Common Mode Signals and the Twisted Pair

People dealing with networking and instrumentation would have often come across the term “twisted pair”. This usually consists of two insulated wires twisted tightly together and is the most common method used to prevent noise on the wires from being carried into a device. This applies to most cables carrying signals related to communication on RS-485, RS-422, computer-network, video, audio and telephones systems. In technical terms, these signals are known as Common Mode Signals.

When you have a local common or ground, any signal with this as the reference and appearing on both the lines of a two-wire cable, with equal amplitude and in-phase, is called a Common Mode Signal. Therefore, if one of the wires is tied to the local common, the common mode signal will be non-existent. There are three ways such signals can arise – radiated signals can couple equally with both lines, the driver circuit may have an offset from the signal common or a ground differential may exist between the receiving and transmitting locations.

For an example, consider the three-phase Y-distribution lines of the AC power system. The neutral current of such a system flows through the earth and this can be 10-70% of the total neutral current flowing in the primary circuit. Ground differentials vary between locations and can be as much as several volts to several tens of volts. This differential can cause the three phases to be unbalanced by 0.2VRMS to 5VRMS.

Noise signals may appear in a cable for various reasons. For example, noise can be capacitive coupled from nearby electric fields, inductively from local magnetic fields, electromagnetically from radio signals or conductively from circuit path leakages. However, when you have a twisted pair line, it intercepts the coupled signal equally, making the incident signals appear only as common mode signals. You have a balanced twisted pair line if there is identical impedance from each line to the local common.

When driving audio signals across a pair of twisted wires, either wire has the same chance of being coupled to some unwanted signal or noise as its twisted partner. That means this common mode signal or unwanted signal appears equally on both wires. The audio circuitry is designed to reject this type of common mode signal and this characteristic is known as its CMRR or Common Mode Rejection Ratio, expressed in dB. Achieving circuit balancing is carried out in two ways – most commonly through impedance balancing and through differential balancing.

With impedance balancing, you can achieve better common mode signal (noise) rejection, as there is a balanced connection to ground. The simplest but most effective way is to use two matched resistors from each line to the common ground. The cable wires must also have the same diameter and resistance for the balancing to be most effective.

In differential balancing, the source equipment transmits the normal signal through one of the conductors of the twisted pair and a polarity-inverted signal through the other. Such differentially balanced or symmetrical lines offer the highest common mode rejection ratios, even though the principle remains the same as that of impedance balancing.