided with a screen.

 The conducting wire 1 can be any electrical conductor, such as copper, copper coated with silver or nickel, stainless steel, a copper alloy, silver, nickel or beryllium copper .

 Insulator 2 may be any desired low dielectric insulator, but preferably consists of sintered foamed polytetrafluoroethyl ether. Preferably, a ribbon around the conductive wire 1 will also be wound in the form of a helix.

 The additional layers of insulation 3 may be additional layers of expanded porous polytetrafluoroethylene, which may be reinforced by sintering or applying a fluoropolymer thermoplastic polymer coating, or both. Several layers of these various ribbon constructions can be applied depending on the desired electrical properties.

 The amplitude of the thickness of the additional insulator 3 which is added on desired sections of the cable is an amplitude necessary to maintain approximately the same impedance in the sections provided with a screen as in the sections not provided with screen. Thus, the usual impedance lowering effect that is involved by the screen is counteracted by using a greater thickness of insulation beneath the screen since the added insulator increases the impedance. It is recognized that the additional amount used will be a function of both the type of the screen and the dielectric of the insulation used.

 The screen 4 may consist of any metal screen, such as braided metal, sheathed wire or sheathed metal foil. An example of braided metal or sheathed metal is silver-plated copper. An example of sheathed sheet is an expanded porous polytetrafluoroethylene sheet containing a thin layer of aluminum.

 Preferably, the screen 4 is copper plated with tin braided. The screen can be applied using a braiding or sheathing machine.

The envelope 5 may be any protective plastic layer such as a PTFE resin called
Teflon (registered trademark), or a thermoplastic fluoropolymer.

 In this way, a continuous cable is obtained, the continuous term meaning that the cable has no splice or break.

In a typical cable assembly, the following characteristics are obtained:

<tb><SEP> Characteristic <SEP> Without <SEP> screen <SEP> With <SEP> screen
<tb> Impedance <SEP> 130 <SEP> ohms <SEP> + <SEP> 5% <SEP> Change
<tb><SEP><+ 2t <SEP> to <SEP> from <SEP> of
<tb><SEP> the impedance <SEP> without
<tb><SEP> screen
<tb> Speed <SEP> of <SEP> pro <SEP> 78% <SEP>><SEP> 77%
<tb> pagation
<tb> Attenuation <SEP> 6.1 <SEP> dB / 300 <SEP> m <SEP> 6.6 <SEP> dB / 300 <SEP> m
<tb> Diameter <SEP> of <SEP> wire <SEP> 2,16 <SEP> mm <SEP> 3,55 <SEP> mm
<tb> primary
<tb> Diameter <SEP> of <SEP><SEP> 4,2 <SEP> mm <SEP> Nominal
<tb> Twisted <SEP> pair <SEP> of <SEP> 6.235 <SEP> mm
<tb> Overall <SEP> Diameter <SEP> 4.2 <SEP> mm <SEP> Nominal
<tb><SEP> of <SEP> 7.37 <SEP> mm
<tb> Weight <SEP> 6.6 <SEP> kg / 300 <SEP> m <SEP> 21.2 <SEP> kg / 300 <SEP> m
<tb> Coating <SEP> for- <SEP> 85%
<tb> mant <SEP> screen
<tb> Rigidity <SEP> (number <SEP> 21.2 <SEP> 10 <SEP> N
<tb> of <SEP> Newtons <SEP> of a
<tb> force <SEP> needed
<tb> for <SEP> curl <SEP> one
<tb> sample <SEP> of
<tb> 23 <SEP> cm <SEP> on <SEP> 2,3 <SEP> cm
<tb> to <SEP> the <SEP> help of a
<tb> machine <SEP> Instron
<Tb>

Claims (2)

 1. Continuous electric cable consisting of a pair of insulated conductors (1) twisted around each other, which comprises cable sections (B) surrounded by a metal screen (4), in which the sections ( B) surrounded by a metal screen (4) have in section an insulation (3) thicker than the sections (A) not being surrounded by a metal screen, the thickness of the insulation (3) thicker being such that the impedance of the sections (B) surrounded by a metal screen (4) is approximately the same as that of the section (A) not being surrounded by a metal screen.  2. A method of preparing an electric cable according to claim 1, characterized in that it comprises the following sequential steps  a) applying insulation (2) on a conducting wire (1),  b) applying additional insulation (3) to selected areas of the insulated conductive wire,  c) twist two wires thus isolated one around the other,  d) applying a metal screen (4) to said selected areas,  e) applying a protective envelope (5) on said metal screen.