| 摘要 |
A trunk circuit comprises a first lead pair for carrying two-way signals, a second lead pair for carrying two-way signals, a circuit for electrically coupling an incoming signal from the second lead pair to the first lead pair, a first transformerless circuit for applying an incoming signal from the first lead pair to the second lead pair, and a second transformerless circuit for preventing signals from the first circuit applied to the first lead pair from being reapplied to the second lead pair and for preventing signals from the first transformerless circuit applied to the second lead pair from being reapplied to the first lead pair. Preferably the coupling circuit is comprised of a transformer having a primary winding connected in a low impedance voltage source circuit to the second lead pair, and having a secondary winding connected in a very high impedance-to-direct-current circuit for applying the incoming signal from the second lead pair to the first lead pair. Due to the resulting very low current levels in the transformer, the effective bandwidth of the transformer is substantially increased, allowing use of an inexpensive transformer which can readily be incorporated on a plug-in printed circuit board.
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