Saw filter with improved stop band suppression

1. 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 (IDT 1 ) arranged in the acoustic track coupled to an input, having a first mean finger period (p 1 ) assigned to a centre frequency of a pass band of the filter;

a second transducer (IDT 2 ) arranged in the acoustic track coupled to an output, having the same first mean finger period (p 1 );

a reflector arranged between the first and second transducers having a second mean finger period (p 2 ) assigned to a stop band frequency different from the centre frequency,

wherein the first and second transducers (IDT 1 ,IDT 2 ) 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 (IDT 1 ,IDT 2 ) 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 (IDT 1 ,IDT 2 ) minimizing the free surface between the first transducer (IDT 1 ), the reflector (REF) and the second transducer (IDT 2 ), the shielding structure (SH) being a fully metalized area or comprising a non-reflective finger grating.

2. The filter of claim 1 , wherein the piezoelectric substrate is cut from a piezoelectric crystal with a cut angle chosen to propagate a pseudo surface acoustic wave (PSAW) on the surface of the substrate.

3. The filter of claim 2 , wherein the PSAW propagating substrate (SU) is chosen from a lithium niobate with a cut LN(41±30°)rotY and from a lithium tantalate with a cut LT(36±5°)rotXY.

4. The filter of claim 1 , where the first and second transducers (IDT 1 ,IDT 2 ) have a metallization height relative to wavelength h/λ of 0.5-4%.

5. The filter of claim 4 , where the first and second transducers (IDT 1 ,IDT 2 ) have a metallization height relative to wavelength h/λ of 1%.

6. The filter of claim 1 , wherein the SPUDT cells are chosen from the type Hanma Hunsinger and Floating Electrode Uni-Directional Transducer (FEUDT).

7. The filter of claim 1 , wherein the SPUDT cells comprise 2 fingers per wavelength λ.

8. The filter of claim 1 , wherein the shielding structure (SH) has a trapezoid area, the width of which is increasing in a transversal direction opposite to the width increase of the transducers (IDT 1 ,IDT 2 ).

9. The filter of claim 1 , wherein a damping structure or an absorber (ABS) is arranged on the surface of the substrate (SU) at each of the longitudinal ends of the acoustic track.

10. The filter of claim 1 , wherein each transducer (IDT 1 ,IDT 2 ) comprises a number of n parallel channels extending along the longitudinal direction, the PSAW having a delay time being the same in each channel, and n is an integer 5<n<50.

11. The filter of claim 10 , wherein each channel has a given transversal extent in the transversal direction, and

wherein the finger widths and finger spacings of the respective channel are constant along the transversal extent but are increasing when turning to an adjoining channel.

12. The filter of claim 1 , wherein each of the first and second transducers (IDT 1 ,IDT 2 ) is connected to ground via a respective shunt arm, and

wherein a respective one of two capacitors (C 1 ,C 2 ) each is arranged in each respective shunt arm.

13. The filter of claim 12 , wherein the capacitance of the two capacitors (C 1 ,C 2 ) is independently chosen to be between 1 and 20 pF.

14. The filter of claim 1 , wherein the reflector (REF) comprises reflector fingers with a constant spacing, and

wherein the reflector has two reflector fingers per wavelength λ at the stop band frequency.

15. The filter of claim 1 , comprising a further reflector (REF) having third mean finger period assigned to a second stop band frequency different from the stop band frequency and different from the centre frequency.

16. The filter of claim 15 , wherein further shielding structures (SH) are arranged between one of the reflectors (REF) and a transducer (IDT), or between two reflectors (REF).

17. 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 (IDT 1 ) arranged in the acoustic track coupled to an input;

a second transducer (IDT 2 ) arranged in the acoustic track coupled to an output;

a reflector arranged between the first and second transducers having a mean finger period (p 2 ) assigned to a stop band frequency different from a centre frequency,

wherein the first and second transducers (IDT 1 ,IDT 2 ) 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 (IDT 1 ,IDT 2 ) 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 (IDT 1 ,IDT 2 ) minimizing the free surface between the first transducer (IDT 1 ), the reflector (REF) and the second transducer (IDT 2 ), the shielding structure (SH) being a fully metalized area or comprising a non-reflective finger grating,

wherein the first and the second transducers (IDT 1 ,IDT 2 ) have different metallization ratios (η 1 ,η 2 ) and different mean finger periods (p 1 ) and (p 3 ) such that the centre frequency that is shifted by the different metallization ratio for PSAW is the same in both transducers.

18. The filter of claim 17 , wherein a first metallization η 1 ratio is chosen between 0.15 and 0.40, and wherein a second metallization η 2 ratio is chosen between 0.60 and 0.80.