34
SIGNALS CONVEY INTELLIGENCE
The insulated conductors of every circuit, such as our wire pair, have to a greater or
lesser degree this property of capacitance. The capacitance of two parallel open wires or
a pair of cable conductors of any considerable length is appreciable in practice.
2.5.1.2
Bandwidth (Hz) of a Twisted Pair
. The usable bandwidth of twisted wire
pair varies with the type of wire pair used and its length. Ordinary wire pair used in
the PSTN subscriber access plant can support 2 MHz over about 1 mile of length.
Special Category 5 twisted pair displays a 67-dB loss at 100 MHz over a length of
1000 ft.
2.5.1.3
Bandwidth Defined
. The IEEE defines bandwidth as "the range of frequencies
within which performance, with respect to some characteristic, falls within specific limits."
One such limit is the amplitude of a signal within the band. Here it is commonly defined
at the points where the response is 3 dB below the reference value. This 3-dB power
bandwidth definition is illustrated graphically in Figure 2.11.
2.5.2
Coaxial Cable Transmission
Up to this point we have been discussing two parallel conductors, namely, wire pair. An
entirely different configuration of two conductors may be used to advantage where high
and very high radio frequencies are involved. This is a coaxial configuration. Here the
conducting pair consists of a cylindrical tube with a single wire conductor going down its
center as shown in Figure 2.12. In practice the center conductor is held in place accurately
by a surrounding insulating material that may take the form of a solid core, discs, or beads
strung along the axis of the wire or a spirally wrapped string. The nominal impedance is
75 ohms, and special cable is available with a 50-ohm impedance.
Figure 2.11
The concept of the 3-dB power bandwidth.
Figure 2.12
A pictorial representation of a coaxial cable section.
Summary :
34 SIGNALS CONVEY INTELLIGENCE The insulated conductors of every circuit, such as our wire pair, have to a greater or lesser degree this property of capacitance. The usable bandwidth of twisted wire pair varies with the type of wire pair used and its length. Here the conducting pair consists of a cylindrical tube with a single wire conductor going down its center as shown in Figure 2.12.
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