Saw filter with improved stop band suppression

A filter working with surface acoustic waves comprising:

• a piezoelectric substrate (SU) providing an acoustic track on the surface of the substrate;
• a first transducer (IDT1) arranged in the acoustic track coupled to an input;
• a second transducer (IDT2) arranged in the acoustic track coupled to an output;
• a reflector arranged between the first and second transducers having a mean finger period (p2) assigned to a stop band frequency different from a centre frequency,
• wherein the first and second transducers (IDT1,IDT2) are fan type transducers,
• wherein a width of each transducer finger and a spacing between the fingers of a respective pair of adjacent transducer fingers are increasing in a transversal direction, the transversal direction being normal to a longitudinal direction of the acoustic track but within the substrate plane, the transducers being adapted to provide a relative bandwidth of the transducer of at least 8%, and
• wherein both the transducers (IDT1,IDT2) comprise Single Phase Uni-Directional Transducer (SPUDT) cells such that an acoustic wave is propagated towards a preferred direction;
• and
• a shielding structure (SH) between the first and second transducers (IDT1,IDT2) minimizing the free surface between the first transducer (IDT1), the reflector (REF) and the second transducer (IDT2), the shielding structure (SH) being a fully metalized area or comprising a non-reflective finger grating,
• wherein the first and the second transducers (IDT1,IDT2) have different metallization ratios (η1,η2) and different mean finger periods (p1) and (p3) such that the centre frequency that is shifted by the different metallization ratio for PSAW is the same in both transducers.