#emi interference

Reduce Emissions and Boost Immunity Just by Twisting Cable Leads

Why are some cables conductors twisted and others not?  

Wire twisting is a detail of cable construction that is simple.  But surprisingly many engineers don't really understand its advantages. Most have an idea that twisting is good for electromagnetic interference (EMI), but when asked they are at a loss to explain why.

Four pros

Twisting wire pairs in a cable has four major benefits.  Two are pretty obvious. Two are pretty subtle.

Twisting improves performance when two adjacent wires are carrying signal and return current in opposite directions.  Likewise, for power and return current.

1. Magnetic field cancellation

Magnetic Field Emissions

In the space between two parallel wires carrying current in opposite directions, the magnetic field is additive.  If the wires are not twisted, the magnetic field in the vicinity of the wires is non-zero. Higher current amplitude and greater distance between wires produces greater magnetic field.  A circuit loop intersected by that magnetic field, such as another untwisted wire pair in the same bundle, will experience an induced voltage.

When the wires are twisted together the magnetic field in adjacent half-twists are in opposite directions and tend to cancel.  More twists give better field cancellation.  Over their length the net magnetic field emitted from the wires is near zero, so an adjacent circuit loop experiences near zero induced voltage.

Magnetic Field Susceptibility

As mentioned above, untwisted wire pairs create a circuit loop that if exposed to a magnetic field will experience an induced voltage.  By not twisting the wire pair, the circuits at the ends of the cable will see more induced noise and will therefore be more susceptible. Twisting the wires together causes incident magnetic fields to induce voltage having opposing polarity in adjacent half-twists.  The net effect is lower induced voltage and improved noise immunity.

2. Reduces electric field pickup and radiation

Electric Field Emissions

Most of the time there is a potential difference between wire pairs of a circuit.  Power and return may have ac or dc voltage between them, differential signal pairs carry voltages that are repeated swapping polarity, and single-ended signals alternate between low potential difference (logic 0) and high potential difference (logic 1).

The electric field originates on the conductor having highest potential and terminates on the conductor having lowest potential.  Higher voltage difference produces stronger electric fields.  Greater distance between wires produces broader fringing.  If the electric field terminates on a nearby conductor, current is induced.

When the wire pair carrying the voltage is twisted together the electric field lines from adjacent half-twists are in opposite directions and tend to cancel one another.  More twists improve field cancellation. Over their length the net electric field emitted from the wires is near zero, so adjacent circuits are less affected.

Electric Field Susceptibility

Just like emissions are lowered by twisting, immunity to electric field pickup is improved by twisting.  Wire pairs exposed to an electric field that is not parallel to the wires will have current induced on the wires.  If the wires are twisted together the current induced in each adjacent half-twist is in opposite directions, so the net effect is near zero induced current and circuit noise immunity is improved.

3. Keeps wire close together

Subtle, but important

Twisting has the added benefit of automatically pulling the wires tightly together. And maintaining their separation at a uniform distance, twice the thickness of the wire insulation.

Characteristic impedance of a wire pair is directly related to wire separation.  By maintaining uniform spacing between wires over their length, characteristic impedance is constant, a property very important to signal integrity.

Electric and magnetic fields emitted from and coupled to a wire pair are directly related to distance between the wires.  Minimizing wire separation minimizes emissions and pick up.

4. Balances common mode impedance

Even more subtle, also very important

Often wire pairs are routed near a conducting surface, a ground plane. This is true of wire harnesses in automobiles, aircraft, equipment racks, cable trays, and any installation in a metallic enclosure.

If one wire is closer to the ground plane than the other, the radio frequency impedance of the two wires with respect to ground will be different.  Their common mode impedance will be unbalanced.  

Even if the two wires are uniformly spaced from one another and their end circuits are optimally designed and balanced, overall the circuit will be unbalanced.

In a circuit with common mode imbalance a portion of the differential signal, the signal between the two wires, will appear as common mode noise, noise between the signal conductors and ground plane.  Besides creating signal integrity degradation, radiated emissions from the wires will be higher and the end circuits will be more susceptible to noise.

On the other hand, if the wires are twisted, averaged over their length the wires will be the same distance from the ground plane.  Their common mode impedance will be balanced and the circuit will exhibit lower radiated emissions and greater immunity to noise pickup.

Why Aren't All Wires Twisted?

Untwisted wires occupy less space and are shorter than twisted wires. Untwisted wires are cheaper.  In electromagnetic environments where the benefits of twisted wires may not be needed, untwisted wires may be a suitable choice.  But in most applications, twisting provides significant benefits for minimal cost.

One last note

It isn't always practical to provide a dedicated return wire for every signal wire.  When two, three, or more signal wires share a return wire, twisting the signal wires together with the return wire is still beneficial compared to not twisting.  Just make sure the net signal current through the twisted group is nominally zero.  The circuits will still benefit from partial field cancellation, loop area reduction, and impedance balancing provided by twisting.

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