DE8524004U1 - Dispersive interdigital transducer for acoustic wave devices - Google Patents

Description

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Dispersive interdigital converter for arrangements operating with acoustic whites.

The invention relates to a dispersive interdigital transducer as specified in the preamble of claim 1 .

From the state of the art, for example from convolvers with acoustic waves, for example from Pro. IEEE, Ultrasonics Symposium (1974), pp. 224 to 227 and (1981), pp. 181 to 185, electrical filter-resonator arrangements and the like are known which work with acoustic waves which are present in a substrate of the arrangement in an area of this substrate which is at least close to the surface .

s substrate. Such acoustic waves are known as Rayleigh waves, Bleustein waves, Love waves, SSBW waves, SABB waves and the like, which are generally referred to below as surface waves (although often only the first two types of waves are referred to as surface waves in the narrower sense).

Such arrangements with acoustic waves have piezoelectric properties at least in the area of the interdigital converter used as input converter and/or output converter. An interdigital converter to be used has finger electrodes arranged parallel to one another on the substrate surface, which are usually distributed over two comb structures formed by these fingers and a busbar each.

These comb structures are interdigital

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Such finger electrodes are usually strip-shaped metallization deposits on the substrate surface.

For the acoustic wave running in the area close to the surface of the substrate, such finger electrodes also act as reflectors, and in an undesirable way in an interdigital converter. There are many suggestions as to how such undesirable reflections (occurring on the finger electrodes of an interdigital converter) can be reduced in their disruptive effect or even eliminated. One such measure is the well-known split-finger design of such an interdigital converter, in which, in contrast to a simple interdigital converter, each individual finger there is split into two parallel fingers lying next to each other, which are electrically charged with the same polarity or are both connected to the same busbar. In a simple interdigital converter, the center distance from one finger electrode to the adjacent, oppositely polarized next finger electrode is usually lambda/2. This center distance is also equal to the (center) distance from a piezoelectric excitation center (between two oppositely polarized finger electrodes) to the next neighboring excitation center. This definition also applies in the corresponding sense to an interdigital converter used as a receiving converter. It is usual to make a finger electrode Lambda/4 in accordance with this center distance Lamda/2 and the gap between two neighboring finger electrodes also Lambda/4. This results in for the split finger design of the interdigital converter provided for in the invention (with a distance Lambda/2 between the neighboring excitation centers) a width of Lambda/8 for a single split finger electrode and also Lambda/8 for the gap between two individual split finger electrodes.

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A split-finger version of an interdigital converter is essentially free of the aforementioned interdigital reflections. Strictly speaking, however, this only applies to the acoustic wave with the value of the wavelength lambda^ which is the basis for the dimensioning of the finger electrodes' distances, widths and spacing. Lambda is the wavelength of the substrate. For many applications, namely with resonators and narrow-band filters, this does not pose a problem. This is different with

In such arrangements working with acoustic waves, have a larger bandwidth, in particular a bandwidth that is close to the value of an octave or is larger than one (or more) octaves. For such arrangements, dispersive interdigital converters are to be used that have finger spacing, width, gap and the like measured in units of lambda, whereby for the converter extending over a certain length on the substrate, different lambda values of the specified bandwidth range are used as a basis in different length parts. As a rule, such a dispersive interdigital converter has (seen in one direction of its longitudinal extension) monotonically decreasing (increasing in the opposite direction) finger spacing, width, gap, i.e. the largest wavelength value lambda_ of the lowest

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lowest frequency of the bandwidth range and the other end is the smallest wavelength value ^Lam bda· of the highest frequency of this bandwidth.

P 30 Even if Lambda _&khgr; is, for example, 1.5 times S of Lambda _m ^ _n for the given bandwidth, larger disturbances due to interdigital reflections can be expected in the dispersive converter. A first maximum of interdigital reflections occurs for Lambda

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35 is equal to 2 times Lambda . .

The object of the present invention is to provide embodiments

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for a split-finger arrangement of an interdigital converter that is designed or intended as a dispersive converter for a larger bandwidth range in the above sense, without this new converter allowing interference signals to take effect to a significant extent.

This object is achieved with a dispersive interdigital converter with the features of patent claim 1 and further embodiments and developments of the invention are evident from the subclaims.

The present invention is based on the finding that in a broadband, dispersive interdigital converter interference signals that occur on the interdigital

1Γ len reflections occur, although a split finger arrangement has been selected for this interdigital transducer. The invention is also based on the knowledge that these reflections cannot fundamentally be eliminated and can only be reduced in their effects. In this connection, it was necessary to take into account that broadband, dispersive interdigital transducers (parallel to the axis of the main wave propagation of the transducer) in particular have a relatively large number of consecutive finger electrodes, i.e. the number of reflection points for interdigital reflections is large in comparison to a narrowband transducer in the usual embodiment of the same. In the invention, in the parts of the transducer in question that are tuned to the lower frequency range of the predetermined bandwidth, dimensions of the width of the finger electrodes are provided which correspond to the corresponding dimension of the finger electrodes of the part (of the transducer) that is intended for the highest frequency part of the bandwidth range . A further measure of ^the invention is the dimensioning of the resulting (in the low frequency portion of the converter ⁴ ) changed distances or gaps^! between the finger electrodes

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to correctly determine and correctly divide or distribute.

It should be noted that the invention can also be used for finger-thinned interdigital transducers,

although the finger dilution for dispersive transducers is

less important than is the case for particularly narrow-band converters (with a long length).
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Of particular importance is the application of the invention, i.e. the use of an interdigital transducer designed according to the invention, for surface wave ||

Convolver arrangements, and in particular for

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Convolver arrangements with compensated self-folding. " ff

For convolver arrangements, the large bandwidth has particular advantages.

importance, but broadband interdigital |

Converters are also required in numerous other applications of | arrangements working with acoustic waves. 20

Further explanations of the invention emerge from the following description given with reference to the figures. |

Figure 1 shows a dispersive interdigital transducer in a *■ split-finger arrangement, whereby this transducer is part of an otherwise not detailed arrangement working with acoustic waves. This figure shows known prior art.

Figure 2 shows a schematic representation and a section of a side view of a known dispersive interdigital transducer of Figure 1.

Compared to Figure 2, Figure 3 shows a schematic representation of an inventive

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dispersive interdigital converter, namely a first variant of the invention.

Figure 4 shows a second variant of the invention compared to Figure 3 and

Figure 5 shows a view (comparable to Figure 1) of a converter according to the invention according to the variant of Figure 3.

In Figure 1, 1 represents a dispersive interdigital transducer located on the surface of a substrate. 3 and 4 denote other structures for acoustic waves present on the substrate surface, whereby, for example, structure 3 can be the integration electrode (and beam compressor arrangements) and structure 4 can be the (output) interdigital transducer of a convolver arrangement.

6 and 7 denote individual finger electrodes of the converter and 8 and 9 denote its busbars. According to the split finger arrangement, two adjacent finger electrodes 6 are connected to one busbar and two adjacent finger electrodes 7 are connected to the other (oppositely polarized) busbar. As can be seen from Figure 1, the electrode finger width and gaps (in Figure 1) decrease from left to right parallel to the main wave propagation direction 10 of the converter 1. The distance a decreases accordingly to a _m 4 _n » where a (equal to

Lambda/2) is the distance between the (dashed) lines of the excitation centers occurring in the (transmitting) converter. These excitation centers lie between oppositely polarized fingers 6 and 7. b is the corresponding center distance of neighboring, opposite

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oppositely poled split finger pairs. The (length) portion of the converter indicated in Figure 1 with 11 is tuned to the range of low frequencies (longest wavelength Lambda ) of the given bandwidth. The length portion 12 corresponds to a medium frequency range and the length portion 13 corresponds to the f' high frequency portion (with Lambda - _n ) of the converter 1. \ This representation of Figure 1 or of the converter 1 is

''-N ₁ Zj only of a fundamental nature, in particular with regard to the lOVAntelJle 11 to 13, the number of finger electrodes of the entire transducer and the like.

Figure 2 shows the finger electrodes 6 and 7 and the substrate 2 in section. The busbar 8 is shown in the background, not sectioned. The (current) polarity + is assumed for this and the finger electrodes 6 connected to it. The low (11), medium (12) and high frequency (13) components of the finger electrode arrangement 6, 7 of the converter 1 correspond to Figure 1. c denotes the respective width of the individual finger electrodes 7, 8 and d denotes the space between two adjacent fingers 6 and two adjacent fingers 6 and 7. As can be seen, the dimensions a, b, c and d are not constant over the length of the converter 1. This corresponds to the intended dispersion of the converter 1.

Figure 3 shows a first variant of the inventive embodiment of an interdigital converter 100. The portion 13 of the converter 100 according to Figure 3 corresponds to the portion 13 of a known converter according to Figures 1 and 2. This is again the portion of the converter 100 which is tuned to the high frequencies or to the wavelength lambda^ of the predetermined bandwidth range. With a _m £ _n, the distance is again

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(Lambda_J _ri /2) between two neighboring excitation centers. The distance b . is again the center distance between two neighboring split finger pairs with opposite polarity, C _n ^ _n is the width of the finger electrodes and d « the gap between two neighboring finger electrodes of the same polarity or opposite polarity. These dimensions also apply to the portion 13 of the second variant of the invention according to Figure 4.

In parts 11 and 12 of Figure 3, the new feature of the invention compared to the prior art according to Figures 1 and 2 can be seen. The finger width is designated by c, the space between two finger electrodes of a split-finger pair, i.e. between two finger electrodes of the same polarity, is designated by d. D ₂ denotes the gap (which differs considerably from the dimension d ) between two adjacent finger electrodes 16 and 17 with different polarity. It is essential for the invention that the width c of the finger electrodes 16, 17 is equal to the value c . or not significantly larger, at most not more than about 1.5 times larger than c *. The width b , of the gap between two electrode fingers 16, 17 of a split finger pair according to the variant of Figure 3 of the invention is also equal to d . or at least not significantly larger (approximately not more than 1.5 times larger) than d . For the sake of completeness, it should also be pointed out that c and d , can also be smaller than c . or d .

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However, this requires increased technological effort. The preferred option is c _min = d _min = c = α _&khgr; .

In the converter 100 according to the invention, as well as necessarily for the portion 13 in for the high frequencies, also for the portion 11 for the lowest frequencies

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and correspondingly for all intermediate parts, such as part 12, for intermediate frequencies, dimensions of the width c for the finger electrodes 16, 17 and distances d, between equally poled electrode fingers 16 and 17 are selected. Inevitably, the dimension d ₂ is significantly larger compared to the state of the art because the condition applies: b = d ₂ + 2 . c + d,, ^{where b} max = ^Lambda max ^/2 '

Figure A shows a converter 100a according to the invention. Unless stated otherwise for converter 100a, the statements for converter 100 in Figure 3 apply to this converter 100a. Converters 100 and 100a differ from one another in that the finger electrodes 16 and 17 are distributed differently in converter 100a than in converter 100. However, these two variants are based on the same principle, namely, the width c of the finger electrodes is dimensioned as described for Figure 3. In converter 100a, however, the width b ₂ ' is chosen to correspond to the dimension d ₁ of converter 100. This means that in converter 100a, an electrode finger 16 of one polarity is located closely adjacent to an electrode finger 17 of the other polarity. The dimension d, ¹ of the gap between the finger electrodes 16 on the one hand and the finger electrodes 17 on the other hand of the respective split finger | is correspondingly large.

pair. Here again (b = ) a = b, ¹ + 2 . c + d _o ' = Lambda /2. Also for the part 12 of the wall

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ler 100a according to Figure 4, what was stated above for part 12 of the converter 100 applies.

A converter according to the invention can also be designed over its length partly according to the variant of the converter 100 and partly according to the variant of the converter 100a. For example, the

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part 11 corresponding to part 11 of Figure 3 and part 12 corresponding to Figure 4, whereby part 13 is always designed at least essentially as described for Figures 3, 4 and Figure 2*

Figure 5 shows the converter 100 from above. Details of Figure 5 can be seen from the preceding description.

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Claims (3)

■ · ■> ■■ «crt > * ■ &igr; t a m 1 · Ii ■ ti ■ · · - Ii - VPA 85 &rgr; j 5 5 i, de claims 1. Dispersive interdigital transducer for acoustic wave arrays,
with split finger arrangement, with such a distribution of the finger electrodes and dimensioning of the excitation centers determined by the finger distribution that this dip-perceptive interdigital transducer has a bandwidth that approaches an octave or covers one or more octaves, wherein in the portion (13) of this interdigital transducer designed for the high frequencies (Lambda . ), the distance between adjacent excitation centers (a = Lambda · /2) and the width of the finger electrodes (a = Lambda · /8) and the spacing between any adjacent fingers (d = Lambda _m ^ _n /8) are designed to correspond to these highest frequencies and in the remaining parts (11, 12) of the interdigital converter, the corresponding dimensions are selected to be tuned to lower frequencies of the bandwidth,
characterized by - that at the respective frequency, the distance a between neighboring excitation centers and the distance b between neighboring finger centers of the oppositely polarized split finger pairs (16 and 17) - the width c of the individual finger electrodes (16, 17) and the gap d between the individual split finger electrodes (16 or 17) of the individual split finger electrode pairs are at least approximately equal to those finger width and gap dimensions (c . , d . ) that are present in the portion 13 of the interdigital converter 100 intended for the high frequencies of the bandwidth. - 12 - VPA 85 P 1 5 5 4 OE 2. Dispersive interdigital transducer for arrangements with acoustic waves, with split finger arrangement, with such a distribution of the finger electrodes and dimensioning of the excitation centers given by the finger distribution that this dispersive interdigital converter has a bandwidth that approaches an octave or covers one or more octaves, _v wobgi in the portion (13) of this interdigital converter designed for the high frequencies (Lambda . ) of the absorption taijid«between neighboring excitation centers (a = Lambda · /2) and the width of the finger electrodes (a = Lamdda * /8) and the spacing between any neighboring fingers (d = Lamdba _m - /8) are dimensioned according to these highest frequencies and .in the remaining parts (11, 12) of the interdigital converter; the corresponding dimensions are selected to be matched to lower frequencies of the bandwidth, characterized in that - at the respective frequency, the distance a between neighboring excitation centers and the distance b between neighboring finger centers of gravity of the oppositely polarized split finger pairs (1(5 and 17) - the width c of the individual finger electrodes (16, ü.7) and the gap between adjacent split finger electrodes (16, 17) of opposite polarity are at least approximately equal to the finger width and gap dimensions ( ^c _m 4 _n ; d _m · ) which are present in the portion 13 of the interdigital transducer (100a) intended for the high frequencies ääx bandwidth. 3. A transducer according to claim 1, characterized in that the width c of the finger electrodes - 13 - VPA 85 P 1 554EN or* 17 and the gap (b.) between adjacent finger electrodes 16 or 17 of a respective split finger pair is not larger than 1.5 times the corresponding dimensions of the finger electrodes of the portion 13 of the transducer 1ÖÖ adapted for the high frequencies. A. Transducer according to claim 2, characterized in that the width c of the finger electrodes and the gap d _o ^f is not greater than 1*5 times the corresponding dimensions c . , d · ) of the portion 13 of the transducer 100a designed for the high frequencies.