DE8514190U1 - Electric filter working with acoustic waves - Google Patents

Description

Siemens Aktiengesellschaft Our symbol

Berlin and Munich VPA

85 P 12 95 EN

Electric filter working with acoustic waves

The present invention relates to an electrical filter according to the preamble of

Saying 1.

From Proc. of the IEEE, Vol. 67 (1969), pp. 147 - 157, Fig. 3, an acoustic wave filter is known as a resonator filter, which has an input transducer and an output transducer arranged directly next to each other on the surface of the substrate. Despite the appropriate polarity, a disturbing level of crosstalk between the input and output can be detected in such a filter.

For shielding between the input transducer and the output transducer, arrangements such as those shown in the figure have been provided, as is known from the "1982 Ultrasonics" Symposium 1982 IEEE, pp. 284 - 289. Such a shielding arrangement essentially involves a bar-shaped structure aligned transversely to the main propagation axis of the waves running through the filter as the surface coating of the substrate, which is connected to ground as a reference potential. This surface coating, which is connected to ground and forms a wide strip, is positioned between the input transducer and the output transducer and in itself provides good shielding. At correspondingly high frequencies, however, the supply line inductances of the connection of this shield to the reference potential already have a disadvantageous effect in that undefined electrical

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potentials can occur on this shielding coating and cause corresponding interference between input and output.

Please also refer to Ultrasonics Symposium 1979 IEEE, pp. 841 - 844.

The object of the present invention is to achieve further improvement with regard to reduced crosstalk or 10 reduced interference between the input and output of a filter operating with acoustic waves.

This object is achieved with a filter having the features of patent claim 1 and further embodiments and developments emerge from the subclaims, with particular reference being made to the embodiments according to claims 5 and 6.

The present invention is based on the idea that a further improvement can be achieved by dividing or electrically interrupting the already known strip running transversely to the axis of the main propagation axis of the waves into (at least) two substructures, in accordance with a basic idea of the invention. Without causing any disturbing crosstalk, one substructure can then be electrically connected to the busbar of the adjacent converter structure, which is at the reference potential, and the other substructure can be connected to the busbar of the other converter structure, which is at the reference potential, which is adjacent to this other substructure, with one of these converter structures being the input and the other of these converter structures being the output. This inventive idea ensures that no interference potential based on any unavoidable inductances is transmitted via the shielding structure.

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can occur. This occurs when current flows between the busbar of one converter, which is at reference potential, and the busbar of the other converter, which is at reference potential. This current flow is reliably prevented by the transverse electrical interruption provided according to the invention, without the shielding effect being reduced in any way.

In particular, the invention provides that these substructures consist of strip-shaped coatings that are electrically connected to one another and are similar to the strip-shaped coatings present in the digital structures of the filter. Preferably, the substructures consisting of a number of such strip-shaped coatings each have at least one busbar, preferably one at each end, of which one or the one of the busbars is connected via a connection coating to the aforementioned busbar of the adjacent converter structure, which is at reference potential. The invention is particularly advantageous for use in a filter that has a coupling structure between the input converter and the output converter, such as that shown in Figure 7 in Proc. of the IEEE, Vol. 67 (1979), p. 150. Another particularly preferred application of the invention is in a filter operating with acoustic waves according to DE-OS 35 01 977, in which, in an arrangement according to the figure attached there, there is another structure between the input transducer and the output transducer. This is a collective of strip-shaped, electrically conductive coatings that correspond to the strip-shaped coatings of the digital structures of the filter and that are positioned and designed in such a way that they are reflection-free.

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In the filter according to this patent application, they are intended to serve to equalize the exposure during the photolithographic production of the filter.

Further explanations of the invention emerge from the following description of preferred embodiments based on the attached figures.

Figure 1 shows a first embodiment of the invention with a simple shield.

Figure 2 shows a second embodiment of the invention, in which a coupling structure that is already provided is also designed to be shielding according to the invention. Figures 3 to 5 show sections of further embodiments, the remaining parts of which correspond, for example, to Figure 2, and whose shielding structure designed according to the invention simultaneously fulfills the function according to patent application P 35 01 977.8 or is a guide structure.

Figure 1 shows a filter 1 according to the invention with two interdigital structures 2 and 3, one of which is an input transducer and the other an output transducer. These interdigital structures 2, 3 are constructed in the usual way and can have a wide variety of special designs. These are characterized by the fact that they have electrode fingers made of electrically conductive (metal) coatings on the surface of the piezoelectric substrate. These interdigitally arranged strip-shaped coatings of the finger electrodes are connected in the direction of the main propagation axis 5 of the waves generated in this filter in succession and alternately to a busbar 6 or 7 or to a busbar 8 or 9 of one transducer structure 2 or the other transducer structure 3. For the intended asymmetrical

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When the filter is in operation, the busbars 7 and 8 are connected to reference potential, in particular ground. The other busbars 6 and 9 are the so-called hot connection ends of the filter 1. 13 designates a bar-shaped structure of a surface coating of the substrate 4, which is aligned essentially transversely to the main propagation axis 5. Its two substructures 14 and 15 are electrically separated from one another by a strip 16 which is also aligned transversely to this axis 5 and which has no electrically conductive surface coating of the substrate 4. This strip 16 is, however, preferably designed to be relatively narrow, in particular to save space. The inclined position of the substructures 14 and 15 shown in Figure 1 serves, for example, to reflect wave components reflected at the edges running transversely to the axis 5 at an angle.

As can be seen from Figure 1, connection arrangements 17, 18 are provided on the substrate surface, which electrically connect the busbar 7 to the substructure 14 on the one hand and the busbar 8 to the substructure 15 on the other. With this measure, these substructures 14 and 15 are connected to the reference potential, i.e. these substructures 14 and 15 can reliably act as shielding in the sense of the known bar-shaped surface arrangement. However, no electrical current can flow through these substructures 14 and 15, which would lead to interference voltages due to inductive series resistances that are unavoidable, especially at high frequencies. The shielding system comprises the details 7, 8, 17, 18, 14 and 15. In this embodiment according to the invention, the substructure 14 is connected via the connection assignment 17 to the busbar 7 of the converter structure 2, which is at (ground) reference potential.

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j which is, for example, the input converter of filter 1. Meaning

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Figure 2 shows a further embodiment of the invention, wherein details already described for Figure 1 in ; Figure 2 have the same reference numerals with the same meaning

I IO have. The essential difference between the embodiment 11 according to 1 Figure 2 and that according to Figure 1 is the ^use of a coupling structure 23 consisting of many strip-shaped surface coatings, which consists of a number of strip-shaped surface coatings \i arranged next to one another in the direction of the axis 5. An arrangement with a coupling structure between two interdigital converters 2, 3 on which the embodiment according to Figure 2 is based is known, for example, from the above-mentioned

\ Publications Ultrasonics Symposium (1979), p. 841, [ 20 Fig. 1 and Proceedings of the IEEE, Vol. 67 (1979), p. 150, I bottom line in Fig. 7. In such a

.(oppel structure, the strip-shaped coatings 123 correspond in their design and dimensions as a rule to the finger electrodes of the converter structures 2, 3» . ■. 25 However, no interdigital arrangement with the necessary busbars is provided for a coupling structure. Such a coupling structure causes a gradual decoupling,

30 In the embodiment of Figure 2 of the invention, such a coupling structure is at the same time

also used as a shielding structure (corresponding to the structure: 14, 15 of Figure 1) and specially designed accordingly. This coupling structure 23 is divided into (at least) two substructures 24 and 25 in the invention 35,

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which are separated from one another by a free strip 26. This is, for example, not wider than the distances between the strip-shaped coverings 123 within the individual coupling structures 24, 25 or the converters 2, 3. However, a wider free strip 26 can also be provided, which makes it possible to omit strip-shaped coverings (such as 123) of the known coupling structure that are otherwise present there in a known structure.

A special design measure is that the substructure 24 has at least one busbar 27. The same applies to the substructure 25 with its busbar 28. The busbars 27 and 28 serve to electrically connect the strip-shaped connections 123 of one substructure and the other substructure 25, as can be seen in Figure 2. According to the invention, connection connections 17 and 18 are also provided in the embodiment according to Figure 2, which connect these substructures 24 and 25 to the busbar 7 and 8 of the converter structure 2 and 3, respectively, which are at reference potential.

Even in the embodiment according to Figure 2, an electrical current flow that leads to interference and the occurrence of a corresponding interference voltage between the converters 2 and 3 is excluded.

As can be seen from Figure 2, additional busbars or surface coverings 127 and 128 corresponding to these in their general form can be provided. The surface covering 127 can also be connected to the strip-shaped coverings 123 of the substructure 24. However, there is no absolute necessity for these connections. This also applies to the

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such additional surface covering 128 and in the case of the substructure 25 this second alternative possibility is shown with electrical connections. In particular, these additional* busbar-corresponding surface coverings 127 and 128 serve to achieve a symmetry of the presence and arrangement of surface coverings.

As a coupling structure, this structure 23, which consists of the partial structures 2A and 25 as a whole, with the strip-shaped coatings 123 has a reflective effect. This reflective effect relates to the acoustic waves generated in the filter or running in the direction of the main propagation axis 5. The embodiment of Figure 5 also contains (as in the prior art mentioned above) reflector structures R1 and R2. Due to the reflective effect of the coupling structure 23, the two parts of this filter 21 of the figure that are located to the right and left of the structure 23 (in Figure 2) are coupled to one another acoustically and, due to the piezoelectric properties of the substrate 4, also electrically.

A filter according to the invention can also be designed with partial structures 24 and 25 corresponding to Figure 2 as shields between the converter structures 2 and 3, without these partial structures 24 and 25 being or having to be effective together as a coupling structure. The above-mentioned DE-OS 35 01 977 describes the use of additional structures consisting of strip-shaped surface coatings. In the embodiment specified in this older patent application, such a structure is designated 100, which is located between the two converter structures of the filter.

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The purpose of this additional structure is to achieve equalization for or during the photolithographic production process of the strip-shaped coatings of these structures. For this reason, the strip-shaped coatings of such an additional structure are designed in accordance with the strip-shaped surface coatings of the necessary digital structures (converter structures and reflector structures) of the filter of this older application. This older application also describes the alternative of providing such an additional structure with busbars (designated 106 there). The decisive factor in the filter according to the older application is that this additional structure arranged between the two interdigital converters is designed or dimensioned to be reflection-free for the acoustic wave. This is in contrast to the coupling structure 23 in Figure 2 of the present invention. How such an (additional) structure is designed to be reflection-free is described in detail in the older application, unless already known. In particular, this can be achieved by (compared to the necessary structures of the filter) different dimensioning of the spacing of the strip-shaped coatings or by spacing jumps measured by quarter lengths of the effective wavelength of the acoustic wave (as in the case of structure 200 of the earlier application).

In Figure 3 of the present description of the invention, this further embodiment 2.1 of the invention is only shown in detail. The structure 33 is a completely reflection-free structure and consists of the substructures 34 and 35. However, the strip-shaped coverings of the structures 34 and 35 have selected distances according to the older application. The substructure

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has (center) distances between the individual strip-shaped coverings 123 from one another that differ by a certain amount from the (center) distances of the strip-shaped coverings of the necessary digital structures of the filter, namely they differ from those (center) distances that determine the center frequency or the effective wavelength of the acoustic wave of the relevant filter. The degree of this deviation is chosen so that the frequency of the first zero point of the interdigital reflections of the strip-shaped coverings of this partial structure 34 coincides with the (predetermined) center frequency of the relevant filter.

The substructure 35 can be designed in the same way as the substructure 34. The representation of the

However, Figure 3 shows a second alternative of a reflection-free structure in the f filter, namely with a

such an arrangement of the strip-shaped coatings, in which individual strip-shaped coatings 123 of the partial structure 35, preferably in groups, are arranged in such a way that the (center) distances of individual strip-shaped coatings 123 or of the strip-shaped coatings of individual groups differ by a quarter wavelength (or an odd multiple of a quarter wavelength) from the distances of the strip-shaped coatings of the structures 2 or 3 which determine the center frequency of the filter.

In the sense of the earlier application, the distance between the structure 2 and the partial structure 34 or the distance between the structure 3 and the partial structure 35 is preferably reduced to the distance of the strip-shaped coverages 123 or the finger electrodes of the transducers 2, so that the connection coverages 17, 18 are correspondingly short.

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Further details of a filter according to Figure 3 correspond to those of a filter according to Figure 2. The same applies to the corresponding reference symbols.

For SBAW and SSBW acoustic waves, which are each a special type of acoustic waves running close to the surface of a substrate and which, in the sense of the present invention, fall under the term surface waves used here, an arrangement is known from "1979 Ultrasonics Symposium", pages 783 to 785, in which an additional "guide" structure is arranged between an input transducer structure and an output transducer structure, which are spaced apart from one another on the surface of the substrate. Figure 4 shows a detail of such an arrangement 31. This guide structure provided between the transducer structures serves to prevent the SBAW wave running between these two transducer structures from leaving the area of the substrate near the surface, i.e. from disappearing into the interior of the substrate body." For such a structure, it is sufficient to provide strip-shaped coatings, which, however, have a completely different, specific dimensioning compared to the transducer structures with regard to the resonance of the structure formed by them. The resonance frequency of this structure preventing the SBAW wave from disappearing is approximately 1.25 times the center frequency of the transducer structures or the entire filter, i.e. the individual strip-shaped coatings have comparatively much closer periodic distances from one another.

This is where such a guide structure differs from , for example, a filling structure mentioned above and described in detail in the older application with the special choice of its frequency, which corresponds to the (first) zero point of the interdigital reflections of the filter. In the *) energy trapping grating

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It is mentioned in the publication that this structure intended for the SBAW or SSBW waves is connected to ground and has a shielding effect.

A guide structure 43 as described above is designed in the invention according to the further details shown in Figure 4 in such a way that (at least) two partial structures 4, 45 are formed, which are connected individually to the adjacent converter structure 2, 3 in accordance with the embodiments in particular in Figures 2 and 3, so that the shielding is divided accordingly and in accordance with the invention.

The embodiments of the invention each relate to cases with either a coupling structure (Figure 2), a filling structure (Figure 3) or a guide structure for SSBW waves (Figure 4). Particularly with a coupling structure, the case may arise that this coupling structure has only very few, e.g. only two or four, strip-shaped coatings, depending on the filter design resulting from the specifications. The above-mentioned division into two substructures 24 and 25 would then mean that a respective substructure 24, 25 consists of only two or even only one strip-shaped coating. The latter would correspond approximately to the embodiment according to Figure 1 in terms of the number of strips. In the embodiment according to Figure 1, however, the two substructures 14, 15 are coatings whose strip width is large compared to the width of such a strip-shaped coating 123, namely whose width is approximately a quarter wavelength. Such a narrow strip-shaped coating 123 can have such a low electrical conductivity along this strip-shaped coating that with regard to the required shielding effect

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along this strip already has too much series resistance.

For a case as described above, a further embodiment 41 according to Figure 5 is to be used, namely to leave a coupling structure 24a comprising only a few, e.g. only a maximum of four strip-shaped coverings 123 undivided, ie to use it as one of the (at least) two substructures in the sense of the invention. As a second substructure, it is then recommended to use a different structure, e.g. a filling structure 35a as in Figure 3 or a guide structure according to Figure 4. Such an embodiment is accordingly a mixture of the respective principle, which on the one hand corresponds to Figure 2 (with only a few coupling strips) and on the other hand to the principle of Figure 3 or 4. The two substructures 24a, 35a, each corresponding to the different principles described here, are operated in the invention in such a way that ^r < on the one hand the one substructure (ie

the entire coupling structure, which only has these few coupling strips) is connected to one converter structure 2 and is connected to the reference potential with it and, on the other hand, another structure (filler structure and/or guide structure) is provided as an associated second substructure 35a and is electrically connected to the other converter structure 3 and is connected to the reference potential with it. In the sense of the invention, there is then again a shielding structure 53 and two substructures 24a, 35a which, when working together, achieve the required shielding effect, although these two substructures each have a different function in the filter.

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Claims (1)

&bull; · · · «ti -14- 85 P 12 95 OE 1. Acoustic wave filter with electrical input and output with a number of digital structures, each of which has strip-shaped, electrically conductive coatings on the surface of the substrate of the filter, these digital structures being arranged in the axis of the main propagation of the waves of the filter and the individual digital structures being converter structures or resonator structures, with a connection for a reference potential and with a shielding structure arranged between the digital structures of an input converter and an output converter, designed as an electrically conductive coating on the surface of the substrate, which is electrically connected to the connection for the reference potential, with one of the busbars of the converter structures being electrically connected to the reference potential, characterized in that the shielding structure (13, 23, 33, 43, 53) has two partial structures (14, 15; 24, 25; 34, 35; 44, 45; 24a, 35a) arranged one behind the other in the main propagation axis (5),
that these partial structures have at least one coating which runs essentially transversely to the main propagation axis (5) in the form of a strip, - wherein one of these substructures (14, 24, 34, 44, 24a) is connected by means of a connection assignment (17) to the busbar (7) connected to reference potential (M) of the converter structure (2) arranged adjacent to this substructure and wherein the other of these substructures (15, 25, 35, 45, 35a) is connected by means of a further connection assignment (18) to the busbar connected to the reference potential (M) (8) of the converter structure (3) adjacent to this substructure. &bull;&diams; · # * « 111(1 &bull; · · · 4 4 · I I t*f| * * <·* ··· » t II It t j -15- 85 P 12 95 EN ] 2. Filter according to claim 1, characterized in that the respective connection assignment (17; 18) with regard to its dimensions is a continuation the busbar (7; 8) connected to it. 5 3. Filter according to claim 1 or 2, characterized in that the connection assignment (17) associated with the one partial structure (14, 24, 34, 44, 24a) has a f 10 side of this axis and the connection assignment (18) of the other substructure (15, 25, 35, 4j>, 35a) are arranged on the other side of this axis« 4. Filter according to claim 1 or 2, characterized in that at least one of the partial structures (24, 25j 34, 35; 44, 45; 24a, 35a) of the shielding structure (23, 33, 43, 53) has a number of strip-shaped, electrically conductive coatings (123) arranged next to one another and aligned essentially transversely to the main propagation axis (5). 5. Filter according to claim 1, 2, 3 or 4, geke &eegr; n- ^ characterized in that that the partial structures (24, 25) belong to a coupling structure <23) of the filter (Fig.2), whereby this coupling structure is dimensioned to reflect the acoustic wave, and that one connection assignment (7) passes into the one busbar of one partial structure (24) and the other connection assignment (8) into the! one busbar (8) of the other partial structure (25). (Fig.2) 6. Filter according to one of claims 1, 2, 3 or 4, g e-35 tt
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that the substructures (44, 45) form a management structure (43) (energy trapping grating) with further strip-shaped coverings (123), that one substructure (44) comprises a number of these further strip-shaped coverings (123), that the other substructure (45) comprises a further number of these further strip-shaped coverings (123) and these substructures (44, 45) have busbars (27, 28), of which the respective busbar (27, 28) of the respective substructure (44, 45) merges into the respective associated connection assignment (17, 18). (Fig. 4) I I I I I
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