Twisted Pairs: Why are Wires Twisted?

April 15, 2020

The twisted pair of electrical wires is a cabling technique used in audio and network technology, among many others. Wires are twisted together to accomplish two main goals: reduce outgoing noise and reduce incoming noise related to electromagnetic interference.

How does a twisted pair of wires work to prevent noise? In this article, you’ll learn the basic science behind the twisted pair technology. You’ll also find a video comparing twisted, untwisted, and spaced pairs of wires so that you can visualize and hear the difference for yourself.

Wires are twisted together to accomplish two main goals: reduce outgoing noise and reduce incoming noise related to electromagnetic interference.

Where Does Noise Come From?

Firstly, if we are going to understand how noise is rejected through the use of a twisted pair, it is important to understand where the noise comes from.

Any time an alternating electric current is passed through a wire, a magnetic field is created around that wire. The opposite is also true. Whenever a magnetic field interacts with an electrical circuit, an electric current will be generated within the circuit. This means that a signal in one circuit can “jump” to another nearby circuit, through a phenomenon called induction. 

Imagine a power cable running along the floor. In the United States, power operates at 60 Hz. That means that there will be an alternating electric current flowing through the power cable at 60 Hz. Given that current running through a wire creates a magnetic field around that wire, the power cable will be radiating an alternating magnetic field at 60 Hz. If you place another electrical circuit in close proximity to the power cable, that magnetic field will induce a 60 Hz current onto that circuit. 

This causes a common issue, called crosstalk, in which a signal from one circuit is transferred to another circuit through electromagnetism. To reduce crosstalk is the primary goal of twisting wires.

How Do Twisted Pairs Prevent Noise?

A circuit contains two wires, each wire carrying equal and opposite voltages. If there is a positive voltage on one wire, there will be an equally negative voltage on the other. The voltages on the two wires are the same, but with inverted polarity. The polarity of the electric voltage running through the wire determines the polarity of the magnetic field created around the wire.

Not only is the polarity of the electric voltage opposite on each wire, but the polarity of the magnetic fields radiating from each wire is also opposite. When equal but opposite forces meet, they cancel.

You can see that illustrated in this image. These two waves are identical in frequency and amplitude, but opposite in polarity. When one wave is positive, the other is negative. When added together, they counteract one another and result in no voltage at all.

Twisted Pairs Reduce Outgoing Noise

Placing the wires next to one another will greatly reduce the energy of the magnetic fields created by each wire. 

That’s because the positive wire creates a positive magnetic field and the negative wire creates a negative magnetic field. The two magnetic fields interact and work against one another. This is called destructive interference.

Twisted Pairs Reduce Incoming Noise

Remember that magnetic fields caused by other cables or devices can inject noise into a circuit. Placing the wires of a circuit in close proximity can also help to prevent this from happening. 

The signal passing through the circuit is opposite in each wire of the circuit. When the signal reaches the destination, the device that receives the signal expects to see opposite voltages on each wire. 

However, any magnetic field that interacts with the wires along the path from source to destination will generate a voltage that is the same in each wire. The receiving device will see that there is a common voltage, and reject it. This is called common mode rejection. 

The Benefit of Twisting Wires

In both cases, placing the wires close to one another reduces the amount of noise entering or exiting the circuit. However, placing the wires near one another isn’t nearly as effective as twisting them, and here’s why. 

Destructive interference and common mode rejection only result in complete cancellation when the two opposite energies are perfectly aligned. 

When the wires are simply next to one another, they don’t truly occupy the same position. This means that a nearby circuit will almost always be closer to one wire than the other. Likewise, a magnetic field passing by will almost always interact with one wire before the other. 

When the opposite signals are not perfectly aligned, some cancelation occurs, but not complete cancellation.

Ideally, the two wires will occupy the same exact position so that any nearby circuits or noise sources are the same distance from each wire. This isn’t possible in the real world, however. 

Coaxial cables are one of the best attempts at achieving this goal. The center conductor shares an axis with the outer sleeve. This is quite expensive to produce, difficult to terminate, and limits a cable to carrying only one channel. 

Twisting wires is a great compromise that allows a cable to carry many channels while still placing the wires in very close proximity, rejecting a significant amount of noise. 

Of course, twisted wires are not occupying the same position. But here’s what is accomplished: along the length of the cable, the two wires alternate. This results in each wire sharing the same average position.

A nearby circuit or noise source may be closer to one wire at some points, but will be closer to the other wire at other points. It’s not a perfect method, but it’s extremely effective!

How Effective are Twisted Pair Wires?

Learning why twisted pairs work is one thing, but seeing and hearing the difference for yourself is also useful. In this section, you’ll find a video that shows how effective twisted wires are at rejecting noise compared to spaced wires.

In this video, I put a signal onto a red and black wire that is opposite polarity in each wire. I use a tool called an induction amplifier that transduces magnetic energy into electric energy and plays it through a speaker. 

You’ll hear that the noise is most extreme where the wires are furthest apart, a bit quieter when the wires are close together, and almost non-existent when the wires are twisted!

The tool I use in this video is a cable tracer. It is very useful for troubleshooting. These tools allow you to hear the signal on a wire by holding the wand in proximity to the wire.

Twisted pair technology is applied virtually everywhere you look. It’s a simple principle that, when applied appropriately, acts as the foundation for many very complicated technologies. This is the case with many things in audio and network technology. That is why learning basic underlying principles is a great way to grow your understanding of more advanced concepts.

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