| 摘要 |
Disclosed is a passive, highly efficient, low noise coupling device that includes a balun (14) and noise reduction circuitry uniquely configured for converting an unbalanced video signal on a 75 ohm transmission line or connector to a balanced signal on a 100 ohm transmission line or connector and vice versa. The device efficiently allows use of a 100 ohm unshielded twisted pair, such as a conventional telephone cable, for multiplex transmission of video signals or the like in the frequency range of 50 to 500 megahertz. The preferred embodiment has a circuit board with a 75 ohm unbalanced side and a 100 ohm balanced side separated by a balun having a toroid core with a very high permeability. A common mode choke (16) for noise rejection is inserted in the balanced side. The choke has a dual toroid core, one core of high permeability and one of low permeability, and has windings with a 100 ohm characteristic impedance. A common mode toroid shunt (18) on the device's balanced side provides further noise reduction. Circuit board traces connecting the components are impedance-matched to either the balanced side or the unbalanced side as appropriate.
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