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Engineering & Design – Cable Manufacturing Gets Up to Speed

The speeds of today's technologies are placing heavy demands on wire and cable. This has led to some significant advances in the manufacturing process.


May 1, 2000  


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The components of wire and cable have stayed relatively consistent throughout the years: start with some copper, insulate it, perhaps pair and/or cable those components together and then jacket it.

Yet, while the individual parts of cables may not have changed significantly, the speeds of today’s technologies necessitate that wire and cable be made to higher standards than ever before. This has led to some notable advances in modern wire and cable manufacturing and materials over the past several years.

TODAY’S REQUIREMENTS

In the early days of plastic insulated wire, a specific gauge size of wire was selected and insulation was applied to it. This insulated conductor was either twisted with another similar conductor to form a pair or was cabled with a several others to form a bundle. Typically, each conductor was a different colour for easy identification. This finished product then had AC or DC power, voice or an alternate low frequency type signal on it. The process did not require much more thought than that.

As technology developed, frequencies increased. The demands placed on the physical wires have gone from essentially DC and 20 kHz signals to many MHz. These technology changes require cables that can perform to ever-increasing frequency specifications.

Today, the methods used in manufacturing data cables are extremely critical to producing wires that are capable of carrying high-speed signals.

A LOOK AT CONDUCTORS

The core of any wire product is the conductor itself. Conductors are usually comprised of a copper-based metal that is soft and pliable. In the past, copper was simply drawn down to a specific size by a wire drawing operation. The conductor was taken up on a large spool and then delivered to the extruder for insulating. This was a great idea for many products, and is still a viable and frequently used method. However, a conductor drawn perfectly round and then wound on a large supply spool experiences significant weight and force as more and more conductor is taken up on that spool. This pressure can deform the soft copper enough to ruin the electrical performance of a finished high-tech product.

In order to solve this problem, a method known as “in-line drawing” was developed. This allows the copper conductor to be drawn down from a slightly larger diameter to the final size as the conductor is pulled through the extruder. By using this technology a perfectly round supply conductor is re-formed.

WIRE INSULATION

The next step in the process involves insulating the wire. First, the correct insulation type must be selected. For pure DC and low speed applications, a PVC type of plastic is traditionally chosen. However, PVC is a poor insulator for high-speed applications. Today, a high-grade polyethylene or polypropylene is most often selected for a non-plenum NEC application; a flame-retardant compound such as FEP may be chosen for plenum use.

The insulation is formed onto the wire by a piece of equipment known as an extruder. An extruder forces plastic through a set of tooling that sizes and shapes the plastic around the conductor. To achieve the highest performance, it is critical that the conductor be placed precisely into the centre of perfectly round plastic.

For many non-critical applications, it is only necessary for the wire to stay chiefly in the centre of the plastic so that the conductors have a minimum amount of dielectric insulation around them. This will prevent conductors from shorting to each other. This process requires a trained extruder operator to set up the machine, measure the conductor size, get the conductor centred, and then just let the machine run.

This is no longer the case with high-tech wiring. The insulation must be constantly monitored and adjusted to maintain centring and finished diameter. In fact, the extrusion process for this application nearly always requires computer monitoring and continuous automatic adjusting to have a capable finished product.

However good the insulated single may be, it is most likely not perfect. Any imperfection will show up in future testing of the product. Even if the single is perfect, the challenge remains to keep it damage-free during subsequent manufacturing processes. In traditional manufacturing, the two singles would be placed into a machine that spins the conductors around each other to form a pair. This is great for traditional DC or AC type circuitry, but the high-tech electronics of today operate in what is known as “balanced mode”. This means each conductor of a pair is carrying a signal equal in magnitude, but opposite in phase, to its opposing conductor. Therefore, the two signals must travel down the two conductors in exactly the same way.

For best transmission, the two conductors must be as nearly identical to one another as possible. This is referred to as “pair balance”. If the two conductors are not identical, the signals in the conductors will be received differently and the balance will be lost.

LOSING BALANCE

Balance can be lost in traditional twinning in a couple of ways. First of all, the tension on the two conductors must be equal as they travel into the twinner. If one conductor has more tension on it, the looser conductor will have a tendency to wrap itself around the tighter one. In addition, as the twinner twists the two conductors into a pair, the process adds twist into the individual conductors. This is very damaging to a finished pair’s performance. Not only does the imperfect pair have some degree of variation from the way it was made, but during this process the variations become more random in relation to the other conductor in the pair.

There are a few solutions to this problem. One commonly used processing method is referred to as “pre-twisting” or “back-twisting”. This process tries to emulate a planetary or neutralizing cable in one small package, made specifically for twinning pairs. The best idea is to actually affix the singles of the pair together so that it is not possible for the two conductors to ever change their relationship with one another.

Not only is the process of twinning the pairs together important, but the length of the twists is also critical. In order to prevent the coupling of high frequency signals from one pair to another, different twist lengths must be used on each pair. If you look at any Category 5 or higher rated cable, you can easily see the difference in twist ratios. Without these differences, the signal on one pair couples onto the other pairs and causes large amounts of noise, known as crosstalk.

The quality of the singles and the balance of the pairs also affect the amount of crosstalk that occurs within a cable. Tight quality control throughout the entire manufacturing process is the key to having a successful finished product.

PAIRING AND JACKETING

The next step in processing high-tech cables involves cabling the pairs together to make an overall assembly. Whether this involves two pairs, four pairs or 25 pairs, it is critical to the cable’s electrical performance. Each pair must have a different twist ratio to that of any nearby pair, in order to minimize signal coupling.

Conductor lengths must also be considered. When twisting each pair at a different rate, the shorter lay pairs will have more physical conductor length in them than the longer lay pairs. For example, the tightest lay pair in a 1000 foot spool may have up to, or more than, 25 feet more conductor length than the shortest pair. This contributes to “delay skew”. If the pairs have very different lay lengths, the signals travelling on different pairs will reach the end of the cable at different times.

Finally, the cabled pairs must be held together by an overall material, usually a PVC, known as the “sheath” or “jacket”. This may appear to be a simple procedure, but the material must be chosen on several criteria. The compound selected must be able to meet the NEC flame rating of the overall assembly and be electrically compatible with the core it will be covering.

After choosing the correct compound, the mat
erial must be applied to the cable with the appropriate tightness. If the jacket is applied too tightly, the poor electrical properties of the PVC may damage the cable’s overall performance. If the jacket is too loose, it will not properly hold the core together to ensure continued electrical performance of the cable after it is removed from the spool.

EACH PART IMPORTANT

There is no part of a high-speed data cable that can be taken for granted. Traditional manufacturing technologies used over the last several decades are not able to produce today’s high performance cables. These products require special attention and capability — from the drawing conductor to the application of the jacket.

This also accounts for many of the reasons why today’s data cables cannot be installed in the same manner as older cables. Anything that disrupts the careful assembly of the overall cable design during installation may cause serious repercussions to the overall performance of a high-speed data cable.CS

Wynn D. Roth, RCDD, is a Technology Development Specialist with Belden Electronics Division in Richmond, Indiana. Mr. Roth joined Belden in 1993 and has held a number of positions with the company, including Product Design Engineering, Product Development Engineering and Product Marketing Specialist.


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