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 (A) 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 (R1) and a second reference signal (R2) for performing the quadrature demodulation, and one of the two reference units comprises a first phase shifter for generating one of the reference signals (R) 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 (R) 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 (A) 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 (R1) and a second reference signal (R2) for performing the quadrature demodulation, and one of the two reference units comprises a first phase shifter for generating one of the reference signals (R) 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 (R) 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. <EOS>