Tone operated switching arrangement

摘要

Disclosed is a switching arrangement operable by a tone of a predetermined frequency. An automatic gain control amplifier has an input connectable to a source of signals and an output connected to an input of a bandpass filter which rejects all frequencies except those of a narrow band centered on the predetermined frequency. The bandpass filter is connected to an input of a phase locked loop which produces a change in voltage from a first level to a second level at its output only in response to a narrow band of frequencies centered on the predetermined frequency. The output of the phase locked loop is connected to a timer which only produces a pulse at its output upon receipt of the second level of voltage for a predetermined time. A pulse from the timer is fed to a flip-flop which changes state and operates a relay. The arrangement is particularly useful in a loop-back unit whereby equipment may be disconnected from a receive line and a transmit line and the two lines connected together for testing purposes.