| 摘要 |
Apparatus for reducing the noise immunity of a tone selective receiver to prevailing noise conditions in a mains network, through which signalling takes place between a network unit and a plurality of terminal units subordinated to it, all of which are connected to the network. The tone selection circuit 3 sends a data flag signal DF, the duration of which is measured in a first time measurement circuit. If this duration is short relative the duration of the tone of the transmitted tone signal, more and more capacitors are connected into an intergrating circuit 7 included in the tone selection circuit. The integrating circuit 7 thus determines the noise immunity of the tone selection circuit, until short-duration DF signals are no longer detected. The invention includes the use of a large number of capacitors C1-C2n in the integrating circuit. In a corresponding manner the capacitors are disconnected stepwise each time a second time measurement circuit has determined that no interference has occurred during a given predetermined time period following the connection of a new capacitor into the integrating circuit. |
