DE8623186U1 - Surface acoustic wave filter - Google Patents

Description

Siemens AG

Surface acoustic wave filter /

The present invention relates to a surface acoustic wave filter according to the preamble of claim 1,

Surface wave filters that work with acoustic, near-surface waves are known. Acoustic near-surface waves are those waves that are less precisely referred to as surface waves, but which include not only surface waves in the narrower sense (SAW waves; Rayleigh waves), but also Bleustein waves, SSBW waves (surface skimming bulk waves), STW waves (surface transient waves), Love waves, and the like. Such arrangements have transducers for converting electrical signals into such waves or for converting such waves into electrical signals. These are interdigital transducers that have interdigitally arranged finger electrodes that are (usually) each connected to a busbar. The basic type of such an interdigital converter thus consists of two bus bars running parallel to one another, from which narrow metallization strips on the surface of a substrate body, referred to as finger electrodes, extend at least essentially at right angles, each directed towards the other bus bar and connected to the relevant bus bar. In this basic type, successively adjacent finger electrodes are alternately electrically connected to the bus bars.

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The way such a transducer works is that by applying an electrical signal between these two busbars (the signal is then present between the adjacent finger electrodes), the acoustic (mechanical) wave is generated between these finger electrodes in the piezoelectric material through the piezoelectric effect. Such an interdigital finger electrode arrangement has a basic resonance that corresponds to the periodicity of the fingers. Such an interdigital transducer usually emits acoustic waves in the material in two directions perpendicular to the finger electrodes. However, such an interdigital transducer also has the disadvantageous property that reflections of the generated acoustic wave occur within the transducer, namely at its finger electrodes. For this purpose, the use of point finger electrode arrangements for transducers was already planned more than a decade ago. The use of such split finger arrangements has the disadvantage that the maximum achievable fundamental resonance frequency is only half as high as with a converter of the basic type, namely under the assumption of equally wide finger electrodes. The maximum possible fundamental frequency for such converters is in fact determined by the technological limit for the production of electrode fingers with the smallest possible width. This dimension is currently around 1 ym with a distance of around 1 pm (clear width between adjacent finger electrodes). A preferred ratio is to make the finger electrode around 1.2 times as wide as the clear distance.

From the publication IEEE Transactions on Sonics and Ultrasonics, Vol. SU-22 (1975), pp. 395-401, for example, transducer arrangements of various designs are known from Figure 3. In Figure 3, the top line shows a transducer arrangement of the basic type. The third line shows a similar

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if nicely described late finger type arrangement. The converter in the second row shows an arrangement that lies in principle between these two converter arrangements described above. The fourth row finally shows a split finger arrangement with additional electrode fingers that are not connected to any of the busbars, ie they have floating potential. _To the right of these converter arrangements in this figur 3 is the position of the resonance frequencies of the converters.

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1 is given as the fundamental resonance frequency of the associated converter. The lines below show the ever smaller fundamental frequencies and the associated (higher) harmonics or overtone frequencies. The split finger wall 1 according to line 3 has a fundamental frequency (as already mentioned above) that is only half as high as the converter in line 1 and a resonance at the third overtone. The same applies to the other lines. However, with regard to this figure 3, it should be noted that operating the respective converter at its overtone is significantly less practical. It can therefore be stated that the further development of a converter in line 1 in the direction of the converters in the following lines 2, 3 and 4 of figure 3 in this publication leads to an increasing reduction in the maximum usable fundamental frequency.

Another way to eliminate internal reflections in a basic type interdigital transducer is to provide unequal (clear) distances within the finger electrode arrangement of the transducer. Assuming the finger width of such a transducer is lambda/4 (lambda = wavelength of the acoustic wave at fundamental resonance) and thus the narrow distance between adjacent fingers is also chosen to be equal to lambda/4, the wider clear distance between electrode fingers in this transducer is lambda/2, which causes destructive interference for reflected parts of the acoustic wave. A

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Such a transducer has the same fundamental frequency as a transducer of the basic type with a comparable finger width. However, such a transducer has relatively strong secondary maxima that can lead to interference. Another disadvantage of this transducer is the unequal distance between the electrodes. Unequal gaps lead to irregularities in the structure produced during the exposure process that must be used to produce the structure, which causes further additional interference with the image quality of a soxCusn nsnuicis.

The object of the present invention is to find a solution to the problem of suppressing internal wave reflections, which, however, does not entail a reduced maximum (fundamental) frequency of the filter or its converter and which does not cause any additional technological difficulties when used (compared to known converters).

This object is achieved with a filter with a transducer having the features of claim 1 and further embodiments emerge from the subclaims. A particularly advantageous development of the invention is a filter with several transducers with a different number of finger electrodes in the groups of the respective transducer and with, in particular, slightly different finger periodicity of the respective transducers among each other.

The present invention is based on the consideration that the converter of the filter according to the invention should have uniformly large clear distances between the finger electrodes. With this converter, however, not only can this advantageous condition be met for the filter, but there is also the advantage, at least for many concepts, that a resonance frequency that can be used for operation (for the fundamental wave excitation) is used.

G 36 23 186.3 - 5 - VPA 86 P 1569 EN

is available, which is even higher than with a converter of the basic type, and this with the same (minimum) width of the finger electrodes and the same width of the clear distances. The two adjacent frequencies that occur with a converter according to the invention are not harmonics or overtones, but rather two fundamental frequencies that are slightly different from one another. Both fundamental frequencies are practically equivalent and are symmetrical to the fundamental frequency of the basic type with the same finger dimensions. The invention therefore provides a fundamental frequency with a comparatively higher frequency value. The invention achieves that with the same technological effort and only the same minimal interference due to production with the same finger width, the "limit frequency" for the use of such filters is increased. The invention therefore not only reduces internal reflections, but also extends the frequency range of the use of such filters or their converters.

The development of the invention already mentioned above is a filter whose at least two transducers have a different number of finger electrodes for the groups of the respective transducer. For example, one transducer of the filter has finger groups with four finger electrodes each and the other transducer or another transducer of this filter has ten finger electrodes per group. This leads to the symmetrically arranged fundamental frequencies of a transducer of the type according to the invention being at different distances from one another. By dimensioning one transducer with regard to its finger periodicity so that its upper fundamental frequency is the same as the upper fundamental frequency of the other transducer, one achieves

that the lower fundamental frequencies of the two converters are different from each other.

Two alternatives to this are to make the lower fundamental frequencies the same size or to make the upper fundamental frequency of one converter the same size as the lower fundamental frequency of the other converter. These alternatives have respective advantages in terms of suppressing the other fundamental frequency(s).

The attached figure serves to further explain the invention.

The figure shows a filter according to the invention with two converters as indicated, in the design with a different number of finger electrodes on the two converters. In the figure:

1, 2 converter in version with

Groups with four or ten finger electrodes each

3, 4, 5 Alternating groups of the converter

6, 7 Alternating groups of the converter

10 finger electrodes

11 busbars

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Claims (7)

Expectations 1. Surface acoustic wave filter,
characterized by a transducer with interdigitally arranged finger electrodes and with busbars with electrical connections on piezoelectric material, such finger electrodes being substantially of equal width and equally spaced from one another and being connected alternately in their sequence to the busbars as in a transducer of the basic type, and wherein the finger electrodes (10) of the converter (1; 2) are present as groups (3, 4; 6, 7) with a predetermined equal number of at least four consecutive finger electrodes, which are alternately connected to the busbars, and successive groups (3-4, 4-3; 6-7, 7-6 ...) in the converter (1, 2) are arranged in such a way that these adjacent groups (for a converter of the basic type) operate in antiphase with each other. 2. Surface acoustic wave filter according to claim 1, characterized in that at least one of the two groups located at the two ends of the converter has a smaller number of finger electrodes (10) connected to a respective busbar than the predetermined number of finger electrodes, 35. Surface acoustic wave filter according to claim 1 or 2, characterized in that the number of finger electrodes per group is not greater than about 20. t t &igr; Il Iff 4 If I *'■■*· i?i ■· * * &igr; f &igr;&igr;&igr;.| ■■ G86 23 186. 3 - 8 - VPA 86 P 1569 EN 4. Surface acoustic wave filter according to claim 1, 2 or 3, characterized in that the minimum number of finger electrodes of a respective group is 10. 5. Surface acoustic wave filter according to claim 1, 2, 3 or 4,
characterized by at least two such transducers having different numbers of finger electrodes in a group. 6. Surface acoustic wave filter according to claim 5, characterized in that these transducers are dimensioned such that, due to the respective finger electrode spacings, the upper fundamental frequency of a first transducer is equal to the lower fundamental frequency of a second other transducer. 7. Surface acoustic wave filter according to claim 5, characterized in that in the case of two such transducers, the finger electrode spacings are dimensioned such that the lower or the upper fundamental frequencies of these transducers coincide. JIII I * &eacgr; 4 I The Ml I ti II f I