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Introduction
1) When network equipment and terminals are equipped
with different type of cable and connectors. It makes the direct connection
a problem. For example, using cable like coaxial, twinaxial, STP or UTP
is not possible to terminate to different connectors like BNC, Twinaxial,
or RJ45.
2) Balun (Balanced-unbalanced)
is created to match different cable impedance and connectors. A good Balun
should be able to match the cabling system's impedance and attenuation,
and minimize signal distortion. Balun is a word derived from terms applied
to BALanced to UNbalanced transmission lines, and generally refers to
an impedance matching device for interfacing two lines having different
characteristic impedance and often one balanced line to one unbalanced
line. Balun is used to connect balanced transmisson lines (such as UTP
cable) with unbalanced transmission lines (such as coaxial cable). Inside
the Balun is a transformer having two windings matching the respective
impedance of the lines and they are often tuned to provide optimum matching,
maximum return loss, minimum insertion loss and maximum transfer of the
signal with minimum waveform distortion and signal reflection. For example
a Balun can match IBM STP cable (150 ohm balanced) with a coaxial cable
of IBM3270 terminals (93 ohm unbalanced)
3) Baluns are used very extensively for computer cabling
in particularly on some of the older mainframe and mini computers which
were designed to connect via coaxial cables. When the twisted pair cabling
systems were introduced for premises wiring, Baluns were used to convert
the coaxial type equipment to run on twisted pair cables. Very large numbers
of these Baluns were used for older IBM3270 terminals, System 34, 36,
38 and AS400, Wang, Fujitsu Computer Systems. Besides, Baluns are also
used to convert 75 Ohm video signals to run on twisted pair cables for
security, video transmission and video conferencing.
4) Impedance Matching Device (IMD)
usually are used to connect both balanced lines, but with different impedance.
For example, IBM Cabling System (150 ohm balanced) needs to attach to
a 10Base-T terminal (100 ohm balanced).
5) Being different from the IMD, Adapters do not contain
transformers. They simply provide mechanical adaption and electrical contact
between two types of connects UTP with RJ45 modular connector (100 ohm
balanced).
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