JP2003198318A - Interdigital converter compensating diffraction of surface wave - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an interdigital converter for surface wave which minimizes the diffraction effect. <P>SOLUTION: In this interdigital converter, the acoustic track of the interdigital converter is divided into (n) pieces of partial tracks which are arranged in parallel with one another by (n) pieces of partial converters. The converters have drive functions being materialized by the grading of electrode digit cross length and have small apertures A<SB>j</SB>and relatively large diffraction, and are electrically connected with each other. The partial converters have an aperture A<SB>j</SB>each, and all the partial converters have the same drive functions. Numerical formula 1 materializes to the given total apertures of the interdigital converter, (n) is an integer of n≥3, and it is so constituted as to have relatively small diffraction as a whole. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interdigital converter that corrects surface wave diffraction.

[0002]

Interdigital converters are used to generate acoustic surface waves on a piezoelectric substrate. This transducer usually consists of multiple independent electrode fingers,
These electrode fingers are arranged in parallel in parallel in a predetermined raster depending on the surface wave frequency to be generated. In a normal electrode finger transducer, the electrode fingers are alternately connected to busbars having different polarities. In this case, the excitation intensity of the surface wave at the position X corresponds to the crossing length at the position X of the electrode finger pair extending from the different generatrix. The overall transducer excitation is synthesized from the contributions of all transducer electrode finger pairs. The total number of electrode fingers of one transducer may be hundreds. If a different excitation intensity is selected for each electrode finger pair, the electrode finger crossing function can be realized in the transducer by weighting the electrode finger crossing length. Further, besides the weighting of the electrode finger crossing length, the excitation strength can be achieved by the electrode finger width, the deviation of a predetermined raster, or the polarity reversal (= thinning weighting) of each electrode finger.

The desired excitation function is, for example, the function sin (x)
Follow / x. This function can be approximated, for example, by weighting the electrode finger cross length, and the effective electrode finger cross region in the transducer may have one main lobe and multiple side lobes. However, for the short electrode finger cross lengths required in the approximation to such transfer functions, diffraction effects usually occur. At that time, some of the surface wave deviates from the main propagation direction across the electrode fingers, which not only loses some of the energy, but also distorts the signal in the receiving transducer, resulting in the desired signal waveform. A signal waveform different from is received. This is especially an obstacle in such transducers, especially in the video and audio field, where an accurate electrode finger crossing function must be guaranteed. The surface wave filter used to deal with this uses such a weighted transducer, but it must be free of distortion and therefore the distortion caused by surface wave diffraction must be corrected. I have to. Usually this is done by software that takes into account diffraction effects in the calculation of the transmitted signal.

Even in the case of unweighted interdigital converters, too small an aperture causes disturbing diffraction effects. Therefore, it is always desirable to provide an interdigital converter with a sufficiently large aperture.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an interdigital converter for surface waves in which the diffraction effect is minimized.

[0006]

SUMMARY OF THE INVENTION According to the present invention, the above object is to electrically excite each other with an excitation function realized by weighting of electrode finger cross lengths and with a small aperture A _j and a relatively large diffraction. The connected n sub-transducers divide the acoustic track of the interdigital converter into n sub-tracks arranged parallel to each other, each sub-transducer having an aperture A _j . And all sub-transducers have the same excitation function, and for a given total aperture of the interdigital converter,

[0007]

[Equation 2]

And n is an integer of n ≧ 3, and is solved by an interdigital converter configured such that the interdigital converter as a whole has a relatively small diffraction.

Advantageous embodiments of the invention emerge from the further claims.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, instead of a conventional interdigital converter which realizes a desired excitation function by weighting of electrode finger cross lengths, n partial converters arranged in parallel are provided. To propose. Note that these n partial transducers are electrically connected to each other, and their partial apertures A _j are added to the desired total aperture A. At least three partial transducers are needed to achieve the desired effect.

By using such an interdigital converter according to the invention (converter as a whole), even if the partial converter itself has a relatively strong diffraction, it is generally parallel and straight over a wide area. , One wavefront of the transducer is obtained. In this wavefront, disturbing diffraction effects are largely eliminated. The "spherical" wavefront, which is characteristic of the individual transducers, is converted by the invention into a nearly straight wavefront. This effect (reduction of diffraction across the transducer) improves as the number n of partial tracks increases. The diffraction of the whole transducer is smaller than that of the partial transducer.

The interdigital converter according to the invention therefore advantageously has at least five partial converters, each of which is a partial track of the entire converter.

In the above, the number n of possible partial transducers is limited only by the manufacturing technique used. The interdigital converter according to the present invention has a large number of partial converters, and the size of the added gap (= distance from each bus bar facing each other to the electrode finger end) is, as a whole, the effective electrode finger crossing length. Greater than. The rational and technically feasible upper limit on the number of partial tracks of the interdigital converter according to the invention is currently around 15, but in other manufacturing processes it is higher, for example up to 30. Good.

As described above, the partial converters are arranged in parallel in parallel, and the two adjacent partial converters can use the busbar between them as a common busbar. The partial converters are electrically connected to each other,
Connect all converters in parallel,
Alternatively, all the partial converters can be connected in series.

However, it is also possible to connect some of the partial converters electrically in series and to connect the remaining partial converters in parallel. In this way, the impedance of the interdigital converter according to the invention can be adjusted to the desired value. The impedance, ie the wave-making resistance of the converter, then increases as the number of partial converters connected in series increases. Conversely, by connecting the maximum number of partial converters in parallel, an interdigital converter whose impedance is minimized can be obtained. In this way, when the partial converters are connected in parallel, the impedance can be further reduced by increasing the number of converters. Alternatively, in the case of series connection, the impedance can be further increased.

In another embodiment of the invention, the partial track apertures are variously selected. However, even in this case, the total of the apertures A _j is equal to the total aperture A. Advantageously, the partial tracks with different apertures are arranged such that a symmetrical distribution is obtained about a central axis parallel to the propagation direction of the surface waves. In such a symmetrical arrangement, the partial track with the largest partial aperture A _j is centrally located, but with a small partial aperture A _j.
The partial track with is located at the edge of the acoustic track of the entire transducer. It is also possible to place the partial track with the smallest partial aperture in the center of the whole transducer, while the partial track with the largest partial aperture at the edge.

The excitation function may be weighted so that a gradient is generated when the electrode finger crossing length is weighted. With such a form of weighting, the straight line following the center of gravity of the effective electrode finger crossing area is not parallel to the central axis but forms an angle.

The interdigital converter according to the invention therefore has a gradient electrode finger crossing function in all converters. However, it is also possible to have different gradient orientations within the partial transducer. For example, side-by-side partial tracks can have alternating gradient orientations, so that the gradient is “upward” in one partial track and “downward” in the next partial track when viewed in the direction of propagation of the surface wave. , And again "up".

However, it is also possible for the gradient to be "up" in one group of sub-transducers arranged in parallel, whereas it is "down" in another group. Here, it is advantageous to make groups of approximately the same size with the same gradient orientation face each other by making the orientation of the gradients symmetrical with respect to said central axis of the transducer in the whole interdigital converter according to the invention. is there.

The partial transducers belonging to the partial acoustic track are
Are arranged in parallel in parallel, advantageously such that adjacent busbars of adjacent partial converters are in electrical contact,
In particular, two adjacent partial converters are arranged so as to share the busbar in the middle.

Depending on the connection of the partial transducers to the whole transducer, the individual partial transducers are mirrored, if necessary, on the X-axis parallel to the direction of propagation of the surface wave, so that all the transducers are reflected. Are excited in the same phase, and an integral wavefront is generated during excitation.
In this way, a homogeneous wave propagation is ensured and diffraction is suppressed in this wave propagation.

The interdigital converter according to the invention has a further weighting in addition to the weighting of the electrode finger crossing lengths in the partial transducers, the excitation function again being the same or at least approximately in all transducers. , The current weighting scheme applies equally to all transducers. Another weighting like this is
It can be selected from thinning weighting, position weighting, and electrode finger width weighting. By using this weighting method, it is possible to construct a recursive converter in which the center of excitation and the center of reflection are shifted with respect to each other. As a result, the reflected component of the surface wave constitutively overlaps the surface wave excited in the same direction only in one propagation direction, while in other propagation directions the reflected component of the wave radiates in the same direction. The directly excited wave component that is generated is canceled. Such a converter has a preferential radiation direction and, in the extreme case, may be a unidirectional converter, a so-called Spudt converter. The interdigital converter according to the present invention also has an advantage as a recursive converter. This is because the diffraction is suppressed even in this case, and correction of this phenomenon is not necessary.

This is at the same time because the correction of the diffraction in the conventional interdigital converter requires an adjustment which leads to a modification of the electrode finger crossing function for excitation, which at the same time makes the electrode finger crossing function for excitation originally symmetrical. Even results in that the actually obtained electrode finger crossing function is no longer symmetric. However, since the interdigital converter according to the present invention can work without correction,
Using the transducer according to the invention, a 100% symmetrical electrode finger crossing function can be realized.

In the following, the invention will be explained in more detail on the basis of an embodiment and the eight figures attached thereto.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 schematically shows an interdigital converter IDT, known per se, with electrode finger crossing length weighting. The excitation function of the transducer is realized by an effective electrode finger crossing UE, and the excitation function at point X corresponds to an electrode finger crossing with a corresponding electrode finger crossing length at point X. The effective electrode finger crossing region, which is delimited by lines in the figure for the sake of clarity, now has three lobes, whose centers of gravity are not symmetrical about the central axis of the transducer IDT. In another schematic view of this weighted transducer on the right side of FIG. 1, the asymmetric (possibly antisymmetric) distribution of the electrode finger crossing region along the axis X is indicated by the diagonal line S, The diagonal line S corresponds to the slope of the illustrated converter IDT. A represents the aperture of the transducer.

Such weighted n interdigital converter IDTs are arranged in parallel and in parallel, and their partial apertures are the total aperture A which is the sum of the individual apertures A _{1 to} A _n of the entire converter. Scaling to match the initial desired aperture results in a multiple parallel interdigital converter MPW. The interdigital converter MPW according to the invention has, in a simple embodiment, identical partial converters TW _{1 to} TW _n , each having the same aperture A _j .

Unlike FIG. 2, FIG. 3 also shows an interdigital converter according to the invention which consists of n (for example n = 6) partial converters TW. Unlike Figure 2,
Some converters are mirrored. This is recognized in the schematic by the orientation of the straight line representing the slope S.
The "original" subtransducer TW and the "mirrored" subtransducer T
The division of the interdigital converter according to the invention into W and W is preferably effected symmetrically, for example as shown in FIG.

FIG. 4 shows another embodiment of the present invention. In this embodiment, the aperture A _j of the partial converter TW is different, unlike the simplest case of FIG. In the figure, for example, the middle sub-transducer TW3 has the largest aperture, while the first sub-transducer TW1 has the smallest aperture like the bottom sub-transducer. In this case, as in the case of FIG.
In principle, the excitation function of a partial transducer is a reflection or slope S
It is not changed by the change of the orientation or the aperture.

FIG. 5 shows the simplest way of electrically connecting the partial converter TW to the multiple parallel interdigital converter MPW according to the invention. In the figure, all the partial converters TW indicated by blanks are connected in series. This is achieved relatively easily by placing the partial transducers directly in parallel with no spacing, as shown in the figure on the right. At that time, two adjacent partial converters TW
Share the bus that lies between them. When the electrical terminals T1 and T2 are connected to the top and bottom busbars of a multiple parallel interdigital converter, a series connection is automatically obtained in this configuration.

However, it is also possible to connect the partial converters electrically in parallel, as shown in FIG. This may be done by correspondingly connecting subtransducers spaced apart from each other, or in the same way in a simplified manner between two adjacent subtransducers with an intermediate busbar (common to each column in the figure). It can be done by sharing (corresponding to the side). Advantageously, the partial transducers next to each other are mirror images of the image and are elements that are symmetrical about the X axis. This is particularly advantageous. This is because adjacent partial converters having different or opposite polarities are connected to the terminal T1.
And T2, and each second partial transducer is corrected by the mirror image configuration.

FIG. 7 shows another embodiment of the present invention. In this embodiment, some partial converters are connected in series, while the remaining partial converters are connected in parallel. According to the figure, the partial converters 1 and 2 are connected in series, these partial converters being designated by the same number in the figure. The partial converters 3, 4 and 5 are connected in parallel, the common bus between the partial converters 2 and 3 and the common bus between the partial converters 4 and 5 are connected to each other, likewise The common busbar between the partial converters 3 and 4 is connected to the busbar of the partial converter 5 shown at the outermost bottom in the figure.

FIG. 8 shows an example of use of the interdigital converter MPW according to the invention in a surface wave filter, for example in a three-converter configuration as shown.
Here, the weighted interdigital converter MPW according to the invention may be an input converter, which is usually surrounded on both sides by short output converters IDT1 and IDT2.
The output converter is unweighted and has an aperture A which corresponds to the total aperture of the multiple parallel interdigital converter MPW. Moreover, the total aperture of the multiple parallel interdigital converter MPW is obtained from the total sum of the partial apertures A _j of the partial converter TW including the necessary bus.

If the weighted input converter formed as an interdigital converter according to the invention is formed as a recursive converter or a unidirectional converter, the use of the second output converter is It is unnecessary. Based on the wave propagation direction which is then preferentially or excluded, all acoustic waves generated by the input transducer are converted back, thereby achieving a lossless transmission of the signal supplied to the input transducer: One output converter is sufficient.

Although the invention has been described in terms of relatively few embodiments for clarity, the invention is not limited to the embodiments shown in the drawings. In particular, the structure of the partial converter is not limited to the embodiment, and the weighting method is not limited to the excitation function having the gradient as shown in FIG. Similarly, it is not necessary to select the normal electrode finger configuration for the partial transducer. It is also possible to additionally implement split electrode finger configurations or so-called position weighting or electrode finger width weighting and decimation weighting, individually or in combination, for the orientation of the electrode finger crossings in the partial transducer.

[Brief description of drawings]

FIG. 1 schematically shows a weighted converter.

FIG. 2 shows schematically an interdigital converter according to the invention.

FIG. 3 shows another interdigital converter according to the invention in which the partial converters are distinguished by the orientation of the gradients.

FIG. 4 shows another interdigital converter according to the invention with different apertures.

FIG. 5 shows an interdigital converter according to the invention in which the partial converters are connected in series.

FIG. 6 shows an interdigital converter according to the invention in which the partial converters are connected in parallel.

FIG. 7 shows an interdigital converter according to the invention in which some of the partial converters are connected in series and some are connected in parallel.

FIG. 8 shows a surface wave filter with three interdigital converters and an interdigital converter according to the invention as a central converter.

Claims (12)

[Claims] 1. An interdigital converter (MPW) for correcting surface wave diffraction, having an excitation function realized by weighting of electrode finger cross lengths, each having a small aperture A _j and a relatively large diffraction. N sub-transducers (T
W) divides the acoustic track of the interdigital converter into n sub-tracks arranged parallel to each other, each sub-transducer having one aperture A _j , The partial converters have the same excitation function, and for a given total aperture (A) of the interdigital converter, And n is an integer of n ≧ 3, and the interdigital converter (MPW) has a relatively small diffraction as a whole. 2. N sub-transducers (T) connected to each other.
W) is provided and 30 ≧ n ≧ 5 is satisfied.
The interdigital converter described. 3. The interdigital converter according to claim 1, wherein all of the n partial converters (TW) are electrically connected in parallel or completely connected in series. . 4. The interdigital converter according to claim 1, wherein the n partial converters (TW) are electrically connected in part in series and in part in parallel. 5. The apertures (A _j ) of the partial transducers (T _j ) have different sizes, and the arrangement of the apertures of different sizes is parallel to the propagation direction (X) of the surface wave. The interdigital converter according to claim 1, wherein the interdigital converter is symmetrical with respect to a central axis of the interdigital converter. 6. The partial transducer (TW) according to claim 1, wherein each of the partial transducers (TW) has a gradient, and the orientation of the gradient with respect to the central axis of the partial transducer (TW) is different for each partial transducer. 1
An interdigital converter according to item. 7. Two partial converters (T) which are respectively adjacent to each other.
7. The interdigital converter according to claim 1, wherein W) utilizes a busbar arranged between the partial converters as a common busbar. 8. Interdigital converter according to claim 7, wherein the n partial converters (TW) are excited in phase. 9. The n sub-transducers (TW) have, in addition to the electrode finger cross length weighting, another weighting selected from decimation weighting, position weighting and electrode finger width weighting. 9. The interdigital converter according to any one of 1 to 8. 10. The interdigital converter according to claim 1, which is composed of a recursive partial converter (TW). 11. A 1-track interdigital converter (IDT) of aperture A, which is configured as an input converter or an output converter of a SAW filter and serves as a corresponding output converter or input converter.
1, IDT 2) are provided. The interdigital converter according to any one of claims 1 to 10. 12. In order to realize a symmetrical transfer function,
Use of an interdigital converter according to any one of claims 1 to 11 in a transversal filter.