When should twisted pair wires be used?

Question

I know several applications where twisted pairs are used and why they are used in those applications, but I would like to know more about the advantages and disadvantages of using twisted pair wires so I can choose appropriately based on what I am designing.

As an example, I understand the use of a twisted pair in differential signaling. This helps reduce noise because both wires receive the same interference which can then be removed by a differential amplifier.

I also remember reading quite a while ago about an induction forge where the power output was significantly lower than anticipated. The guy figured out that the separation of the power leads to the electronics enclosed a large area, creating extra inductance (which acted as a low-pass filter decreasing the power factor). By taping the power wires in a bundle, the stray inductance was removed.

It seems that twisted pairs offer benefits when inductance or induction are concerns, but this is where my understanding terminates.

What are the advantages and disadvantages of twisted pairs, and for what applications does this make them beneficial?

EDIT: The issue of inductance does not appear to be addressed elsewhere, indicating one application for which twisted pairs are helpful, which isn't addressed outside this question. Similarly, a comment raised the issue of added capacitance, which is not addressed in the linked question.

Answer 1

There are two key advantages to twisted pairs

• Reduced inductance \$ L = N^2 \cdot \frac{\mu \cdot Ae}{l}\$ by twisting the wires together you are reducing \$ Ae \$ the enclosed area and so the inductance.

• Twisting the wires together means they are close together and so any noise picked up in one conductor should also be picked up in the other. A differential measurement should not see it.

The obvious down-side is cost. You will pay a little more if you buy a twisted pair or you can twist them your self but if you consider your time is not free this is an extra cost too.

Answer 2

Elaborating on @ThePhoton's comment by @WarrenHill's request.

The black X's are an increasing magnetic field pointing into the page. The integral form of Faraday's law says $$\oint \textbf{E} \cdot dl = -\int_S \frac{\partial{\textbf{B}}}{\partial{t}} \cdot d \textbf{s}$$ In words this means that there is voltage induced around loops of the wire. As illustrated below, the voltage induced on one wire in one loop is cancelled by the voltage induced in the same wire on the next loop.

This works best if the magnetic field is changing at a constant rate along the length of the twisted pair. If the change is not constant along the length of the twisted pair, then the induced voltage in one loop will imperfectly cancel the induced voltage in the next loop. The twist rate must be smaller than the wavelength of the EMI you are trying to block. You can imagine if you were dealing with an electromagnetic wave with a wavelength equal to the twist rate, you would get a lot of coupling as the sign of the magnetic field would change loop to loop, causing the induced voltages to add up rather than cancel.

Answer 3

This Flux diagram should not be confused with current flow.

The current in the wires does not change at all. Each current flows in the same direction for the length of the cable.

Twisting is a mechanical technique to ensure that the two independant wires remain closely coupled. The effect twisting has on the mag/e fields is that it rotates them in space. Think right hand rule and so on.

At any given point along the cable is a differential parallel pair therefore cancelation occurs due to the proximity to each other. Side x side, Over x Under, Diag in either direction or at any angle. It does not matter as long as they remain closely coupled.

The coupling distance does have some affect on the quality of the cancelation of the common mode interference.

Ribbon cable is a good example of a diff pair over a "gnd" plane but its not the only one. Another is a tri-twist with a diff pair coupled with a "gnd" wire sometimes also referred to as a shield when its made of foil and wrapped around the outside rather then entangled with the diff pair.

Also important is how the wires are terminated including the shield and their consistancy over the length of the cable.Termination is related to the cable's own impedance. It is in someways like two equal series resistors.

The pair's twist rate or pitch has little to do with the diff signal's frequency as much as the cable's capacitance does which should be matched to the application otherwise the signal could be attenuated within the cable.