A signal processing unit is described relates to a communication system by radio waves with frequency modulation comprising a signal processing unit, a transmission stage for transmitting a transmission signal in response to a modulated control signal and in response to a first frequency reference signal, a modulator connected between the processing unit and the transmission stage for forming the modulated control signal in response to an output signal of the processing device and in response to a second frequency reference signal and means for providing the first frequency signal to the transmission stage and for providing the second frequency reference signal to the modulator wherein the first and second frequency reference signal are derived from a signal oscillator.