A vibronic sensor for determining and/or monitoring at least one process variable of a medium in a container, comprising at least one mechanically resonant unit, one drive/receiver unit, and one electronic unit, wherein the drive/receiver unit is configured to excite the mechanically resonant unit to mechanical vibrations using an electrical excitation signal (U A), as well as to receive the mechanical vibrations of the mechanically resonant unit and convert them into an electrical reception signal (U E), wherein the electronic unit is configured to generate the excitation signal (U A) on the basis of the reception signal (U E) and then determine the at least one process variable from the reception signal (U E), wherein the electronic unit comprises a first adaptive filter, and wherein the electronic unit is configured to adjust the filter characteristics of the adaptive filter in such a way that a targeted phase shift (ΔΦ soll) occurs between the excitation signal (U A) and the reception signal (U E), characterized in that the sensor exhibits a detection unit that is configured to determine a phase shift (ΔΦ) between the excitation signal (U A) and the reception signal and/or an amplitude of the reception signal (U E) on the basis of a quadrature demodulation, the detection unit comprises a first reference unit and a second reference unit for generating a first reference signal and a second reference signal for performing the quadrature demodulation, and one of the two reference units comprises a first phase shifter for generating one of the reference signals with a phase shift of +/- 90° relative to the reception signal (U E), wherein the first phase shifter is a second adaptive filter or an allpass filter or a Hilbert transformer, and/or that one of the two reference units comprises a second phase shifter for generating one of the reference signals with a phase shift of 0° relative to the reception signal (U E), wherein the second phase shifter is a multiplier or an adaptive filter. wherein the drive/receiver unit is configured to excite the mechanically resonant unit to mechanical vibrations using an electrical excitation signal (U A), as well as to receive the mechanical vibrations of the mechanically resonant unit and convert them into an electrical reception signal (U E), wherein the electronic unit is configured to generate the excitation signal (U A) on the basis of the reception signal (U E) and then determine the at least one process variable from the reception signal (U E), wherein the electronic unit comprises a first adaptive filter, and wherein the electronic unit is configured to adjust the filter characteristics of the adaptive filter in such a way that a targeted phase shift (ΔΦ soll) occurs between the excitation signal (U A) and the reception signal (U E), characterized in that the sensor exhibits a detection unit that is configured to determine a phase shift (ΔΦ) between the excitation signal (U A) and the reception signal and/or an amplitude of the reception signal (U E) on the basis of a quadrature demodulation, the detection unit comprises a first reference unit and a second reference unit for generating a first reference signal and a second reference signal for performing the quadrature demodulation, and one of the two reference units comprises a first phase shifter for generating one of the reference signals with a phase shift of +/- 90° relative to the reception signal (U E), wherein the first phase shifter is a second adaptive filter or an allpass filter or a Hilbert transformer, and/or that one of the two reference units comprises a second phase shifter for generating one of the reference signals with a phase shift of 0° relative to the reception signal (U E), wherein the second phase shifter is a multiplier or an adaptive filter.