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Interference in Balanced and Unbalanced Circuits

With respect to ground as the reference plane, two separate types of lines and networks exist: first, those that are completely unbalanced, such as the coaxial cable which operates with the shield at ground potential; and second, those circuits that are carefully balanced with respect to ground as discussed in this chapter.

In the unbalanced circuit, interfering noise and crosstalk are controlled by carefully shielding the circuits, the sheath acting as the shield in the coaxial cable. In the balanced circuit no shielding is provided; the balance (including the transpositions) keeps the induced voltages equal, and, since they are in the same directions, they cancel.

Extreme care must be taken in connecting balanced and unbalanced lines (or other equipment) to each other. When this is done, these two basically different circuits should be isolated metallically from each other by transformers (or repeating coils), and for best results these should have a grounded shield between the primary and the secondary.



Last Update: 2011-05-30