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It is common knowledge in electrical and electronic cable design that twisting wire pairs together is usually a good thing. However, the benefits of twisting and the degree to which twisting is valuable is less well understood.

This article illustrates the effects that twisting can have on cable radiation.

Get More Background

In an earlier article we discussed in conceptual terms, how cable twisting improves electromagnetic compatibility (EMC) and reduces electromagnetic interference (EMI). In this article, using EMI Analyst™ software, we explore tangible examples that quantify the EMC/EMC benefits of wire twisting. The previous article, How Cable Twisting Improves EMI can be found in the download area of the EMI Software website.

Which Wires to Twist Together?

Twisting can reduce cable radiation when two adjacent wires are carrying signal and return current in opposite directions. The benefit is greatest when the opposing current has equal magnitude and is 180-degrees out of phase. Fields emitted by adjacent half-twists tend to cancel. Even if the wires carry unequal current or if the current is not completely out of phase, partial field cancellation is still beneficial.

Likewise, when a wire pair having a voltage differential is twisted together the electric field lines from adjacent half-twists are in opposite directions and tend to cancel one another. Over the cable length, net electric field emitted from the wires is lower.

We Are Talking Differential Signals Here

Twisting reduces radiation caused by differential mode signals. Twisting does nothing for common mode signals.

Differential mode signals are time-varying voltage and current waveforms on the wires. Signals in this context may carry information or may be an unintended byproduct of circuit operation. Differential mode current flows one direction on one wire and the opposite direction on the other wire. Differential mode voltage is the potential difference between the wires.

Common mode signals, also time-varying, exist between the wire pair and another conductor, usually ground, chassis, or some other reference. Common mode current flows in the same direction on both wires and returns on the third conductor. Common mode voltage is the potential between both wires and the third conductor.

Let’s See an Example

The five graphs below show how radiated field levels decrease as the number of turns increase. In general, more twists beget lower radiation.

The circuit for this example is a digital driver that produces a 3-volt, 1 MHz trapezoidal waveform. The output of the driver is connected to a digital receiver through a 1-meter long unshielded wire pair cable in free space. The wires are #22 AWG with 1 mm spacing between them. There is no common mode signal component. Twisting is varied from 0 to 100 uniformly spaced twists per meter. The limit line shown on the graphs is for reference only.

0 Twists Per Meter

5 Twists Per Meter

10 Twists Per Meter

40 Twists Per Meter

100 Twists Per Meter

The Effect of More Twists

Clearly, more twists equate to lower radiation. However, there is diminishing return. For this example, the first five twists provide about 25 dB improvement. It takes another 95 twists, 100 twists total, to get another 25 dB. For any wire pair, there is a physical limitation to the maximum number of twists per meter.

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