Symmetrical cable

Symmetrical cable; The double conductors of the core wire are made in a protective sleeve. Symmetrical cables are made of several pairs of double conductors called core wires in a protective sleeve. In order to reduce the interference between each pair of conductors, each pair of conductors is made into twisted wires, which is called twisted pair. The core wire of the symmetrical cable is thinner than the open wire, with a diameter of 0.4mm~1.4mm, so its loss is larger than the open wire, but the performance of the symmetrical cable is relatively stable. Symmetrical cables are widely used in subscriber access circuits in wired telephone networks. The amplitude-frequency characteristic of symmetrical cable is low-pass type, and the crosstalk increases with the increase of frequency, so the degree of multiplexing is not high.

Insulation layer of symmetrical cable

The conductors in symmetrical cables are generally copper wires with a diameter of 0.25 mm~1.4 mm. The diameter of symmetrical cable used in the city is usually 0.4 mm~0.9 mm; The symmetrical cable diameter for long-distance use is usually 1.2 mm. In a few cases, symmetrical cables with aluminum cores also appear. The insulating materials for symmetrical cables are divided into three categories: 1) Paper: Paper insulated cables have three forms: paper bag, pulp and rope. The method of rope insulation is to coil the rope on the wire and then wrap 1-2 layers of paper tape. 2) PVC: PVC has large dielectric constant and dielectric loss, and is only used on short distance symmetrical cables. However, the cable made of this material is flame retardant and can be used for indoor wiring. 3) Polyolefin and polystyrene: These two materials have good high-frequency performance, and are often used in cables that transmit signals of megahertz level or below the third group. Polystyrene is usually made by means of rope. Polyolefins are mostly solid, foam or foam skin. For easy identification, different colors are usually selected for different insulation layers.

Symmetrical cable stranding mode

Different stranding methods. 1) Two or four insulated wires are twisted into two or four wire groups with appropriate torque. For a 4-wire group, when the position of the 4 wires is square from the cross section, we call it a star-twisted 4-wire group, where the 2 wires on the diagonal form a circuit. 2) When twisted into two 2-wire groups and then twisted into four wires, we call it multiple-pair twisted 4-wire groups. 3) Layered stranding: stranding several wire groups into cable cores in layers. 4) Unit formula: first twist several wire groups into independent units, and then twist several units into cable cores. On the outside of the wire group, different color ties are often used to distinguish different wire groups.

Symmetrical cable sheath

In order to ensure the safety of symmetrical cables, the outer part of the cable core will be wrapped with tape, sometimes with shielding layer and sheath. 1) Sealed sheath is required for cables with paper insulation and polystyrene rope insulation. Lead sealing sheath was used in the early stage, and aluminum sealing sheath was also used later. In some cases, aluminum-polyethylene bonded sheath is also used, which is semi-sealed. 2) Indoor cables are sheathed with PVC or low-halogen (or halogen-free) materials. On the outside of the sheath, different outer protective layers need to be added according to different use environments to ensure the safety of communication cables.

Symmetrical cable reduces interference

In the design and manufacture of symmetrical cables, scientific and technological workers have taken various measures, but there will still be some mutual interference between wire loops. In order to reduce the impact of near-end crosstalk between loops on transmission lines in different directions, scientific and technological workers have thought of other methods: 1) two-cable system: two loops not in the same cable conduct two-way transmission at the same frequency. 2) Single cable system: only one loop is used for two-way transmission at different frequencies. 3) Internal shield: sometimes the cable core is separated into two parts by a special shield in a cable to replace two cables. We call this kind of cable internal shielded cable.