Laterally coupled bulk acoustic wave filter with improved passband characteristics

Laterally coupled bulk acoustic wave filter comprising,

• a vibration layer for carrying bulk acoustic waves,
• electrode means comprising a first electrode coupled to the vibration layer for exciting to the vibration layer at least one longitudinal wave mode having a first frequency band and one shear wave mode having a second frequency band, and a second electrode coupled to the vibration layer for sensing the filter pass signal, the first and second electrodes being laterally arranged with respect to each other,
• an acoustic reflector structure in acoustic connection with the vibration layer,
• the reflector structure comprising a stack of acoustic impedance layers,
• wherein
• the thickness of the topmost layer of the stack closest to the vibration layer is larger than the calculatory layer thickness of a corresponding layer in a quarter-wave longitudinal reflector structure, whose reflectance is at its highest for the second frequency band,
• the thickness of at least one of the layers of the stack below the topmost layer is smaller than the calculatory layer thickness of a corresponding layer in a quarter-wave longitudinal reflector structure, whose reflectance is at its highest for the second frequency band.