By David Herres
Capacitive and inductive reactances are present throughout electrical equipment in proportion to the frequencies used. This is particularly true in long transmission lines. But then everything is transmission line at high frequency, including active as well as passive devices. A simple carbon resistor in the RF range and above has capacitive as well as inductive elements. The leads constitute the plates of a capacitor and the resistive material is the dielectric layer. Because it conducts current, the resistor is an inductor, surrounded by a rapidly fluctuating magnetic field. The same is true of semiconducting devices such as diodes and transistors.
Two wires sitting in the same cable or raceway can couple electromagnetically at high frequencies because of their proximity. Mutual inductance is the primary problem but electrostatic interference from outside sources can be a problem as well. Placing the cable in grounded metallic conduit or metallic shielding greatly reduces these problems.
At very high frequencies, electromagnetic coupling becomes a fact of life in point-to-point chassis wiring, printed circuit board traces and even within microchips. Good design can prevent losses from parasitic capacitive and inductive reactance. Conductors including traces should be short and straight. A slight bend is equivalent to a partial winding, boosting inductance significantly. Consider the equation for inductive reactance, XL = 2πfL. When f has a high value, so does XL and it rises proportionally with f.
In high bandwidth data cabling, two conductors in close proximity are, in effect, plates of a capacitor. Accordingly, capacitive reactance between the two conductors tends to shunt out the high-frequency signal. Similarly, each conductor has inductance and at higher frequencies the inductive reactance is high. Inductive reactance is a series phenomenon, so it tends to weaken the signal. This unfortunate fact limits the upper frequency of signals that a given conductor can convey.
An effective remedy, employed in category – as in Cat 5, Cat 5e (enhanced), Cat 6 and Cat 6a (augmented) – cabling, is to use twisted pairs of conductors, as in the near-by illustration. How can twisting conductors mitigate reactive loss?
The answer is in the strategy of differential transmission. The technique originated in nineteenth century telephone technology when high-powered, brush-type motors became commonplace. These motors were problematic, especially in large cities which at the time were great centers of manufacturing, because they generated significant electrical noise. The noise tended to inductively couple into long telephone lines. Telephone engineers devised the strategy of line transposition as a countermeasure, where lines crossed and changed places typically every third pole.
Today’s high bit rates need a much tighter twist. In fact, the more recent versions, notably Cat 6a compared to Cat 5e, employ a denser twisting.
Another means of heading off the effects of inductive coupling is to employ differential mode transmission. Here two conductors carry equal but opposite phases of the same signal. Magnetic fields tend to induce current equally in two wires that are the same distance from the fields. Twisting the two wires reduces the effect because, in the course of a half twist, first one wire, then the other is closer to the source of the interference. Thus the effects on the two wires cancel out
The twisted pair strategy also minimizes the emission of magnetic and electrostatic interference. And because eliminating this emission also eliminates a loss of energy, it further reduces signal attenuation.
For these reasons, unshielded twisted pair (UTP) has become the dominant medium in Ethernet networking, having replaced the coaxial cable which once was standard for early versions.
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