Apparatus for determining and/or monitoring at least one process variable of a medium, particularly a level, a density or a viscosity of a medium in a vessel, with - a structure capable of mechanical vibration during operation which projects into the vessel and has at least one vibration property that depends on the process variable, - an electromechanical transducer, - which excites the structure to perform mechanical vibrations using an excitation signal supplied to the transducer at the input side, and - which converts the resulting vibrations of the structure to an electrical reception signal that reflects the vibration, and outputs it at the output side, and - electronics, -- which comprise an apparatus connected to the transducer at the input side and designed to generate the excitation signal, -- which has a first filter connected to the transducer at the output side, said filter filtering a useful signal out of the reception signal, and -- which determines and/or monitors the process variable using the useful signal, characterized in that - the first filter is a bypass filter in the form of a switched capacitor filter, which has at least one capacitor that is switched at a switching frequency (f sc), and whose center frequency (f 0) can be adjusted via the switching frequency (f sc), and - the electronics comprise an apparatus, which, during operation, adjusts the center frequency (f 0) to the frequency (f S) of the excitation signal by regulating the switching frequency (f SC) of the capacitor depending on the current frequency (f S) of the excitation signal. - a structure capable of mechanical vibration during operation which projects into the vessel and has at least one vibration property that depends on the process variable, - an electromechanical transducer, - which excites the structure to perform mechanical vibrations using an excitation signal supplied to the transducer at the input side, and - which converts the resulting vibrations of the structure to an electrical reception signal that reflects the vibration, and outputs it at the output side, and - which excites the structure to perform mechanical vibrations using an excitation signal supplied to the transducer at the input side, and - which converts the resulting vibrations of the structure to an electrical reception signal that reflects the vibration, and outputs it at the output side, and - electronics, -- which comprise an apparatus connected to the transducer at the input side and designed to generate the excitation signal, -- which has a first filter connected to the transducer at the output side, said filter filtering a useful signal out of the reception signal, and -- which determines and/or monitors the process variable using the useful signal, -- which comprise an apparatus connected to the transducer at the input side and designed to generate the excitation signal, -- which has a first filter connected to the transducer at the output side, said filter filtering a useful signal out of the reception signal, and -- which determines and/or monitors the process variable using the useful signal, - the first filter is a bypass filter in the form of a switched capacitor filter, which has at least one capacitor that is switched at a switching frequency (f sc), and whose center frequency (f 0) can be adjusted via the switching frequency (f sc), and - the electronics comprise an apparatus, which, during operation, adjusts the center frequency (f 0) to the frequency (f S) of the excitation signal by regulating the switching frequency (f SC) of the capacitor depending on the current frequency (f S) of the excitation signal.