a system and method for activating vehicle sensors which are based on inductive loops . a voltage controlled oscillator is scanned throughout the required frequency range while a balanced mixer , low - pass filter , and detector are arranged to show the presence of a signal from an inductive loop by the amplitude of the resultant signal at a difference frequency . the voltage controlled oscillator is then made to dwell at or near the frequency of the sensor loop by adjusting so as to keep the amplitude of that low - frequency difference as high as possible .