| 摘要 |
Electronic subscriber line interface circuits contain sensitive current identification circuits (S) which monitor the current (I) in the subscriber loop. The ringing of a subscriber is signalled by rhythmic interruption of an AC ringing voltage which produces an alternating current in the subscriber loop. AC ringing voltages are normally de-activated near to the zero transition, but, in the switching phase, there is the risk that, as a result of the capacitive load of the connected subscriber line interface circuit, a current flows which deceptively simulates a loop current and leads to erroneous loop identification. This erroneous response is eliminated according to the invention by means of a comparator (K2) with a very low reference voltage (UV) whereby, following evaluation of the loop current (I) by a current identification circuit (S), the ring tripping is carried out in the immediate vicinity of the zero transition of the loop current and the cause of the erroneous loop identification is eliminated. <IMAGE> |
