Information Technology Reference
In-Depth Information
FIGURE 5.2
Twisted wire causes interfering electromagnetic fields to couple equally to each wire in the
pair and cancel out at the load.
The twisting of the two wires causes interfering electromagnetic fields to cou-
ple equally to each wire in the pair, as shown in Fig. 5.2, thus effectively canceling
out the resulting interfering signal. This mode of operation is referred to as “bal-
anced” transmission. For proper cancellation, it requires that a desired signal be
applied to the wire pair by a balanced driving circuit and load. The equally coupled
interfering signal is ignored by the balanced load, although a common-mode com-
ponent of the interfering signal may exist. A balanced circuit is shown in Fig. 5.3.
As an additional benefit, electromagnetic emissions from a balanced twisted
pair are reduced (note we said “balanced!”). This prevents the high frequencies of a
LAN signal from interfering with other devices. However, some radiation of the sig-
nal does occur. The transmitted signal amplitude is kept low to maintain the spuri-
ous emissions within acceptable limits. Longitudinal balance of the entire channel is
important for low emissions as well as for susceptibility to outside interference.
UTP/STP Wire Sizes
The conductor diameter in twisted-pair wires is referred to by wire gauge. The com-
mon standard is the American Wire Gauge (AWG).
Desired
signal
Receiver
load
Balanced transmission
Interfering common-mode signal
Accepts desired
balanced signal and
rejects undesired
common-mode interference
FIGURE 5.3
A balanced circuit is used to cancel the effects of common-mode interference.
