Any electrical system with considerable speeds or sensitive/aggressive lines must keep EMI as a primary concern. Systems can experience poor or intermittent performance due to noise-induced signal distortion without carefully placing components, traces, and cables. While there are numerous methods to prevent EMI, one of the simplest and most efficient methods is twisted wire pairs – a technology that’s as straightforward as it sounds. Twisted pair cabling typically uses four or more twisted wire pairs, with or without shielding, to ensure system connectivity meets compliance requirements for safety and reliability.
Comparing Network Wiring Options |
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| Pros | Cons | Max length (m) | Cost | Speed | |
| Unshielded twisted pair (UTP) |
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100 | Low | Up to 1000 Mbps |
| Shielded twisted pair (STP) |
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100 | Mod. low | Up to 100 Mbps |
| Coaxial |
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~200 – 500 | Mod. low | Up to 100 Mbps |
| Fiber optic |
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>1000 | High | Up to 100 Gbps |
Understanding Twisted Pair Cabling
Twisted pair cabling is wrapping wires to cancel the crosstalk between signal pairs. As signals propagate across their transmission media, they are susceptible to noise from nearby conductors’ coupling, which affects signal integrity. Twisting the pairs ensures that, throughout the transmission path, the proximal effects of nearby conductors are distributed equally between the two wires. Since coupling effects should be theoretically identical to both wires (i.e., common mode), a differential comparison can detect and cancel the noise-introduced signal. For manufacturing purposes, twisted pairs are straightforward and inexpensive compared to other designs and can operate at distances up to 100 meters.
Do The Twist |
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How does twisting improve signal transmission?
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System assembly must account for environmental and design factors that can cause excessive loss. It’s necessary to understand that systems with tightly constrained cable assemblies can experience failure or unacceptable performance due to mechanical installation conditions. Excessive bending or untwisting at the terminations can need budgeting during design speccing to provide sufficient leeway. Likewise, temperature effects gleaned at room temperature will present issues at the extremes of ratings; an increase in temperature will practically translate to additional wire length that adds several dB of loss.
Additionally, newer cabling standards unlock better performance in exchange for thicker copper gauging. While a thicker conductor supports a lower insertion loss at a higher frequency due to the skin effect, additional copper surface area is necessary to carry the same signal energy. This choice increases the per-length weight of twisted pair cabling, which can add extra tension at the connectors, potentially accelerating wear without some compensating mechanical accommodation.
In general, twisted pair cabling is highly advantageous due to its small external diameter, making it ideal for cramped wire harnesses and enclosure openings. A wiring duct that houses hundreds of cables can do so most effectively (regarding density) with twisted pair cabling. It’s the cheapest option per meter with a straightforward installation, particularly for older infrastructure, and its adoption continues to grow due to its compatibility with extant systems. However, twisted pair cabling without shielding suffers increased EMI susceptibility in exchange for a smaller cable size. Regardless of shielding, twisted pair cables have greater loss than comparable coaxial or fiber optic solutions, thus necessitating more frequent signal boosting; this requirement tends to work against the total cost advantage of twisted pair cabling and can complicate long cable runs in tight spaces.
To Shield or Not to Shield?
For fabrication, twisted pair configurations are available as shielded (STP) or unshielded (UTP). The latter is the simpler and cheaper construction that provides only an outer jacket to physically contain the many twisted wire pairs. Importantly, this method has to use distinct rates of pair twisting (known as the pitch of the cable) to prevent nearby twisted pairs from experiencing an uneven background noise: two (or more) twisted pairs with the same pitch would have a wire from each pair repeatedly neighboring each other throughout the transmission path. STP avoids this issue by using a sheath underneath the jacket and surrounding each twisted wire pair.
STP cabling can also be necessary for wire harnesses with multiple twisted pair cables where crosstalk between cables is a concern. At higher speeds, this form of crosstalk becomes the dominant noise source and, combined with the signal strength following insertion loss, establishes the system’s signal-to-noise ratio (SNR). The SNR directly affects the bit-error-rate (BER) of the transmission: in a digital (i.e., binary) mapping of a 1:1 scheme, the baud rate (discrete voltage levels) and the total data rate are equal. It’s possible to encode more bits per voltage level (i.e., 4, 8, 16, …, 2n, …) to reduce the effect of the bit-error rate, but this reduces the signal bandwidth by a factor of n – a compromise between accuracy and data throughput. Maintaining the SNR with STP cabling reduces or ideally eliminates the need to increase the baud rate to minimize transmission error.
Is springing for the cost difference between STP and UTP worthwhile? For most local area network (LAN) applications, the tradeoff for speed with STP is negligible, arguably a point in its favor (i.e., adequate data transfer rates with more robust shielding). UTP and STP are the cheapest communication technologies, so the increase may be marginal overall, especially if the total amount of cabling necessary is minimal. However, additional costs may be associated with rewiring or integrating into extant UTP systems; speaking to a cable manufacturer can put a finer point on the pricing.
Pair With an Experienced Contract Manufacturer
Twisted pair cabling is one of the premier cable connection methods due to its cost and manufacturing flexibility. While simple in construction, it utilizes the basics of electromagnetic theory (occasionally alongside shielding) to effectively transmit data while preventing undue interference to or from nearby signals. Determining the best solution – unshielded, shielded, or a completely different cabling solution – requires the knowledge of an experienced cable manufacturer and wire harness assembler for cost and performance. Here at VSE, we’re a team of engineers committed to building electronics for our customers. We’ve been realizing life-saving and life-changing devices alongside our valued manufacturing partners for over forty years.
