PURPOSE: To extract a measurement signal from a driving signal for converter by providing a demodulator which demodulates part of the driving signal reflected by a sensor.

CONSTITUTION: A driving signal generator 2 supplies an appropriate driving signal to a resonator 1 through a coupling means 3. The driving signal supplied from the generator 2 is composed of a carrier signal modulated by a modulated signal which is DC-biased so that the percentage modulation can become ≤100% and composed of a repeatable signal. Therefore, the amplitude of the modulated carrier signal does not drop to zero. A return signal from the resonator 1, on the other hand, is coupled with the first input of a subtracting and amplifying state 5 by means of a coupling means 4 and the second input of the stage 5 is received from the generator 2 as a reference signal through a coupling means 6. The reference signal corresponds to the driving signal and the stage 5 is positioned so that the stage 5 can subtract the reference signal from a contaminated signal and can output a completely converted resonant modulated signal as its output signal 7.