| 摘要 |
<p>A carrier loop comprises first and second carrier channels coupling signals in opposite directions and having at each end thereof a hybrid with one pair of opposing ports connected to the adjacent ends of associated channels. A balancing network is connected to a port of the other pair of opposing ports of one hybrid. A first relay adjacent the one hybrid has a pair of normally open contacts connected across the balancing network and has pairs of normally closed contacts connected in series with associated terminals of the other output port of the other pair of opposing ports of the one hybrid. A second relay adjacent the other hybrid has pairs of normally open contacts connecting a test signal source to associated terminals of one port of the other pair of opposing ports of the other hybrid, and has pairs of normally open contacts connecting an impedance network and a meter to one port of the one pair of ports of the other hybrid and to the associated end of the second channel, respectively, that is normally connected to this one port. The operation of the loop is tested by energizing the second relay to connect the test signal source and impedance network to associated ports of the other hybrid and the meter to the end of the second channel, and by transmitting a control signal over an associated test channel to energize the first relay to short-circuit the balancing network and open-circuit the output port of the one hybrid to produce 0 dB return loss in the latter. This causes the test signal that is coupled through the other hybrid to be transmitted on the first carrier channel and coupled through the one hybrid, onto the second carrier channel. The returned test signal on the second channel is applied to the meter to provide a measure of the operativeness of the carrier loop.</p> |
