What is the difference between star-quad strand cores and twisted strand cores?

Twisted-Pair Stranding

It is often necessary to transmit signals via two pairs of cores. This often creates the risk of signals in the core pairs influencing each other. This phenomenon is called the near-end crosstalk. To counteract this, twisted pair stranding is used. Twisted-pair stranding is when you twist two cores together to form a pair and then strand together those pairs of cores.

The disadvantage of this type of stranding is that the cable becomes thicker and less round than with cores wound in layers. For applications in energy chains, the cable diameter is important. An e-chain® offers only limited space and the bend radius of a cable is also determined by the outer diameter. Furthermore, a two-pair cable is mechanically not as stable as a cable with cores wound in layers.

In situations where the near-end crosstalk of electrical signals can be neglected, cores wound in layers is the most cost-effective option. For applications in the energy chain, this is also the  most stable option mechanically.




Advantages of Star Quad Stranding

There is a little trick to overcome this problem. If you want to combine good near-end crosstalk and durability, lay the two pairs of cores crosswise. This produces the same effect with a cable whose cores are wound in layers. This is called a four-star, also known as a star quad. In this type of stranding, the cores are not arranged in 1, 2, 3, 4, but the first core is followed by the third, then the second and then the fourth core. Mechanically, one now has the advantage of a thin cable and the same stable structure of a cable stranded in layers with four cores. Crosstalk is just as well prevented as with a twisted-pair data or bus cable. Especially in cable tracks or in energy chains, this type of data cable is preferable to a twisted-pair cable.
Conclusion

Four-core data cables and also bus cables such as Profinet cables can be stranded in one layer. This is often the case only at first glance. On closer inspection, however, it is noticeable that the cable is stranded in a star quad. This has a positive effect on the mechanical load capacity in moving applications.

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Advantages

When star-quad cable is used for a single balanced line, such as professional audio applications and two-wire telephony, two non-adjacent conductors are terminated together at both ends of the cable, and the other two conductors are also terminated together. Interference picked up by the cable arrives as a virtually perfect common mode signal, which is easily removed by a coupling transformer or differential amplifier. The combined benefits of twisting, differential signalling, and quadrupole pattern give outstanding noise immunity, especially advantageous for low-signal-level applications such as long microphone cables, even when installed very close to a power cable. It is particularly beneficial compared to twisted pair when AC magnetic field sources are in close proximity, for example a stage cable that can lie against an inline power transformer.[7]

Disadvantages

The disadvantage is that star quad, in combining two conductors, typically has more capacitance than similar two-conductor twisted and shielded audio cable. High capacitance causes an increasing loss of high frequencies as distance increases.[8][9] The high-frequency loss is due to the RC filter formed by the output impedance of the cable driver and the capacitance of the cable. In some cases an increase in distortion can occur in the cable driver if it has difficulty driving the higher cable capacitance.

The capacitance of a four-conductor quad-star cable is roughly equal to the capacitance of a standard two-conductor cable about 1.5 times as long. The increased capacitance of the star quad cable is not usually a problem with short cable runs, but it can be an issue for long cable runs. For example, an 8 m (25 ft) star-quad cable has a capacitance of 150 pF/m[10] for a total capacitance of 1200 pF for the entire length of cable. With a 150 Ohm source impedance and 1200 pF load capacitance, the frequency response of this RC circuit is -0.02 dB at 20 kHz. If the cable were 80 m instead of 8 m, then the frequency response would be -0.2 dB at 20 kHz, and -3 dB at 88 kHz.


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