An acquisition circuit for providing a first and a second validation signal based on an alternator output signal has a coupler being adapted for receiving the alternator output signal and for providing a coupling signal having information on the phase of the alternator output signal, the coupler having a capacitive element for coupling the alternator output signal and the coupling signal, the coupler having a resistive element for coupling the coupling signal and a reference signal. The acquisition circuit further has a detector adapted for receiving the coupling signal and for providing the first and second validation signals, wherein the relation between the validation signals is based on the information on the phase of the output signal, wherein the frequency of the alternator output signal is derivable from the validation signals and the validity information is derivable from the relation between the validation signals.