Low-frequency alternating current signal detector, in particular for central office line interface circuits

Claim 1

A low-frequency alternating current signal detector for recognizing tone signals in a central office line interface circuit connecting a private telephone installation to an analog telephone network subscriber line, said low-frequency alternating current signal detector comprising: an input amplifier, an input of said input amplifier being connected to an input of a bandpass filter and to an input of a first rectifier-filter, a second rectifier-filter is connected between an output of the bandpass filter and an output of a threshold presence detector, an input of said threshold presence detector receives the output of the bandpass filter and when said threshold presence detector detects an output signal from said bandpass filter which is at a level higher than a threshold level, said threshold presence detector enables a comparator to compare an input signal and the output signal of said bandpass filter, such that when the output signal of the bandpass filter has an amplitude higher than that of the input signal of said bandpass filter, an expected alternating current signal is determined to be present and the comparator generates a binary signal in response to the presence of the expected alternating current signal, wherein said comparator is connected to a common point to which an output of said first rectifier-filter and an output of said second rectifier-filter are supplied along with the output of said threshold presence detector.