Balance is a tricky thing. Athletes historically have been concerned with building their strength and endurance. However, balance has taken a backseat until recently, with activities like yoga becoming more popular. Balance helps prevent injury by promoting proper posture during other activities, helping strengthen lesser-utilized muscle groups, and curbing injury.
Balance also plays a role in electronics, but in a much different capacity: electromagnetic interference (EMI). EMI plays a huge role in designing individual devices and those nearby to ensure they comply and “play nice.” While many methods exist to implement balance into a PCB, a twisted pair can offer performance in a cost-effective package for various uses. However, twisted pair wire advantages and disadvantages should be discussed between product development and manufacturing teams to optimize the design.
At-a-glance Twisted Wire Pair
| Advantages | Disadvantages |
|
Common-mode rejection |
Deformation |
|
Ease of implementation |
Inter-pair Skew |
|
Crosswalk reduction |
Asymmetry |
Twisted Pair Wire Advantages and Disadvantages
Balanced lines help reject noise by ensuring that both lines are subjected to the same external electric fields over the length of their travel. While this doesn’t prevent noise on the lines, it ensures that noise equally influences both signals. Since the noise added to both signals is constant, these common-mode signals can be removed with a differential amplifier. By wrapping the wire pair together, designers ensure the signals are, on average, experiencing nearly uniform field strength, provided the twist rate is sufficient.
Advantages |
|
| Common-mode rejection | A differential amplifier can isolate and remove noise by exposing both signals to the same fields. |
| Crosstalk minimization | The longer the length of wire spends near an aggressor signal, the greater the coupling. While the primary goal of twisted pairs is to balance the lines, a secondary effect of doing so is halving (on average) the amount of length that one signal can couple to an aggressor line. |
| Ease of implementation | Twisted pairs require minimal processing and can be introduced into a wide range of wire harnesses. Alternate constructions are available that can prove advantageous in more trying installations (e.g., high vibration/shock). |
Disadvantages |
|
| Deformation | The twisted pair’s common-mode properties arise from the wires’ orientations to one another. The necessary tension between maintaining this arrangement and the minimum bend radii of the wires play an important role in the long-term reliability of the twisted pair. |
| Inter-pair skew | Cables may contain multiple twisted pairs; the twist rate between pairs must differ in these cases. This prevents crosstalk from same-side conductors but has the effect of changing the timing between the pairs. Therefore, applications that require multiple differential signals may suffer from using twisted pairs (either due to noise or timing mismatches). |
| Asymmetry | Material differences between wires or shields in a pair or even between nearby pairs can lead to coupling between common- and differential-mode signals, ultimately resulting in an imbalance between the two lines. |
Different Design Options for Twisted Pairs
The shielding and construction of the twisted pair also play a significant role in its functionality. The biggest delineating factor is between shielded and unshielded cables: shielded cables contain an extra sheath between the wires and the outer jacket.
There’s a cost-performance comparison for shielded and unshielded cables, but both find regular use:
- Unshielded: Twisted pairs can be combined into cables containing up to 25 separate pairs for telephone and internet connections of a single residence. Outdoor lines will further subdivide these groups of 25 twisted pairs into individual bundles that are then twisted to form the greater cable. To offset crosstalk, twisted pairs within a bundle contain varying twist rates.
- Shielded: Shielding of twisted pairs serves two major purposes. First, it acts as a barrier for EMI originating from or coupling to the twisted pairs. Second, it offers a conductive path to ground in the event of coupling that further electrically isolates the twisted pairs; this may include a drain wire which makes intentional contact with the shielding material.
Shielded twisted pairs come in separate varieties:
- Individual: Each twisted pair within the cable possesses its shielding, equivalently known as pair in metal foil (PiMF). This configuration isolates individually shielded pairs from EMI and crosstalk.
- Cable: All twisted pairs are encompassed with a shielding foil, also known as an overall foil. This prevents EMI on the cable, although twisted pairs can still exert or be susceptible to internal influence.
- Full shield: A combination of individual and overall shielding, full shielding can provide the most comprehensive protection against EMI and crosstalk concerns.
Beyond shielding, twisted pairs can take on alternate forms for enhanced mechanical properties. Bonding is available to ensure the twist configuration after wrapping despite field usage that may otherwise impact the twist due to shock and vibration. This can be extended to a ribbon wire where twisted pairs are bonded and then to adjacent bonded pairs for easy cable assembling. For permanent installations, a solid core twisted pair provides a hardier connection but lacks some of the flexibility of traditional twisted wire setups.
Your Contract Manufacturer Keeps Your Cable Assemblies Straight
Twisted pair wire advantages and disadvantages have many considerations that assemblers will want to weigh before committing to a particular style. Compared to optical and coaxial cables, twisted pairs may offer cost-effective performance for networking applications’ short- and medium-length runs.
Wire harnesses require some compromises between space, flexibility, and electrical performance, and an expert assembler will be able to effectively plan out the array that best suits the design intent of the PCB. Here at VSE, we’re a team of engineers committed to building electronics across many life-saving and life-changing industries for our customers. Coupled with our professional manufacturing partners, we strive to deliver the highest PCB performance and reliability, whether your manufacturing needs are prototyping or mass production.
