Abstract: To excite a resonator so that it oscillates at its resonant frequency, a self-oscillating system is used having a digital delay element (such as a digital memory) or an analog delay element (such as a bucket-brigade device) inserted in a feed-back loop. Detecting the oscillation signal and exciting the resonator is done periodically and alternately. Thus, the oscillation signal is detected while the resonator oscillates freely. In the embodiment disclosed, a coil (1) serves both to detect the oscillation signal and to excite the resonator which is in the form of a string (2). A switch (3) is controlled so as to apply the detected signal to a delay circuit (7) during each first period and to supply the delayed signal as an exciting signal to the coil (1) during each second period.