Basic knowledge of cables

Basic constructions
Wire and cable consists, for the most part, of four basic constructions:

• Single conductor
• One conductor, bare or insulated.
• Multiconductor
• Multiple insulated wires
• Twisted pairs
• Two insulated wires usually twisted together
• Coaxial cable
• Insulated center conductor with a shield and the jacket overall.

Solid and Stranded
Conductors come in two variations, solid and stranded. Solid (Figure 1) offers a slightly lower resistance. The key to solid conductors is better performance at high frequencies.

Figure 1

Stranded (Figure 2) offers greater flexibility that is limpness, and greater "flex-life", or flexes until failure.

Figure 2

Multiconductor
As the name implies, multiconductor cables consist of many conductors. (Figure 3.) They are common in control applications but are rarely used for signal applications, and therefore, we will not be focusing on them for this paper.

Twisted Pairs
Twisted pairs (Figure 4) consist of two insulated wires twisted together. They are specifically intended for carrying signals and were invented in the 1880’s for wiring up the early telephone systems. Twisted pairs offer low noise pick-up and low noise emission from a cable because it is a balanced line and because a balanced line offers "common-mode noise rejection".

What is a Balanced Line?
A balanced line is a configuration where two wires are kept close together, usually by twisting them (Figure 5). Conductors need to be the same length, the same size, with a constant distance between them.

Figure 5

It should be noted that the signal, at any instant in time, is exactly the same but opposite polarity on the two wires. Another way of saying that is, if you note the signal voltage at any point of the cable, they should add up to zero.

When Noise Appears….
Noise is a fact. It is electromagnetic radiation and can come from many sources including fluorescent lighting, motors, car ignition systems, equipment such as hospital analyzers, transmission equipment from CB’s, truck, taxis, radio and television broadcasters, and natural sources such as the sun.
When noise appears, and hits the two wires in our twisted pairs (Figure 6), the electromagnetic radiation of the noise induces a voltage in both wires. However, the direction is the same or "common mode" in both wires.

Figure 6

When the two noise signals reach either end of the cable, there is either a passive balancing device (such as a transformer shown in Figure 6) or the equivalent active balanced input. As you can see, the two noise signals on the two wires cancel each other out. In this way, the noise cancels out and the signal (which is "differential mode") can continue through.

Coaxial Cable
Coaxial cable, or coax, for short, is the other most popular cable configuration. The signal on the cable is not the same on the two conductors since the shield carries both ground and signal. The two conductors are not the same size, same resistance. Therefore, coax is not a balanced line. It is an unbalanced line.
Coax does not have the inherent noise rejection of twisted pairs. However, performance of coaxial cable can be far superior to twisted pairs. First, coax has extremely stable performance. The various parts of the cable are "locked together". (Figure 7) This, as we will discuss later, gives much better performance especially at high frequencies.

Figure 7

Shields
Shields are added to twisted pairs, or multiconductor cables, to help prevent in ingress (interference) or egress (radiation) of noise. Shields are an inherent part of coax cable. There are six basic shield configurations:

• Unshielded
• Twisted pairs, especially in data, are often unshielded. Coax, by definition, cannot be unshielded.
• Serve/spiral shields
• Serve or spiral shields are simply wound around the inner conductors.
• Braid shields
• Conductors woven or braided around a core. Most effective from 1,000 Hz to 50 MHz.
• French braid shields
• This is a combination of serve and braid. Here two serves are braided along one axis.
• Foil shields
• These are the simplest, cheapest, and easiest to apply. They are most effective above 50 MHz
• Combination shields
• Combined foil and braid shields are effective at low and high frequencies.

Unshielded
Unshielded cable is appropriate where no noise is present, such as no crosstalk from adjacent wires. Or it can be used if you don’t care if there is noise, that noise cannot substantially affect the signal on the cable.
Unshielded cables are especially prevalent in the data world where pairs have very tight twists, or may use conductors that are bonded together. Such high-performance pairs are good to excellent at not picking up or radiating noise.

Serve/Spiral Shields
Serve or spiral shields can be made to be ultra-flexible. However, serve shields can open up when flexed, which compromises shield effectiveness.
A spiral of wire obviously affects the inductance of the shield. Therefore spiral shields are rare in video and are usually audio only. There is a double spiral serve, also known as a "Reussen" shield. This configurations "shorts out" the inductive effect of a signal spiral, but the shield can still open up when flexed. This double serve is common on many European and Japanese audio snakes. The ultra-flexibility of the cables is a key. However, it should be realized that users of this cable type often trade flexibility for performance.

Braid Shields
Braid shields are formed by spinning wires or groups of wires around a core. This slow and labour-intensive process makes braiding the most expensive single step of cable manufacturing. Single braid coverage of up to 95% can be realized. Double braid coverage can be up to 98% coverage. Since braids always have "holes" where the wires cross, 100% coverage not possible with braid.
Braid shields are most effective at frequencies from 1,000 Hz to 50 MHz. For these frequencies, the low resistance of a braid gives good coverage. Below 1,000 Hz there is no standard braid material, which is effective. The wavelengths are so long, and the low frequency energy so pronounced, that the only effective shielding is solid steel conduit. In addition, at 60 Hz, even steel conduit gives 27 dB of noise reduction.
At frequencies above 50 MHz, braid becomes "wavelength dependant" where the holes look larger and larger as the wavelength gets smaller and smaller. The effective coverage of a braid gets worse and worse, especially compared to a foil shield, which has no holes.

French Braid Shields
French braid shields are a combination of serve and braid. A French braid consists of two serves braided along one axis. This gives cables excellent flexibility, rivaling those Reussen shield in European and Japanese cables. Yet, unlike Reussen shields, lab tests indicate that French braids have excellent RF performance. This may be partly because the braiding "shorts out" the inductive effect of serve shields and "shorts out" the RF noise too.
Because it takes the same number of steps as a regular braid, French braids are the same cost as regular braid. Maximum coverage of a French braid is 98%.

Foil Shields
Foil shields are the easiest and cheapest to apply. They can be applied as fast as the cable will run. Foil shields actually consist of two layers, a metal layer and a plastic substrate of polyester. This can be easily seen since the foil is silver on one side and colored (red, blue green or other colours) on the plastic side.
Since foil, shields lack the mass and low resistance of a braid shields, the exhibit poor to average low-frequency performance. However, after 50 MHz, foil shields have excellent high frequency coverage. Since foil is a continuous sheet of metal, coverage can be 100%.

Combination Shields
Combination shields consist of foil and braid combined. Occasionally there can be more than one layer of each, such as "quad" cable television cable, so called because it has two layers of foil and two layers of braid. Because of this, combination shields are the most expensive of all. However, they also give the best broadband coverage, since it contains a braid for low frequencies and a foil for high frequencies
The difference between broadcast coax cables, which often contain foil and braid in digital applications, and CATV/broadband cable is that CATV cables use low coverage braid (sometimes as low as 40). The reason is that these cables only operate above 50 MHz. At those frequencies, braid shields are ineffective. It is actually the foil shield that is doing all the noise reduction. The braid shield is there to give the F-connector something to grab onto. It’s a reliability issue, not a performance issue. CATV braids are aluminium belying their low cost and indicating that this braid is not included for performance.
Combination braids are required for digital video such as SDI or HD. The Broad frequency range of SDI (135 MHz) or HD (750 MHz) make a combination shield a requirement. That being said, it should be notes that double-braid cables (such as Belden 8281) can still operate at these high frequencies. It is simply that the effective distance they can run is severely reduced compared to cables with foil + braid (among other improvements). Most precision digital cables contain 95% braid + 100% foils