Transversal filter

1. A transversal filter, comprising

a piezoelectric substrate;

at least one acoustic track formed on the piezoelectric substrate; and

at least one SAW input transducer and at least one SAW output transducer arranged in each track;

wherein at least one input transducer and at least one output transducer has a distributed excitation comprising

a continuous excitation function having a main lobe and at least one tail lobe with a sign of excitation different than the main lobe; and

an excitation strength S in the main lobe being greater than an excitation strength G of the at least one tail lobe by a factor of x, where 8≧x≧2, x and x is a real number;

wherein the at least one input transducer of each track is divided into a main lobe sub-transducer and a tail lobe sub-transducer by electrically separating at least one bus bar of the input transducer into two parts;

wherein the main lobe and tail lobe sub-transducers adjoining each other in a longitudinal direction; and

wherein the main lobe of excitation being assigned to the main lobe sub-transducer, the at least one tail lobe being assigned to the tail lobe sub-transducer.

2. The filter of claim 1 , wherein at least one transducer is recursive and has a preferred direction of wave propagation.

3. The filter of claim 2 , wherein the input transducer is withdrawal weighted and comprises SPUDT cells.

4. The filter of claim 3 , wherein the input and output transducers are of the same type.

5. The filter of claim 1 , wherein the at least one acoustic track comprises at least two tracks;

wherein a finger arrangement in the at least two tracks is essentially identical;

wherein the at least two tracks directly adjoin each other in a direction transversal to a propagation of an acoustic wave; and

wherein the finger arrangement and circuiting of the input and output transducers are such that an excited acoustic wave has a continuous wave front over the entire aperture of the filter.

6. The filter of claim 1 , comprising first and second tracks,

in the first and second tracks, the main lobe sub-transducer of the at least one input transducer being a unique transducer with an aperture summarizing apertures of the first and second tracks,

the main lobe sub-transducer being electrically connected to two input terminals;

the tail lobe sub-transducers of the input transducer in the first and second track being electrically connected in series between the two input terminals.

7. The filter of claim 1 , wherein a capacitor is circuited between an input bus bar of the main lobe sub-transducer and an input bus bar of the tail lobe sub-transducer.

8. The filter of claim 7 , wherein the capacitor comprises a metallization having an interdigital finger structure, fingers of the interdigital finger structure being rotated relative to electrode fingers of the input and output transducers.

9. The filter of claim 1 , comprising

a first track and a second track forming a first filter unit;

a third track and a fourth track, forming a second filter unit; and

the first and second filter units directly adjoining each other and being electrically connected in series or parallel.

10. The filter of claim 1 , having a constant aperture over a length of the filter, the aperture comprising a length of input transducer plus a length of output transducer, the transducer being not apodized.

11. The filter of claim 1 , where S=x G and 3≦x≦5.

12. The filter of claim 11 , where x is about 4.

13. The filter of claim 1 , wherein each track comprises an input transducer and an output transducer;

the input transducer and output transducer being similarly divided in sub-transducers;

a tail lobe sub-transducer of the input and output transducer directly adjoining each other;

the sub-transducers of the output transducer being circuited between two output terminals;

wherein an electrical connection of the sub-transducers between two input terminals is symmetrical to an electrical connection of sub-transducers between the two output terminals.

14. The filter of claim 1 , wherein the substrate comprises a high coupling material.

15. The filter of claim 14 , wherein the substrate comprises lithium tantalate having a rot xy 112 cut.

16. The filter of claim 9 , wherein the first filter unit is identical to the second filter unit.

17. The filter of claim 16 , wherein identical filter units are shifted against each other in a longitudinal direction such that their phases are matching and the filter has a continuous wave front.

18. A transversal filter, comprising:

a piezoelectric substrate;

at least one acoustic track formed on the piezoelectric substrate; and

at least one SAW input transducer and at least one SAW output transducer arranged in each track;

wherein at least one input transducer and at least one output transducer has a distributed excitation comprising

a continuous excitation function having a main lobe and at least one tail lobe with a sign of excitation different than the main lobe; and

an excitation strength S in the main lobe being greater than an excitation strength G of the at least one tail lobe by a factor of x, such that S=x G where 3≦x≦5.

19. The filter of claim 18 , where x is about 4.

20. A transversal filter, comprising:

a piezoelectric substrate;

at least one acoustic track formed on the piezoelectric substrate; and

at least one SAW input transducer and at least one SAW output transducer arranged in each track;

wherein at least one input transducer and at least one output transducer has a distributed excitation comprising

a continuous excitation function having a main lobe and at least one tail lobe with a sign of excitation different than the main lobe; and

an excitation strength S in the main lobe being greater than an excitation strength G of the at least one tail lobe by a factor of x, where 8≧x≧2, x and x is a real number;

wherein each track comprises an input transducer and an output transducer, the input transducer and output transducer being similarly divided in sub-transducers;

wherein a tail lobe sub-transducer of the input and output transducer directly adjoin each other;

wherein the sub-transducers of the output transducer being circuited between two output terminals; and

wherein an electrical connection of the sub-transducers between two input terminals is symmetrical to an electrical connection of sub-transducers between the two output terminals.