Electroacoustic transducer having a piezoelectric substrate with electrode fingers divided into four groups

Abstract

An electroacoustic transducer having an alternative finger structure is provided. The number of fingers of a cell of length A is divisible by four. The electrode fingers of the cell are divided into four groups. The distance (.sub.2) between the second group and the third group is less than the distance (.sub.1) between the first group and the second group and less than the distance (.sub.3) between the third group and the fourth group.

Claims

1. An electroacoustic transducer, comprising: a piezoelectric substrate (PSU) having electrode fingers (EF) arranged over the piezoelectric substrate (PSU); and a cell of length having N electrode fingers (EF), wherein is the acoustic wavelength, the N electrode fingers (EF) of the cell are divided into four groups arranged along the propagation direction of the acoustic wave, the first group comprises n1 fingers (EF), the finger spacing within the first group is D1, and the distance between the first and second groups is 1, the second group comprises n2 fingers (EF), the finger spacing within the second group is D2, and the distance between the second and third groups is 2, the third group comprises n3 fingers (EF), the finger spacing within the third group is D3, and the distance between the third and fourth groups is 3, the fourth group comprises n4 fingers (EF), the finger spacing within the fourth group is D4, n1+n2+n3+n4=N, 2 is not equal to 1 or 2 is not equal to 3, D1=D2=D3=D4=2, and N is four times an integer k>=2.

2. The electroacoustic transducer according to claim 1, wherein 2<1 and 2<3.

3. The electroacoustic transducer according to claim 1, wherein a preceding finger (EF) is arranged before the cell and the distance between the preceding finger (EF) and the first group is less than 1.

4. The electroacoustic transducer according to claim 1, wherein a preceding finger (EF) is arranged before the cell, and an excitation center (EC) is arranged between the preceding finger (EF) and the first group.

5. The electroacoustic transducer according to claim 1, wherein a subsequent finger (EF) is arranged after the cell, and the distance between the fourth group and the subsequent finger (EF) is less than 3.

6. The electroacoustic transducer according to claim 1, wherein a subsequent finger (EF) is arranged after the cell, and an excitation center (EC) is arranged between the fourth group and the subsequent finger (EF).

7. The electroacoustic transducer according to claim 1, wherein an excitation center (EC) is arranged between the second group and the third group.

8. The electroacoustic transducer according to claim 1, wherein n1=n2=n3=n4.

9. The electroacoustic transducer according to claim 1, wherein 1=3.

10. The electroacoustic transducer according to claim 1, wherein k is either 2, 3 or 4.

Description

(1) FIG. 1 shows an embodiment with four fingers per wavelength ,

(2) FIG. 2 shows an embodiment with eight fingers per ,

(3) FIG. 3 shows a calculated transfer function S.sub.21 for reduced distances (solid curve) between two electrode fingers of an excitation center, and for increased distances (dashed curve) between the fingers of an excitation center,

(4) FIG. 4 shows calculated curves of the reflectivity S.sub.11 at the input port of a transducer for reduced distances (solid curve) and increased distances (dashed curve),

(5) FIG. 5 shows the reflection S.sub.22 at the output port for reduced distances (solid curve) and increased distances (dashed curve),

(6) FIG. 6 shows the real part of the admittance of a transducer for reduced distances (solid curve) and increased distances (dashed curve),

(7) FIG. 7 shows the imaginary part of the admittance of a transducer for reduced distances (solid curve) and increased distances (dashed curve),

(8) FIG. 8 shows the magnitude of the admittance of a transducer for reduced distances (solid curve) and increased distances (dashed curve).

(9) FIG. 1 shows a conventional (split finger) cell of an electroacoustic transducer of length in a first acoustic track AT1 and, for comparison, an optimized cell of length of a second transducer in an acoustic track AT2. The transducer cell of the track AT1 is essentially a conventional split finger transducer cell, as known for example from US 2003/0057805 A1. 1 denotes the distance between the first group, which comprises only one electrode finger, and the second group, which likewise comprises one electrode finger. 2 describes the distance between the finger of the second group and the single finger of the third group. 3 describes the distance between the single finger of the third group and the single finger of the fourth group.

(10) In contrast thereto, the transducer structure of the acoustic track AT2 represents an optimized finger arrangement, the fingers of the first group being shifted to the left toward the excitation center EC by the amount d. The finger of the second group is shifted to the right toward the excitation center EC by the amount d. The finger of the third group is shifted to the left toward the excitation center between the second finger and the third finger by the amount d. The fourth finger is shifted to the right toward the excitation center between the fourth finger and the subsequent finger by the amount d. Overall, all the fingers of the corresponding group are thus shifted toward the closest excitation center. The distance 2 between the second group and the third group is less than the distance 1 between the first group and the second group, and less than the distance 3 between the third group and the fourth group.

(11) Independently of the number of cells and the number of fingers per cell, it is possible that: 1=3 and 2=4.

(12) Since each of the four finger groups comprises only a single finger, there are no spacings of the fingers within a group.

(13) In contrast to FIG. 1, FIG. 2 shows two cells, in which each of the four groups comprises precisely two electrode fingers. In each group, there is therefore precisely one spacing between the first finger and the second finger. The spacings within the group are correspondingly denoted by D1, D2, D3 and D4. The quantities 1, 2 and 3 denote, as before, the distances between the corresponding groups. The finger arrangement of the acoustic track AT1 shows a cell of length of a transducer, in which the spacings of the groups are equal. In contrast thereto, the acoustic track AT2 comprises a finger arrangement in which the finger next to an excitation center EC are shifted toward the excitation center by an amount d. The other respective finger of a group is likewise shifted in the direction of the excitation center, but by the amount 3d.

(14) In configurations of cells with more than two fingers per group, the finger next to an excitation center would be shifted by an amount d. The subsequent finger of the same group would be shifted by the amount 3d. The third finger of a group would be shifted by the amount 5d. In general, the i.sup.th finger of a group is shifted in the direction of the excitation center lying closest to the group by the amount (2i1)d. This applies for the situation when the finger spacings in all groups are equally large and the distance 2 between the second group and the third group is equal to the correspondingly set finger spacing D1=D2=D3=D4. Precisely one further parameter, namely d, is thus obtained, from which the offset of each individual electrode finger (2n1)d is derived. The arrangement of all the electrode fingers of a cell of length is therefore well defined by a single parameter. Overall, a transducer which is optimized in relation to the reflectivity and the electrical capacitance is obtained. A corresponding cell may thus be adjusted by variation of a single parameter in a method for the optimization of a transducer. Furthermore: With a given minimum spacing d between the fingers, the cell with equal distances between the fingers D1=D2= . . . has the maximum static capacitance and the maximum reflection.

(15) The offset d is in this case positive when a finger next to an excitation center is shifted in the direction of the excitation center. If they are shifted in the opposite direction, i.e. away from the excitation center, the offset d is negative.

(16) FIG. 3 shows the transfer function S.sub.21 for positive d (solid curve) and for negative d (dashed curve). Although the absolute length of the cell, which corresponds to the wavelength , is maintained, the characteristic frequencies of the low-frequency edge of the transmission range are shifted. Thus, for positive d, the low-frequency edge has a smaller transition width. Furthermore, the waviness in the passband can be reduced.

(17) FIG. 4 shows the reflectivity S.sub.11 of a correspondingly configured transducer, with positive d (solid curve) and negative d (dashed curve). The frequency-dependent impedance is shown in the Smith chart on the left in FIG. 4. The right-hand part of FIG. 4 shows that the transducer is essentially transmissive for frequencies in the range of the passband. The curves furthermore differ depending on the direction in which the electrode fingers are shifted.

(18) FIG. 5 shows the reflectivity S.sub.22 at the output port in a similar way to FIG. 4.

(19) FIGS. 3, 4 and 5 relate to a ladder-type filter having a metallization height of 200 nm, an average finger period (pitch) of 5 m and a metallization ratio eta of 0.5.

(20) FIG. 6 shows the real part of the admittance of a corresponding transducer with positive d (solid curve) and negative d (dashed curve).

(21) FIG. 7, in contrast to FIG. 6, shows the imaginary part of the admittance.

(22) FIG. 8 shows the magnitudes of the admittance, on the one hand for positive d (solid curve) and for negative d (dashed curve).

(23) FIGS. 6, 7 and 8 relate to a resonator with a metallization height of 400 nm, a pitch of 5.0 m and a metallization ratio eta of 0.5.

(24) All of FIGS. 3 to 8 relate to transducers having cells with four fingers per , i.e. a cell structure as shown in FIG. 1 in the acoustic track AT2.

(25) An electroacoustic transducer according to the invention is not restricted to one of the exemplary embodiments described. Transducers having additional cells and additional metallization structures or layer systems on a piezoelectric substrate, or on the electrode fingers, or between the electrode fingers and the piezoelectric substrate, which contribute to guiding acoustic waves in the acoustic track, likewise represent exemplary embodiments according to the invention.

LIST OF REFERENCES

(26) AT1, AT2: acoustic track BB: busbar d: offset of an electrode finger next to an excitation center D1, D2, D3, D4: finger spacings within the first, second, third and fourth groups EC: excitation center EF: electrode finger F: frequency PSU: piezoelectric substrate S.sub.11: reflectivity at the input port S.sub.21: transfer function S.sub.22: reflectivity at the output port 1, 2, 3: distances between the first, second, third and fourth finger groups : wavelength of the acoustic wave