Electroacoustic filter comprising low-pass characteristics

Abstract

An electroacoustic filter has improved low-pass characteristics. The filter includes a first electroacoustic converter, an electroacoustic element and a grid structure between the converter and the element. The grid structure is acoustically active in one frequency range that lies above the acoustically active frequency range of the first electroacoustic converter.

Claims

1. An electroacoustic filter, comprising: an acoustic track; a first electroacoustic transducer arranged in the acoustic track, the first electroacoustic transducer being acoustically active in a frequency range around a center frequency f.sub.1; an acoustic element arranged in the acoustic track, the acoustic element being acoustically active in the frequency range around the center frequency f.sub.1; a grid structure arranged in the acoustic track between the first electroacoustic transducer and the acoustic element, wherein the grid structure is acoustically active in a frequency range around a center frequency f.sub.2, where f.sub.2>f.sub.1 and wherein the grid structure has a grid pitch P.sub.G, which is selected in order to dissipate energy of a frequency component f>f.sub.1; and wherein the grid structure is arranged as a bulk wave conversion structure, wherein the electroacoustic filter functions using acoustic surface waves or guided bulk acoustic waves.

2. The electroacoustic filter according to claim 1, wherein the acoustic element comprises a second electroacoustic transducer, a reflector, and/or a deflecting structure.

3. The electroacoustic filter according to claim 1, wherein f.sub.2 lies in a range around 2*f.sub.1.

4. The electroacoustic filter according to claim 1, wherein the first electroacoustic transducer includes an area having a grid pitch of P.sub.1; the acoustic element includes an area having the grid pitch of P.sub.1; and the grid pitch of the grid structure P.sub.G<P.sub.1.

5. The electroacoustic filter according to claim 4, wherein the grid pitch of the grid structure P.sub.G comprises a pitch selected from the group consisting of a pitch P>0.5*P.sub.1, a pitch P>0.5*1.030*P.sub.1, and a pitch P>0.5*1.035*P.sub.1.

6. The electroacoustic filter according claim 1, wherein the acoustic element includes split fingers having an area of split fingers having a grid pitch of P.sub.1, and wherein the grid pitch of the grid structure comprises a pitch selected from the group consisting of a pitch P>P.sub.1, a pitch P>1.030*P.sub.1, a pitch P>1.035*P.sub.1, and a pitch P>1.20*P.sub.1.

7. The electroacoustic filter according to claim 1, wherein the first electroacoustic transducer, the acoustic element, and/or the grid structure is designed as fan-shaped structures of a FAN filter.

8. The electroacoustic filter according to claim 1, wherein the grid structure comprises a metallization, a dielectric material, or recesses in the material of the acoustic track.

9. The electroacoustic filter according to claim 1, further comprising a phase structure in the acoustic track between the first electroacoustic transducer and the grid structure and/or between the grid structure and the acoustic element, wherein the velocity of an acoustic wave in the phase structure deviates from the velocity of the acoustic wave in the acoustic track outside the phase structure.

10. An electroacoustic filter, comprising: an acoustic track; a first electroacoustic transducer arranged in the acoustic track, the first electroacoustic transducer being acoustically active in a frequency range around a center frequency f.sub.1; an acoustic element arranged in the acoustic track, the acoustic element being acoustically active in the frequency range around the center frequency f.sub.1; and a grid structure arranged in the acoustic track between the first electroacoustic transducer and the acoustic element, wherein the grid structure is acoustically active in a frequency range around a center frequency f.sub.2, where f.sub.2 lies in a range around 2*f.sub.1.

11. The electroacoustic filter according to claim 10, wherein the acoustic element comprises a second electroacoustic transducer, a reflector, and/or a deflecting structure.

12. The electroacoustic filter according to claim 10, wherein the grid structure is a reflecting structure and/or a bulk wave conversion structure; and the electroacoustic filter functions using acoustic surface waves or guided bulk acoustic waves.

13. The electroacoustic filter according to claim 10, wherein the grid structure has a grid pitch P.sub.G, which is selected in order to dissipate acoustic energy of a frequency component f>f.sub.1.

14. The electroacoustic filter according to claim 10, wherein the first electroacoustic transducer includes an area having a grid pitch of P.sub.1; the acoustic element includes an area having the grid pitch of P.sub.1; and the grid structure includes an area having a grid pitch of P.sub.G, where P.sub.G<P.sub.1.

15. The electroacoustic filter according to claim 14, wherein the grid pitch P.sub.G of the grid structure comprises a pitch selected from the group consisting of a pitch P>0.5*P.sub.1, a pitch P>0.5*1.030*P.sub.1, and a pitch P>0.5*1.035*P.sub.1.

16. The electroacoustic filter according claim 10, wherein the acoustic element includes an area of split fingers having a grid pitch of P.sub.1 and wherein a grid pitch of the grid structure comprises a pitch selected from the group consisting of a pitch P>P.sub.1, a pitch P>1.030*P.sub.1, a pitch P>1.035*P.sub.1, and a pitch P>1.20*P.sub.1.

17. The electroacoustic filter according to claim 10, wherein the first electroacoustic transducer, the acoustic element, and/or the grid structure is designed as fan-shaped structures of a FAN filter.

18. The electroacoustic filter according to claim 10, wherein the grid structure comprises a metallization, a dielectric material, or recesses in the material of the acoustic track.

19. The electroacoustic filter according to claim 10, further comprising a phase structure in the acoustic track between the first transducer and the grid structure and/or between the grid structure and the acoustic element, wherein the velocity of an acoustic wave in the phase structure deviates from the velocity of the acoustic wave in the acoustic track outside the phase structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The electroacoustic filter and underlying ideas are described in greater detail below based on exemplary embodiments and associated schematic figures.

(2) FIG. 1 shows a schematic embodiment of the electroacoustic filter having the grid structure between the first electroacoustic transducer and the acoustic element;

(3) FIG. 2 shows one specific embodiment, in which the acoustic element is designed as a second electroacoustic transducer;

(4) FIG. 3 shows one specific embodiment, in which the acoustic element is designed as a deflecting structure;

(5) FIG. 4 shows one specific embodiment, in which the first electroacoustic transducer and the acoustic element are equipped as split-finger transducers;

(6) FIG. 5 shows one embodiment, in which the first electroacoustic transducer, the acoustic element, and the grid structure between them are FAN-shaped;

(7) FIG. 6 shows one embodiment, in which the grid structure includes two phase structures;

(8) FIG. 7 shows the frequency-dependent attenuation coefficients of a grid structure; and

(9) FIG. 8 shows the insertion loss of an electroacoustic filter having a grid structure.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(10) FIG. 1 schematically depicts one specific embodiment of an electroacoustic filter EAF including a first electroacoustic transducer TD.sub.1 and an acoustic element AE. The first electroacoustic transducer TD.sub.1 and the acoustic element AE are arranged in the acoustic track AT. A grid structure GS is arranged between the first electroacoustic transducer TD.sub.1 and the acoustic element AE. The first electroacoustic transducer TD.sub.1 is acoustically active in a frequency range around a center frequency f.sub.1, wherein the center frequency f is determined by the pitch P.sub.1 via the transducer structure.

(11) The grid structure GS comprises structured strips which, for example, are manufactured from a metallization in which the first electroacoustic transducer is formed. The grid pitch of the grid structure GS, i.e., the center-to-center distance between the strips, is P.sub.G. If P.sub.G is suitably set relative to P.sub.1, the grid structure GS is then transparent to acoustic waves of the frequency range around the center frequency of the electroacoustic transducer, while acoustic waves of higher frequency are reflected and/or converted into bulk waves.

(12) The influence of corresponding undesirable frequency components on the acoustic element AE is therefore eliminated or at least reduced.

(13) FIG. 2 schematically depicts one specific embodiment of an electroacoustic filter EAF, in which the acoustic element AE is designed as an electroacoustic transducer TD.sub.2. The electroacoustic filter EAF thus comprises the first electroacoustic transducer TD.sub.1 and the second electroacoustic transducer TD.sub.2 as well as the grid structure GS between them. One of the two transducers may be an input transducer, while the corresponding other transducer is an output transducer. Thus, a two-port filter having an improved low-pass characteristic may be obtained.

(14) FIG. 3 shows one specific embodiment of the electroacoustic filter EAF, in which the acoustic element AE is designed as a deflecting structure EADS. Acoustic waves which, for example, are transmitted by the first electroacoustic transducer and, passing through the grid structure GS, reach the acoustic element AE, may thus be deflected in a different direction. It is possible that the deflecting structure EADS is oriented such that electroacoustic waves are reflected back to the first electroacoustic transducer. The deflecting structure EADS then constitutes a reflector.

(15) Furthermore, it is possible that the grid structure GS is staged. The grid structure GS then comprises areas which are offset by a distance with respect to another area of the grid structure GS. Residual reflections which are possibly present may then be suppressed. At an offset of =P.sub.G/2, for example, residual reflections may be suppressed via destructive interference.

(16) FIG. 4 schematically depicts one specific embodiment, in which the acoustic element AE is designed as a second electroacoustic transducer TD.sub.2. The first electroacoustic transducer TD.sub.1 and the second electroacoustic transducer TD.sub.2 are designed as split-finger transducers. The distance of the finger centers of adjacent fingers P essentially determines one-quarter of the wavelength of the acoustic wavelength at the operating frequency of the first electroacoustic transducer. The same applies to the second electroacoustic transducer TD.sub.2. The distance of the finger centers of adjacent fingers of the grid structure GS is of the same order of magnitude as the structural finger distance P of the first electroacoustic transducer TD.sub.1. The electrode fingers of the transducers and the grid structure GS have essentially the same distance from the adjacent electrode fingers and may be implemented via the same manufacturing steps and using the same type of manufacture. Nevertheless, the grid structure GS is essentially transparent to the operating frequencies of the first and the second electroacoustic transducer TD.sub.1, TD.sub.2, but impermeable to higher frequencies, in particular, twice the frequency.

(17) FIG. 5 schematically depicts one specific embodiment of the electroacoustic filter EAF, in which the first electroacoustic transducer, the second electroacoustic transducer, and the grid structure GS are fan-shaped. The distance of the finger centers and the width of the fingers increase from one side of the acoustic track to the other side of the acoustic track, i.e., along the aperture. The information with respect to the frequencies or grid pitches therefore applies only to transverse areas of the acoustic track which correspond to one another. In this respect, transverse areas are areas which are arranged adjacent to one another in the longitudinal direction relative to the acoustic track and which have a defined distance from one side of the acoustic track, for example, from the area of the busbar. The various areas RE are represented by lines running in parallel along the direction of propagation of the acoustic waves.

(18) Like FIG. 4, FIG. 5 shows the option of stub fingers in the first electroacoustic transducer and in the second electroacoustic transducer. The stub fingers are arranged next to fingers having the same polarity and essentially do not contribute to the excitation of acoustic waves.

(19) FIG. 6 schematically depicts one specific embodiment of the electroacoustic filter EAF, in which a phase structure PS is arranged between the first electroacoustic transducer and the grid structure GS. A second phase structure PS is arranged between the grid structure GS and the second electroacoustic transducer as an acoustic element. Both phase structures may contribute to reducing or harmonizing phase differences caused by the fan-shaped design of the transducers and the grid structure in different areas RE of the acoustic track.

(20) The phase structures PS may also comprise the metallizations from which at least the first electroacoustic transducer is structured.

(21) All arrangements which influence the propagation velocity of an acoustic wave are essentially possible as phase structures. The propagation velocity may thus be increased or reduced locally.

(22) FIG. 7 depicts the attenuation a in dB per wavelength in a LiNbO.sub.3 substrate which experiences a surface wave, plotted over the normalized grid pitch, wherein the onset frequency for the bulk wave conversion is realized by attaining a value of the attenuation coefficient which is non-zero, here, for example, at approximately P/=0.65 (see, e.g., B. Fleischmann, VDI Progress Reports, Series 10: Information/Communication Technology No. 274, VDI-Verlag, Dsseldorf, 1994, pp. 81-82).

(23) FIG. 8 shows the frequency-dependent attenuation plotted over the frequency, in which various stretching factors s, i.e., 1.2, 1.25, and 1.3, were used for the grid stretching. The frequency 2f to be attenuated is 900 MHz, depicted by the perpendicular line. A sufficient bulk attenuation exists as of an attenuation of 1.25. This stretching corresponds to a normalized grid pitch of P.sub.G/=1.25/2=0.625. The substrate material is LiTaO.sub.3 X 112.2 Y.

(24) An electroacoustic filter is not limited to one of the described exemplary embodiments. Combinations of features and variations of the examples which, for example, comprise additional metallization structures, also constitute exemplary embodiments according to the present invention. In particular, any aforementioned features may be combined in order to obtain electroacoustic filters which should satisfy specific requirements.