The circuit for a frequency-following oscillator is described for use in obtaining a commensurate reference for a two-phase lock-in amplifier. This device is useful in tracking signals which drift slowly in frequency and which are apt to be either intermittent or momentarily small in amplitude, as in the Brownian motion of a mechanical oscillator. Digital hysteresis is used to reduce the number of tracking corrections made when the signal and the reference oscillator are close in frequency and to increase the number of corrections when they are not.