Resonator structure and a filter comprising such a resonator structure

A filter comprising at least one thin film resonator (1200, 1300, 1400), where a certain wave mode is piezoelectrically excitable and which resonator comprises at least two conductor layers (110, 120) and at least one piezoelectric layer (100) in between the conductor layers, said resonator having a first area over which said conductor layers and piezoelectric layer extend, which first area is a piezoelectrically excitable area of the resonator, characterized in that

• the resonator comprises a frame-like zone (2, 4) of any shape which frame-like zone confines a center area (3) within the first area of the resonator,
• the resonator is adapted to operate in the thickness extensional wave mode as a TE mode and in such a way that a cut-off frequency of the piezoelectrically excited thickness extensional wave mode in the layer structure of the frame-like zone is different from the cut-off frequency of the piezoelectrically excited thickness extensional wave mode in the layer structure of the center area, and
• the resonator is adapted in such a way that a width of the frame-like zone and acoustical property means of the layer structure in the frame-like zone are arranged so that displacement relating to the piezoelectrically excited strongest resonance mode is substantially uniform in the center area of the resonator.