Patent ID: 7012979

Claim:
A tone signal detection circuit for a receiving circuit for detecting at least one signal having a predetermined tone signal frequency (f E ) which is contained in a received analog input signal, comprising: (a) a reference signal generator for generating an analog converter reference signal V ref which consists of a reference DC (V refDC ) and a periodic reference AC (V refAC ) having a variable fundamental frequency (f G ), which is superimposed on the reference DC (V refDC ); (b) an analog/digital converter for converting the analog input signal into a digital data stream in dependence on the analog converter reference signal (V ref ); and (c) a digital control circuit which adjusts the variable fundamental frequency (f G ) of the reference signal (V ref ) generated by the reference signal generator in accordance with the predetermined tone signal frequencies (f G ) of the tone signals to be detected and evaluates the digital data stream output by the analog/digital converter for detecting a data pattern corresponding to the tone signal.