Device for determining and/or monitoring a process variable of a medium

An apparatus for determining and/or monitoring at least one process variable of a medium, comprising:

• at least one mechanically oscillatable unit;
• a driving/receiving unit, which excites said mechanically oscillatable unit to execute mechanical oscillations, starting with an electric, excitation signal, and which receives the mechanical oscillations of said mechanically oscillatable unit and converts them into an electric, received signal;
• an electronics unit, which, starting with a electric, received signal, generates the electric, excitation signal, and sends such to said driving/receiving unit, said electronics unit is provided with at least one all-pass filter for tuning the phase difference;
• and
• at least one control unit, which controls said all-pass filter, using closed-loop or open-loop control, such that the phase difference between the excitation signal and the received signal essentially corresponds to a predetermined, desired value, wherein:
• said electronics unit is embodied in such a manner that it generates the excitation signal such that a predeterminable phase difference exists between the excitation signal and the received signal.