What is cable that can carry a wide range of frequencies with low signal loss?

7.8 Twisted pair cables

A twisted pair cable gives much better transmission performance. To start with it is a balanced cable. This means that an equal and opposite current flows down each of the two conductors of the twisted pair, there is no common earth return for all the signals as in RS 232 multicore. As current flows down any conductor it creates an electromagnetic field around it. This field can be a cause of pickup, or crosstalk, into adjacent conductors, and can be a major source of interference. The higher the frequency of operation, the worse the crosstalk phenomenon becomes. By having an equal but opposite current flowing in each conductor, a large part of the external magnetic field is cancelled out, greatly improving the crosstalk performance of the cable (Fig. 7.6).

The twisted pair cable is also more immune to external noise and crosstalk from other conductors. Induced noise tends to be common mode, i.e. the noise current flows down each conductor, in the same direction and of the same magnitude. Several tricks in the receiver design can cancel out this common mode noise as the receiver is only looking for differential signals on the two conductors of the pair, i.e. equal and opposite (Fig. 7.7).

Twisted pair is the dominant cable type in structured cabling and Local Area Networks. There can be any number of pairs within a cable. Structured cabling generally uses four but sometimes twenty-five pairs of 24 AWG are used for backbone or zone distribution systems. Cables used for voice telephony frequently go up to very high pair counts, such as 4000 pairs, but often with smaller conductors.

As we have already mentioned, twisted pair cables can be screened (shielded) or unscreened (unshielded). Unscreened cable is usually referred to as UTP. The cable can be screened with an overall screen of aluminium foil or copper braid or even both placed around the pairs and under the sheath (jacket), when it is often referred to as FTP or S-FTP or ScTP. Each pair can also have its own screen of foil in which case the cable is often referred to as STP or PIMF. It should be noted however that users and suppliers alike often mix up this terminology.

Other manufacturing methods to improve cable performance are:

15.2.4 Pair cables for HF carrier systems

High frequency symmetric pair cables have been developed for analogue carrier telephone systems providing 12, 24, 36, 48, 60 and 120 channels on each pair, necessitating operation at up to 550kHz. These cables have polyethylene insulated pairs in star quad formation, and conductor sizes from 0.9mm to 1.3mm. A typical cable is described in CCITT Rec. G.611 (CCITT, 1989) from which Table 15.5 is an extract.

Table 15.5. Pair cables for analogue transmission

ParameterValueConductor diameter1.2mmMutual capacity26nF/kmCharacteristic impedance @ 60kHz178ΩCharacteristic impedance @ 120kHz174ΩCharacteristic impedance @ 240kHz172ΩAttenuation @ 10°C, 120kHz2.0dB/kmAttenuation @ 10°C, 240kHz2.9dB/kmAttenuation @ 10°C, 552kHz4.8dB/km

(From CCITT G.611)

Far end crosstalk between pairs operating in the same direction has to be of the order of 70dB and this is achieved with the use of capacitive crosstalk balancing frames during cable installation.

2.5 Electrical isolation of British Telecom circuits

All of the above BT cable pairs are terminated by BT on isolation links (BT reference, Links Isolation Type 5A) in the locations detailed in Section 2.1 of this chapter, to permit easy electrical disconnection of the cable pairs from equipment installed in the station.

If the calculated rise of earth potential during electrical fault conditions exceeds 650 V AC RMS, then isolation equipment is provided on every working circuit to isolate the off-site BT circuits electrically from the power station telecommunication equipment. This prevents the transfer of the rise of earth potential but permits through-transfer of speech and signalling information, thus preventing damage to BT plant external to the power station and also the transfer of electrical potential to BT personnel working on external BT plant.

Figure 8.7 shows a circuit diagram of an exchange line isolating equipment and Fig 8.8 shows a circuit diagram for isolating equipment typically used for grid system control, telephony and protection of telecommunication circuits.

Copper cables for LANs

In Chapter 7, we met all the cables that we use in LANs, so for now it will be enough to concentrate on the use rather than delve back into their construction and operation.

10.3.1 Copper cable

There are two typical types of copper wire cables used for communication, twisted pair and coaxial cables. The wires may be shielded or unshielded depending on the design types and needs. The so-called wired networks establish a connection between various devices through cables and routers. Copper cables as mean of a transmission medium are susceptible to interference, noise, and signal attenuation, mostly attributed to the electromagnetic coupling fields. The physical constraints of the environment bring in unfavorable influence on the choice of the copper cable. However, the dual conductor dependency and the material (copper) characterize the physical components of copper-based cables used for transmission lines. Previously used to transport voices, the technology has been expanded to serve high-speed data services (ADSL, VDSL, and so on…) [8]. A wired transmission medium provides high-speed connectivity but poses constraints like immobility and extensive cabling [9].

The characteristics of copper cables create a level of limitation on the signal flowing in the channel. Considering the metal components of copper cables, with the induced resistance, the electrical conductivity characteristic of the medium affects the speed of data transfer during transmission. The signal in the copper medium can generate harmonics and degrades the signal over time. Copper conductor cables are also sensitive to noise, cross-talk, and electromagnetic interference, unlike fiber optic links where interference is minimal. A couple of techniques have been implemented to improve performance in the copper transmission medium, such as twisting of copper cables or wire transposition [6]. The copper medium can induce a high amount of attenuation, thus limiting the coverage footprint.

Important Hypothetical Reference Circuits Defined by CCITT and CCIR

Hypothetical Reference Circuit on Symmetric Cable Pairs

This circuit is 2500 kilometers long and is set up on a symmetric-cable-pair carrier system. For each direction of transmission, it has a total of three pairs of channel modulators and demodulators, six pairs of group modulators and demodulators, and six pairs of supergroup modulators and demodulators.

Fig. 2A shows that there are 15 modulations and 15 demodulations for each direction of transmission, assuming that single-stage translations are used. There are six homogeneous sections of equal length. (G.322)

Fig. 2. Typical hypothetical reference circuits for systems using frequency-division multiplex.

Hypothetical Reference Circuit Over Active Intercontinental Communication-Satellite Systems

This circuit has no fixed length. For intercontinental connections, satellite links should be capable of spanning 7500 kilometers. For great-circle distances up to 25 000 kilometers, it will be necessary to connect two or three satellite links in tandem. The basic hypothetical reference circuit shall consist of one earth-satellite-earth link, as shown in Fig. 3. It shall contain one pair of modulation and demodulation equipments for translation from the baseband to the radio-frequency carrier and back. (CCIR, 352-3)

Fig. 3. Basic hypothetical reference circuit, active intercontinental communication-satellite system (CCIR, 352-3).

Instrument to remote terminal unit

Communication between the instruments and controllers and the RTU or PLC is normally over a twisted pair cable. This cable may be placed in conduit. Often, many pairs of wires are installed in a large cable that connects from the RTU to several instruments. Normally, as in the case of analog current or voltage signals, a single pair of wires goes directly from the instrument to the RTU or PLC. In some cases, as in the case of digital or Fieldbus transmitters, a single wire may be connected from the RTU or PLC to many instruments or controllers. In a few cases, wireless communications are used.

This communication is one of the weakest links in the production automation system. Wires may be cut, damaged, or shorted. Fortunately, it is easy to tell if there is a communication outage. For example, with analog current transmitters, a value of 4 mA represents a zero (0) value of the signal being measured. If the analog current signal goes to 0 mA, this signifies a communication outage. Similar indications exist for voltage, digital, and Fieldbus signals.

Which cable is capable of carrying high frequency signals and also provide a good shield?

Which wired cable supports the highest frequency?

What is the frequency range of twisted pair cable?

What is the difference between STP and coaxial cable?