davidkho,
A balanced signal/cable is one that supplies a voltage with respect to a common point (ground). A balanced cable is a 2 wire cable that supplies a differential voltage, i.e., the voltage is applied between the two conductors, without a common (reference) point such as ground. Ballanced cables have the advantage that spurious noise pickup to to capacitive or magnetic coupling is applied equally to both conductors, so that the net effect on the differential voltage is zero. Unbalanced cables are usually shielded to avoid the effects of noise pickup. A common example of a balanced signal is the 300 Ohm twin lead cable used by older tv antennas. A common example of an unbalanced cable is coaxial cable, in which the shield serves as the 2nd conductor.
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Matching is important to avoid ringing in response to step inputs, and to provide maximum power transfer. from the source to the load. The characteristic impedance of a cable can be very closely approximated by the following equation: Zo = SQRT(Ln/Cn), where Ln is the inductance per unit length of the cable, and Cn is the capacitance per unit length of the cable.
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A balun serves two purposes: It has a balanced(unbalanced) input and an unbalanced(balanced) otuput winding, and it provides a turns ratio that provides impedance matching as well.
Regards,
Kral