the invention relates to a bandpass filter that follows the frequency of an input signal especially for reducing the noise on the receive band of mobile communications devices . more specifically the invention relates to a method and arrangement for tuning a resonator , where an input signal feeds the resonator in such a manner that the resonator oscillates at , the frequency of the input signal , and where the center frequency of the pass band of the resonator is set substantially to the frequency of the input , signal . in accordance with the invention , there is generated a difference signal proportional to the phase difference between the input signal voltage and resonator voltage and the resonance frequency of the resonator is changed using the difference signal in such a manner that said phase difference becomes smaller . using the invention , the resonance frequency of the resonator can be set relatively accurately to the input signal frequency because the phase characteristic of the resonator is at its steepest at the resonance frequency , whereby the phase difference detector reacts strongly on even the slightest changes in frequency .