Optimize signal clarity in an industrial network
OSI Layer One (the physical layer) issues comprise more than 50% of reported problems in network installations, emphasizing that signal clarity is an important consideration when planning a network infrastructure of an industrial application. Twisted-pair cabling is a staple in industrial network infrastructure. More flexible and less expensive than coaxial cable, Cat5e and Cat6 cables are recommended for many installations; therefore, the structure of the twisted-pair cable is critical to ensure high-quality transmission.
How does differential Ethernet work? Cat5e and Cat6 cables transmit signals using differential mode signals, which are opposite polarity and equal amplitude from each wire in the twisted pair. At the receive end of the channel, the equal but opposite polarity signals are evaluated for the "difference" potential. If we had a 2 V signal, the difference in magnitude between the signals would be 4 V. Noise acts as a common mode signal when it is coupled into the balanced pair channel. It is "common" to both wires and is the same amplitude and phase. This interference signal does not propagate separately from the data signal but is integrated into the overall voltage signal to become a composite waveform.
The beauty of differential mode transmission is that it elegantly removes the noise from the source signal when the "difference" between the two wires is calculated. (See diagram.) With a 4 V differential signal (-2 V to +2 V) and a 1 V common mode disturber (+1 V on each wire), the voltage magnitude differential between the two wires is still 4 V (3 V to -1 V). In a perfectly balanced cabling system, the induced common mode signal would appear as two equal voltages that are simply subtracted out by the transceiver, thereby resulting in perfect noise immunity.
In reality twisted-pair cables are not perfectly balanced, and the TIA (Telecommunications Industrial Association) has limits on cable specification intended to make sure at least a base level of twisted-pair balance is maintained. DCR unbalance, capacitance unbalance (CUB), and transverse conversion loss (TCL) are examples of specifications designed to ensure pair balance.
Balancing twisted pairs
Balancing twisted pairs involves numerous strategies, and the better balanced the twisted-pair cabling, the more reliable the signals. Some of the most common challenges when using twisted-pair cabling are:
- Susceptibility to noise: Twisted pairs tend to separate due to movement during installation, flexing, or handling. Each pair can be pictured as an antenna that can receive or transmit signals; thus variations in conductor-to-conductor spacing are cumulative and result in susceptibility to EMI and RFI that degrades signal transmission and network performance.
- Signal reflections: When twisted pairs separate, they create impedance irregularities that can cause signal reflections (return loss). Impedance variations are also cumulative.
- Pair-to-pair crosstalk: All twisted-pair Ethernet cables have crosstalk or pair-to-pair coupling, which is caused because each pair has different twists/in. (lay length). Lay length variation can increase the crosstalk that is cumulative down the length of the cable.
- Connector issues: Twisted pairs can separate inside an improperly terminated connector, which can lead to lack of signal integrity. Tracing degradation inside connectors is a time-consuming and expensive process.
- Lack of mechanical robustness: Like other cable types, twisted pairs are subject to degradation due to stretching and flexing during installation. When tension is applied with unequal force from one conductor to another, issues can occur.
Noise includes ground loops
If a twisted pair is not perfectly balanced, modal conversion of balanced to unbalanced signals is going to occur at RF frequencies. Differential mode signals at 20-30 MHz or higher can convert to common mode signals and vice-versa. The conversion artifacts adversely impact noise immunity from the environment as well as contribute to crosstalk between pairs and between other balanced cables. Only highly balanced twisted pairs can mitigate the modal conversion artifacts.
Signal phase in a balanced pair also is important because each signal must arrive at the end of the pair in proper phase. Both wires must be the same electrical length so the differential process acts on the "equal" but opposite signal.
Three types of noise can be coupled onto twisted-pair cabling: differential, environmental, and ground loop. Differential noise, that is, noise from nearby pairs in a cable, is called NEXT (internal to the cable), while noise from nearby cables is called ANEXT (from adjacent cables). Environmental noise is capacitive or inductively coupled to balanced pairs from external electromagnetic fields from disturbers such as electric motor noise, fluorescent light ballasts, and radio-frequency (RF) sources (in increasing order of severity). Ground loop noise is induced by a difference in potential between conductor ends or physical ground point locations in a building or between buildings.
Shielding can decrease the potential for modal conversion by limiting noise coupled onto the twisted pair from the environment. Shielding acts as a noise attenuator so that disturber signals are as small as possible before they impact the twisted pairs underneath the shield. A shield cannot remove noise; it can only attenuate noise. Balance twisted pairs utilizing differential signals remove noise. The more perfectly balanced they are, the more noise the balanced pairs can remove. The greater the noise source, the more critical it becomes to have good pair balances.
Bonded-pair technology can alleviate many of the challenges of twisted-pair cable, exhibiting consistencies that traditionally have only been possible with coax and twin-lead designs. When conductors are bonded (that is, adjoined along their longitudinal axis), they can provide uniform conductor-to-conductor spacing, uniform twisting of insulated conductors into pairs, and a robustness that assures that the twists of the pair will not loosen up or separate during manufacturing or installation.
Don’t cut corners
Twisted-pair cabling provides multiple benefits in an industrial networking application. However, cutting corners in cable quality can lead to downtime and costly maintenance. It is important to choose balanced-pair data cabling that can protect against noise, signal reflections, crosstalk, and other variables that can impact signal clarity.
Gareis is principal product engineer, Belden Inc. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media.
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