FR2818051A1 - OPTIMIZED SYMMETRY SURFACE ACOUSTIC WAVE FLITERS - Google Patents

Abstract

The invention concerns a surface acoustic wave filter comprising at least an i<th> cascaded input acoustic path (VAi) to an i + 1 output acoustic path (VAi + 1), each acoustic path comprising interdigital electrode transducers and in particular: at least a central transducer (TCi); at least a pair of coupling transducers consisting of a first lateral transducer (T1Li) and a second lateral transducer (T2Li). The invention is characterised in that it comprises a first electrical connection with a first potential (V1) connecting the first lateral transducer (T1Li) of the i<th> acoustic path and a second electrical connection to a second potential (V2) connecting the second lateral transducer of the (i + 1)<th> acoustic path (T2Li+1) and the potentials are out of phase. The central transducer of the i<th> acoustic path being connected to at least an input voltage (ViN). The central transducer of the i + 1<th> acoustic path being connected to at least an output voltage (Vout). Said filter structure enables to reduce cross couplings and to obtain in differential structures very good symmetry, in the field of double mode filters.

Description

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The field of the invention is that of surface wave filters and more precisely that of symmetric dual-mode filters also called Dtv) S ", in particular described by T. MORITA et al; WIDEBAND LOW LOSS DOUBLE MODE FILTERS, 1992 IEEE ULTRASONICS PROC., Pp 95-104, which correspond to an improvement of filters with longitudinally coupled resonators.

In general, filters with resonators with longitudinal coupling comprise two cavities each formed by a transducer located between two reflector arrays, said cavities being coupled longitudinally, as illustrated in FIG. 1. A first cavity is defined by the transducer Ti between the reflectors Ri and R2, the second cavity is defined by the transducer T2 located between the reflectors R2 and R3, the reflector R2 being common to the two cavities. Two main modes of acoustic resonance are thus obtained (a symmetrical mode and an antisymmetric mode) and the difference between the frequencies of these two modes gives the first order the passband of the filter.

The main drawback of this filter is its limitation in passband (and / or the high source load impedances that it requires).

To overcome this drawback, the solution generally used consists in replacing the structure with two transducers as illustrated in FIG. 1, by a structure with three transducers making it possible without excessively increasing the impedance of the filter to widen its passband. Such a structure is illustrated in Figure 2. Three transducers Tu, Tis, Tis are located between two reflectors Ru and Riz. The two side transducers Tu and T13 are electrically connected in parallel. The central transducer T12 having a transduction and reflection function and making it possible to define two cavities coupled longitudinally thanks to the central reflection function. The structure is completely symmetrical with respect to the vertical axis AA 'and the polarities of two electrodes symmetrical with respect to this axis are identical (see the electrodes Ej, Ej + 1 or E, and E, + 1). Thanks to this symmetry, only the symmetrical modes can be excited, the antisymmetric modes not being coupled.

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In this type of filter the frequency difference between the first two symmetrical modes gives the first order the width of the passband, which is wider than the passband of the first structure illustrated in Figure 1.

This type of structure is currently improved in terms of rejection, that is to say attenuation of out-of-band frequencies by cascading several identical or identical type of acoustic channels.

voie acoustique VA2, la première voie acoustique comporte un transducteur d'entrée T22, la seconde voie acoustique comporte un transducteur de sortie T32 et, un second transducteur T23 de la voie acoustique VA, est également connecté électriquement à un second transducteur T33 de la seconde voie acoustique VA2. Dans chacune des voies acoustiques les trois transducteurs sont situés entre deux réflecteurs R21 et R22 (R3, et R32). Ainsi on peut définir dans chacune des voies acoustiques, deux cavités couplées longitudinalement. En effet, en raison des fonctions de transduction et de réflexion des transducteurs centraux, les transducteurs T21, T23, T31 et T33 sont également insérés dans des cavités. Ces cavités sont liées aux décalages spatiaux entre les transducteurs centraux et les transducteurs latéraux et aux réflexions sur les transducteurs et réflecteurs. FIG. 3 illustrates such a structure comprising two acoustic paths VA1 and VA2. A first transducer T2, of the acoustic path VA, is electrically coupled to a first lateral transducer T31 of the

acoustic path VA2, the first acoustic path comprises an input transducer T22, the second acoustic path comprises an output transducer T32 and, a second transducer T23 of the acoustic path VA, is also electrically connected to a second transducer T33 of the second acoustic channel VA2. In each of the acoustic channels, the three transducers are located between two reflectors R21 and R22 (R3, and R32). Thus it is possible to define in each of the acoustic paths, two cavities coupled longitudinally. Indeed, due to the transduction and reflection functions of the central transducers, the transducers T21, T23, T31 and T33 are also inserted into cavities. These cavities are related to the spatial offsets between the central transducers and the side transducers and to the reflections on the transducers and reflectors.

According to the configuration of Figure 4 the central transducer T22 (or T32) has an odd number N, of electrodes as shown in Figure 4 which illustrates the positioning of a central transducer with respect to the two side transducers T21 / T23 or T31 / T33 positioned outside the two borders placed on the drawing. This figure also shows the conventions generally adopted for transducer limits. It is usually considered that a transducer of period p has its two limits placed half a period (left and right) from its last electrode (left and right). In the text when we give distances between transducers, it is to the distance between limits of transducers that we will refer.

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Considering P the period of the central transducer and d the offset between central transducer and side transducers and the sources of the central transducer, positioned on the hot electrodes noted +, the other electrodes being at ground M, it appears that the distances between hot electrodes and the limits of the side transducers are two by two due to the odd number of electrodes. Conventionally in this type of filter, the period P being close to half a wavelength, it appears that the waves emitted by the central transducer 22 are in phase on the left and right borders of the central transducer. Consequently, the polarities of the successive electrodes seen from the two side transducers are identical and the voltages V 1 and V2 referenced between the two acoustic paths VA, and VA2 represented in FIG. 3 are equal.

Due to the small dimensions involved, the electromagnetic parasitic couplings of the input on the midpoints 1 and 2 are not negligible. This is also true for the electromagnetic couplings of the midpoints 1 and 2 on the output. On the other hand, these couplings can have an influence on the voltage at the output of the filter and degrade the transfer function of the filter.

Indeed, by symmetry, the parasitic couplings of the input on the midpoints 1 and 2 give rise to parasitic voltages both equal to CV, N (if C is the coefficient giving the coupling between the input and the points media 1 and 2). In the configuration considered, the output voltage is proportional by symmetry to the sum of the voltages V1 and V2, which means that the influences on the output voltage of the parasitic couplings of the input on the midpoints 1 and 2 are added. phase. Likewise, the parasitic couplings of the midpoints on the output voltage are added in phase.

To reduce the coupling phenomena, the invention proposes to configure the filter to impose opposing voltages at the midpoint 1 and 2.

More precisely and generally, the subject of the invention is a surface acoustic wave filter comprising at least an ith acoustic channel (VA,) of input cascaded to an i + 1 acoustic channel (VA, + 1) of output, each acoustic channel comprising transducers with interdigitated electrodes and in particular:

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- au moins un transducteur central (TC,) ; - au moins une paire de transducteurs de couplage formée d'un premier transducteur latéral (T1 LI) et d'un second transducteur latéral (T2L,) caractérisé en ce qu'il comporte une première liaison électrique à un premier potentiel (V1) reliant le premier transducteur latéral (T1LI) de la seme voie acoustique au premier transducteur latéral (T1LI + 1) de la i + ième voie acoustique et une deuxième liaison électrique à un second potentiel (V2) reliant le deuxième transducteur latéral de la ième voie acoustique (T2LI) au deuxième transducteur latéral de la (i + 1)eme voie acoustique (T2L, + 1) et en ce que le premier potentiel et le second potentiel sont en opposition de phase.

- at least one central transducer (TC,); - at least one pair of coupling transducers formed of a first lateral transducer (T1 LI) and of a second lateral transducer (T2L,) characterized in that it comprises a first electrical connection to a first potential (V1) connecting the first lateral transducer (T1LI) of the seme acoustic channel to the first lateral transducer (T1LI + 1) of the i + th acoustic channel and a second electrical connection to a second potential (V2) connecting the second lateral transducer of the ith acoustic channel (T2LI) to the second lateral transducer of the (i + 1) th acoustic channel (T2L, + 1) and in that the first potential and the second potential are in phase opposition.

The central transducer of the ith acoustic channel being connected to at least one input voltage (VIN).

The central transducer of the i + 1 acoustic channel being connected to at least one output voltage (Veut).

According to a variant of the invention, to obtain the potential inversion, the central transducer of the i th and / or of the i + th acoustic channel has an odd number of electrodes, the first and second lateral transducers having a series of electrodes connected to a potential called an interdigitated hot spot with a series of electrodes connected to a ground potential, the first electrode of the first side transducer, from the central transducer being connected to the hot point, the first electrode of the second side transducer from the transducer central being connected to ground, the distance between the central transducer and the first lateral transducer being equal to an even number of period (P, if P is the period of the electrodes in the transducers) or to an integer of length of wave # near (with # the wavelength corresponding to the central frequency of the filter) to that between the central transducer and the second side transducer and the polarities of the successive electrodes of the first lateral transducer being opposite to the polarities of the successive electrodes of the second lateral transducer.

According to another variant of the invention, the central transducer of the and / or of the i + i th acoustic channel may have an odd number

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of electrodes, the first and second side transducers having a series of electrodes connected to a so-called interdigitated hot spot potential with a series of electrodes connected to a ground potential, the first electrode of the first side transducer from the central transducer being connected at the same potential as the first electrode of the second lateral transducer from the central transducer, the distance between the central transducer and the first lateral transducer being equal to an odd number of period or to an odd number of half wavelengths-to that between the central transducer and the second lateral transducer.

According to another variant of the invention, to obtain the potential inversion, the central transducer of the and / or of the i + i th acoustic channel has an even number of electrodes, the first and second lateral transducers having a series of 'electrodes connected to a so-called hot spot potential, interdigitated with a series of electrodes connected to a ground potential, the first electrode of the first side transducer from the central transducer being connected to the same potential as the first electrode of the second side transducer from the central transducer, the distance between the central transducer and the first lateral transducer being equal to an even number of period (2kP, with k integer, P period of the electrodes in the transducers) or to an integer of wavelength, to those between the central transducer and the second side transducer.

In general, the filter comprising two stages, to function correctly must include either two identical stages or two stages of the same type, i.e. the voltages V1 and V2 on the side transducers of the second stage must give responses in phase opposition. on the output transducer.

All of the filters described above assume that the input and output are simple (an input voltage VIN and an output voltage Veut). The input or output impedances of these filters can typically be 50 ohms making these filters particularly suitable for applications in the field of radio frequencies.

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To make these filters compatible with current amplification circuits, it is possible to adapt these filters, by making them operate in differential. For this one of the central transducers can be split into two separate equal parts, with reversed polarities Want + and Want.

US Pat. No. 6081172 describes, for example, structures with single input and differential outputs, nevertheless suffering from imperfect symmetry.

The proprietor in the patent application: M. SOLAL, J. DESBOIS, P. DUFILIE, Surface acoustic wave filter with reflective networks)), French patent application 92 07883 described a transducer structure allowing differential attack . This structure, of the type of that of FIG. 5, consists in separating the transducer into two equal parts connected one between the mass and the positive voltage and the other between the mass and the negative voltage.

More precisely, this transducer is composed of a continuous series of electrodes Ej and of two series of electrodes E, 1 and E, 2, these two series of electrodes being connected at opposite potentials.

However, if this type of transducer is integrated into a filter comprising two cascaded acoustic paths as illustrated in FIG. 4, the symmetry required to cancel the antisymmetric mode is lost. It is therefore necessary to adapt the structure to two acoustic paths to restore the desired symmetry. Indeed the two half-transducers illustrated in Figure 5, are generally separated by a distance equal to a half wavelength or a period, so the waves emitted to the right by the right transducer Td and referenced 1 are in phase with the waves emitted to the right by the left transducer Tg and referenced 2 ', likewise the waves emitted to the left by the transducer Tg and referenced 2 are in phase with the waves emitted by the left by the right transducer and referenced 1 '. On the other hand, the waves referenced 1 and 2 are in phase opposition and therefore overall the acoustic emissions being in phase opposition, the resultant of these waves for the whole of the filter is zero. To restore correct operation in a structure as illustrated in Figure 4, the polarities of the side transducers can be reversed. The structure of the prior art illustrated in FIG. 6 is obtained. The invention thus applies

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also in the context of differential input output filters, for which it is also sought to reduce the problems of parasitic coupling.

This is why the invention also relates to a surface wave filter, characterized in that it comprises at least one acoustic channel connected to a differential input and / or a differential output (VIN +, VIN- and / or Vout +, Vout -), said acoustic channel comprising a central transducer split into two parts and comprising a continuous series of interdigitated electrodes with, two continuous and adjacent series of electrodes connected to the input voltages (VIN +, VIN-) and / or audited output voltages (Vaut +, Vaut -), the two parts being symmetrical, i.e. the series of polarities of the electrodes starting from the center are opposite, i.e. one electrode + for the first part corresponds to an electrode - for the other and a mass for one part corresponds to a mass for the other, the distance between the + electrodes and the electrodes - is equal to an odd number of periods (or half-lengths wave) and the two side transducers are positioned at the same e distance (within 2kP) from the central transducer.

Advantageously, the filter according to the invention can comprise a lithium tantalate substrate with a cutting angle of between Y + 410 and Y + 430 with Y conventional crystallographic axis.

Advantageously, the thickness of the electrodes can be of the order of 5 to 10% of the acoustic wavelength.

Furthermore, the number of electrodes of the side transducers can advantageously be between approximately 50% and 100% of the number of electrodes of the central transducers.

A further subject of the invention is a surface wave filter, characterized in that the ith input acoustic channel (VAi) and / or the i + 1st output acoustic channel (VA, +,) comprises a series of transducers central (TCI, l and / or TCj) interposed with on the one hand a series of first lateral transducers (T1L ,. j and / or T1L, n, j) and on the other hand a series of second lateral transducers (T2L, ,, and / or T2L, + i. j).

The invention will be better understood and other advantages will appear on reading the description which follows, given without limitation and thanks to the appended figures, among which:

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- Figure 1 illustrates a first example of a filter with longitudinal coupling resonators according to the known art; FIG. 2 illustrates a second example of a filter with longitudinal coupling resonators according to the known art comprising a central transducer and two lateral transducers; FIG. 3 illustrates a third example of a filter with longitudinal coupling resonators according to the known art comprising two cascaded acoustic paths; - Figure 4 illustrates the positioning of the electrodes of a central transducer relative to the side transducers in the third example of filter illustrated in Figure 3; - Figure 5 illustrates a differential input / output transducer of the known art; FIG. 6 illustrates a fourth example of a filter with longitudinal coupling resonators according to the known art comprising two cascaded acoustic paths and a transducer with differential input / outputs; FIG. 7 illustrates a first example of a filter with longitudinal coupling resonators according to the invention with single input / output; FIG. 8 illustrates a second example of a filter with longitudinal coupling resonators according to the invention, with single input / output; FIG. 9 illustrates a third example of a filter with longitudinally coupled resonators according to the invention with single input / output; FIGS. 10a and 10b illustrate examples of a filter according to the invention with differential inputs / outputs; FIG. 11 illustrates an example of a filter according to the invention comprising a number of acoustic channels greater than 2; FIG. 12 illustrates an example of a filter according to the invention in which each acoustic channel comprises several input or output transducers and several pairs of lateral transducers;

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- les Figures 13a et 13b illustrent plusieurs types de séparations entre transducteurs, dans un filtre selon l'invention ; - la Figure 14 illustre la fonction de transfert d'un exemple de filtre selon l'invention ; - la Figure 15 illustre la fonction de transfert d'un exemple de filtre selon l'invention ; - la Figure 16 illustre la symétrie d'amplitude et de phase du même exemple.

- Figures 13a and 13b illustrate several types of separations between transducers, in a filter according to the invention; FIG. 14 illustrates the transfer function of an example of a filter according to the invention; FIG. 15 illustrates the transfer function of an example of a filter according to the invention; - Figure 16 illustrates the amplitude and phase symmetry of the same example.

In general, the filter according to the invention comprises at least two acoustic paths, each acoustic path comprising at least one central transducer and two lateral transducers electrically connected to cascade the two acoustic paths. In all cases, the number of electrodes of the central transducer, the polarities of the transducers, the distances between transducers are chosen so that the voltages at points symmetrical with respect to the axis of symmetry of the filter are in opposition phase two to two. In other words, the transfer functions giving the voltages at the midpoints with respect to the input voltage are opposed as well as the transfer functions giving the output voltage as a function of the voltages at the midpoints. By reciprocity, the reasoning is equivalent if we consider one or the other type of transfer function.

A set of filter examples will be described below, illustrating the invention.

Example 1
According to a first variant, the filter according to the invention can comprise a central transducer with an odd number of electrodes and two lateral transducers whose polarities are reversed. Figure 7 illustrates such a configuration.

The acoustic path VA, comprises between two reflectors R11 and Run central transducer TC, connected to an input voltage VIN, inserted between a first lateral transducer T1 Li and a second lateral transducer T2L ,. Similarly, the acoustic channel VA2 comprises between two reflectors R12 and R22, a central transducer TC2 connected to an output voltage Veut, inserted

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between a first lateral transducer T1 L2 and a second lateral transducer T2L2.

The distance between the central transducer and the first lateral transducer is chosen equal to the distance between the central transducer and the second lateral transducer and designated by the letter d. The electrodes Eiu, which are symmetrical to the electrodes Ë2U with respect to the axis of symmetry of the filter S, in distance are brought to reversed polarities so as to bring the points 1 and 2 to opposite voltages. By symmetry, the parasitic electromagnetic couplings of the input on the midpoints give rise to the midpoints 1 and 2 at parasitic voltages equal to C VIN (if C is the coefficient giving the parasitic coupling of the input to the midpoints) . By the polarities inversions of the transducers, the output voltage is proportional to the difference V1-V2, and therefore the influences on the output signal of the filter of the electromagnetic couplings of the input on the middle points 1 and 2 are in opposition to phase and therefore offset each other.

By symmetry, the couplings of the midpoints on the output voltage of the filter are CV1 and CV2, that is to say since V1 and V2 are in phase opposition, CV1 and-CV1. The parasitic voltages linked to these two couplings therefore also compensate for each other. The invention thus makes it possible to very clearly improve the operation of the filter by compensating for the influence of all the parasitic electromagnetic couplings of the input on the midpoints (and of the midpoints on the output). The only parasitic electromagnetic couplings remaining are those existing from the input to the output, but these parasitic couplings are negligible compared to the previous couplings due to the greater distance. Good out-of-band attenuation is therefore more easily obtained.

Example 2
A configuration close to that illustrated in Figure 7, consists in still having a central transducer having an odd number of electrodes, in positioning the side transducers so that the distance between the central transducer and the first side transducer and the distance between the central transducer and the second lateral transducer are distinct by a value kP (or k lambda / 2) with k odd non-zero integer, as illustrated in Figure 8. The successive electrodes of the

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two side transducers being brought to the same polarities (hot point or ground). The waves arriving on the transducers T1L1 and T2L1 are therefore in phase opposition.

Example 3
According to a third variant, the filter of the invention can comprise a central transducer with an even number of electrodes and two lateral transducers whose polarities are identical. Figure 9 illustrates such a configuration. The electrodes E1L, symmetrical to the electrodes E2u with respect to the axis of symmetry S, of the filter are brought to identical polarities. In this example, the distance between the central transducer and the first lateral transducer is again chosen equal to the distance between the central transducer and the second lateral transducer (to an integer number of wavelength or an even number of periods). It appears in this configuration that the waves emitted by the central transducer TC1 (TC2) are in phase opposition at the level of the lateral transducer T1L1 (T1L2) with the waves emitted by the central transducer TC1 (TC2) at the level of the lateral transducer T2u ( Ï2L2). The successive polarities of the electrodes of the lateral transducers being identical, it follows that the contact points 1 and 2 are thus brought to potentials in phase opposition + V1 and - V1 so as to compensate for the influences of parasitic electromagnetic couplings.

It goes without saying that the preceding examples can be combined with each other, that is to say that the two cascaded paths are not necessarily identical or of the same type, but they are both of one of the types described above.

We have just described examples of single input and single output filters. The invention can nevertheless and advantageously be applied in the context of filters with differential inputs and / or outputs, as will be described below.

Filters with differential inputs and / or outputs

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The first way to work in differential is to connect the central transducer of one (or both) channels between a positive voltage and a negative voltage, that is to say that the electrodes previously connected to the mass on the central transducer are connected. now at a potential opposite to the potential to which the other electrodes of the central transducer are connected, the positions and polarities of the side transducers being unchanged. We then obtain a filter with differential inputs and / or outputs working on the same impedances as the previous single input-output filter.

Example 4
To achieve a differential output, another way consists in splitting the central transducer of the output acoustic channel VA2 into two parts comprising a first set of continuous electrodes connected in the example to ground and interdigitated with two sets of electrodes. respectively connected to a Vaut + output and a Want- output. It can be noted that the ground connection is shown in the figure, but that by symmetry the continuous electrode is naturally grounded and that this connection is therefore not necessary in practice (known art).

According to this example, the input channel VA is identical to that described in example 1, i.e. a central transducer with an odd number of electrodes, lateral transducers Tiu and T2u equidistant by a distance d from the central transducer TC1 and the electrodes symmetrical Water and E2L, brought to opposite potentials.

The output channel VA2 comprises a complex central transducer fed in differential with an even total number of electrodes, and therefore a configuration identical to that of example 3, in which the lateral transducers are equidistant from the central transducer, the electrodes symmetrical by with respect to the axis of symmetry S being at identical polarities, as illustrated in Figure 10a.

It is also possible to obtain a filter with differential inputs and outputs according to the invention by cascading two channels of the type of channel VA2 of FIG. 1 osa, as illustrated in FIG. 1 Ob.

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In all of the examples described above, the acoustic paths of the same filter are therefore identical or different but are always structured so as to create connection points 1 and 2 at voltages in phase opposition.

The invention can also be applied in the case of a number i of channels, greater than 2. According to a preferred embodiment, it can be envisaged to cascade several pairs of channels such as the channels VA, and VA2, as illustrated in FIG. 11.

Likewise, the filter according to the invention can comprise more than one central transducer and more than one pair of lateral transducers acoustically. In the case of several pairs of lateral transducers acoustically, the polarities of these transducers are chosen so that the voltages at points symmetrical with respect to the axis of symmetry of the filter are opposite in pairs.

The advantage of a filter in which the so-called central transducers are interposed with so-called lateral transducers is to reduce losses or to make the filter stiffer.

TlL1, 2/T2L1, 2 ; T1Li. i/T2Li. i (T1 L2, 1/T2L2, 1 ; T1 L2, 2/T2L2, 2) avec V1 : la tension entre le transducteur T1 Lu et le transducteur T1 L2, 1 ; - Vi : la tension entre le transducteur T2Lij et le transducteur T2L2. 1 ; V2 : la tension entre le transducteur T1 Li, 2 et le transducteur T1 L2, 2 ; - V2 : la tension entre le transducteur T2Li, 2 et le transducteur T2L2, 2. Figure 12 illustrates this type of configuration. The filter has an axis of symmetry S, each acoustic channel VA, and VA2 respectively comprise three transducers TC1, 1, TCi. 2, TCi, 3 (TC2. I, TC2, 2, TC2, 3) connected to the input voltage VIN (to the output voltage Veut) and pairs of side transducers symmetrical with respect to the S axis:

T1L1, 2 / T2L1, 2; T1Li. i / T2Li. i (T1 L2, 1 / T2L2, 1; T1 L2, 2 / T2L2, 2) with V1: the voltage between the transducer T1 Lu and the transducer T1 L2, 1; - Vi: the voltage between transducer T2Lij and transducer T2L2. 1; V2: the voltage between the transducer T1 Li, 2 and the transducer T1 L2, 2; - V2: the voltage between transducer T2Li, 2 and transducer T2L2, 2.

In all of the configurations presented, it may be particularly advantageous to separate the different transducers by a larger metal electrode. Indeed, as regards the spaces between the transducers, those skilled in the art know that these spaces constitute

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a break in the periodicity which results in the appearance of losses often attributed to a diffraction in bulk acoustic waves. To reduce these losses, it is known that it is more favorable to replace the free space between the transducers by a metallized zone, which generally amounts to placing between the transducers a metal electrode of greater width than those of the other electrodes. Another, more effective means of reducing these losses at the separation between the transducers consists in carrying out the phase shift gradually by gradually changing the periods of the last electrodes of the transducers, for example by reducing it. This process is conventionally used for making resonators and is used in dual-mode filters from the company EPCOS (eg: filter ref. B4121).

FIGS. 13a and 13b thus respectively illustrate two transducers separated by a large electrode or transducers separated by virtue of the presence of variation of periods, periods chosen to follow a linear law.

We will describe an exemplary embodiment of a filter according to the invention for Extended GSM application.

The filter consists of two cascaded channels further comprising reflectors. The input impedance is 50 ohms and the output impedance is 200 ohms. The filter is formed according to the configuration shown schematically in FIG. 10. The material chosen is lithium tantalate of cut Y + 42. The Fujitsu company, in European Patent No. EP 0845 858 A2, Fujitsu has shown that it was particularly advantageous for these filters to use Lithium Tantalate of Y + 42 cut as substrate. On the other hand, this patent describes configurations in terms of number of electrodes of the transducers, periods of transducers and networks, metallization thickness and also in terms of offset between the transducers making it possible to obtain good performance. The acoustic openings, ie the lengths of the electrodes are the same for the two channels, ie 170 microns. The routes are described in more detail below.

For the non-differential channel:

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- transducteur central : 3 électrodes non périodiques + 27 électrodes périodiques (p = 2. 21 um) + 3 électrodes non périodiques, soit au total 33 électrodes avec des électrodes chaudes aux extrémités ; - transducteurs latéraux : 25 électrodes périodiques (p = 2.21 um) + 3 électrodes non périodiques pour chacun des deux transducteurs latéraux, soit au total 28 électrodes pour chaque transducteur latéral. Les polarités des transducteurs latéraux sont inversées ; - réseaux : 50 électrodes à la masse pour chaque réseau (p =

2. 245 um).

- central transducer: 3 non-periodic electrodes + 27 periodic electrodes (p = 2.21 µm) + 3 non-periodic electrodes, ie a total of 33 electrodes with hot electrodes at the ends; - side transducers: 25 periodic electrodes (p = 2.21 μm) + 3 non-periodic electrodes for each of the two side transducers, ie a total of 28 electrodes for each side transducer. The polarities of the side transducers are reversed; - networks: 50 electrodes to ground for each network (p =

2. 245 µm).

The table below gives the successive periods between electrodes for the separations between transducers:

<tb>
<tb> 2. <SEP> 21 <SEP> m
<tb> p1 <SEP> 2. <SEP> 16 <SEP> um
<tb> p2 <SEP> 2. <SEP> 065 <SEP> m
<tb> p3 <SEP> 1. <SEP> 98 <SEP> m
<tb> p4 <SEP> 1. <SEP> 94 <SEP> m
<tb> P3 <SEP> 1. <SEP> 98 <SEP> m
<tb> p4 <SEP> 2. <SEP> 065 <SEP> m
<tb> P1 <SEP> 2. <SEP> 16 <SEP> m
<tb> 2. <SEP> 21 <SEP> m
<tb>

The side transducers and the central transducer have the same period of 2.21 µm and the separation is made by 3 electrodes for each transducer with a linear variation of 2.21 up to 1.94 µm. The sequences are identical on the left and on the right. There is no offset between arrays and lateral transducers, i.e. the distance between the centers of the successive electrodes of the array and of the lateral transducer are (2.21 + 2.245) / 2.

The period deviations given in the table above correspond to a total distance between the center of the last electrode

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15. 47-14. 35= 1. 12 um soit 1. 12/2. 21=50. 6% de la période des transducteurs. of the side transducer before the separation (ie the change of period) and the center of the first electrode of the central transducer after the separation equal to: 2.16 + 2. 065 + 1.98 + 1. 94 + 1.98 + 2. 065 + 2.16 = 14.35 µm. If this separation is not placed, the distance would be 7 * 2. 21 = 15. 47um. the progressive separation is therefore roughly equivalent, from the point of view of the phase shift to a bringing together of the transducers of

15. 47-14. 35 = 1. 12 um or 1. 12/2. 21 = 50. 6% of the period of the transducers.

With regard to the differential channel, the periods and sequences for the separations are identical. The difference comes from the central transducer which is now separated into two parts each containing 14 periodic electrodes of period 2.21 µm and 3 non-periodic electrodes of identical sequence as given below. For each of the two half-transducers, the electrode closest to the center is at + (or at-). The two side transducers are made in such a way that the electrodes closest to the central transducer are connected to the coupling points (V1 and-V1).

Figures 14 and 15 show the transfer function of the filter (Figure 15 corresponding to an enlargement of Figure 14). Insertion losses of less than 3 dB are obtained throughout the 35 MHz band and wideband rejection better than 55 dB. On the other hand, the rejection at 925 MHz is greater than 20 dB. Figure 16 shows the symmetry of the filter, ie the ratio between the output voltages V + and -V- (amplitude and phase). This symmetry is very good compared to the results usually obtained with filters according to the prior art. Indeed, the amplitude symmetry is between -1. 5 dB and + 0.8 dB in the band and is better than +/- 0. 5 dB in most of the bandwidth. Phase symmetry is between -50 and +8 and is better than +/- 2 across most of the band. These good results are due to the perfect symmetry of the differential path which means that voltages V + and V-see identical electrode configurations. The symmetry error comes on the one hand from the wiring of the filter and on the other hand from the polarities inversions of the side transducers of the input channel, which result in

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slightly different operations. For a filter with differential inputs and outputs and with careful wiring for example with a flip chip type assembly, the symmetry would be perfect.

On the other hand, it has been shown that the most interesting configurations for RF filter applications for radio telephones are those which use a piezoelectric tantalate substrate with a cutting angle between Y + 410 and Y + 43, for a thickness metallization reduced to the acoustic wavelength between 7% and 9% and such that the following relationships are verified.

- number of side transducer electrodes between
60% and 100% of the number of electrodes of the central transducer; - the separation of the central transducer and the side transducers is done gradually; - this separation is equivalent to a course difference of between -45% and -70% of the mean period of the transducers, i.e. the distance between the center of the last electrode of the lateral transducer before the separation and the center of the first electrode of the central transducer after separation is 45% to 70% of the mean period of the two transducers less than the distance that would be obtained if the two transducers were periodic and if no separation was imposed.

Claims (13)

1. Surface acoustic wave filters comprising at least one ith acoustic channel (VA,) of input cascaded to an i + 1 acoustic channel (VA, + 1) of output each acoustic channel comprising transducers with interdigitated electrodes and in particular: - at least one central transducer (TCI); - at least one pair of coupling transducers formed of a first lateral transducer (T1 LI) and of a second lateral transducer (T2L,) characterized in that it comprises a first electrical connection to a first potential (Vi) connecting the first lateral transducer (T1L) of the jth acoustic channel to the first lateral transducer (T1 LI + 1) of the i + th acoustic channel and a second electrical connection to a second potential (V2) connecting the second lateral transducer of the jth channel (T2L,) to the second lateral transducer of the (i + 1) th acoustic channel (T2L, + 1) and in that the potentials are in phase opposition. The central transducer of the jth acoustic channel being connected to at least one input voltage (VIN). The central transducer of the i + 1 acoustic channel being connected to at least one output voltage (Veut). 2. Surface wave filter according to claim 1, characterized in that the central transducer of the ith and / or of the i + th acoustic channel has an odd number of electrodes, the first and second lateral transducers having a series of 'electrodes connected to a so-called hot point potential (V1 / V2) interdigitated with a series of electrodes connected to a ground potential, the first electrode of the first lateral transducer, from the central transducer being connected to the hot point, the first electrode of the second side transducer from the central transducer being connected to ground, the distance between the central transducer and the first side transducer being equal to an even number of periods (2 kP with k integer and P is the period of the electrodes in the transducers) or to <Desc / Clms Page number 19> an integer of wavelength λ near corresponding to the central frequency of the filter, to that between the central transducer and the second lateral transducer. 3. Surface wave filter according to claim 1, characterized in that the central transducer of the i and / or of the i + i th acoustic channel has an odd number of electrodes, the first and second lateral transducers having a series of d. 'electrodes connected to a potential called an interdigitated hot spot with a series of electrodes connected to a ground potential, the first electrode of the first lateral transducer from the central transducer being connected to a potential of the same type (hot point or ground) as the first electrode of the second side transducer from the central transducer, the distance between the central transducer and the first side transducer being equal to an odd number of period ((2 k + 1) P with k integer, P period of the electrodes of the transducers) or to an integer of half wavelength, the distance between the central transducer and the second side transducer. 4. Surface wave filter according to claim 1, characterized in that the central transducer of the i th and / or the i + jth acoustic channel has an even number of electrodes, the first and second side transducers having a series of d. 'electrodes connected to a so-called hot point potential, interdigitated with a series of electrodes connected to a ground potential, the first electrode of the first lateral transducer from the central transducer being connected to a potential of the same type (hot point or ground) as the first electrode of the second side transducer from the central transducer, the distance between the central transducer and the first side transducer being equal to an even number of periods (2kP, with k integer, P period of the electrodes in the transducers) or to a whole number of wavelengths, except that between the central transducer and the second side transducer. 5. Surface wave filter according to one of claims 1 to 4, characterized in that it comprises at least one acoustic channel having a <Desc / Clms Page number 20> central transducer connected to a differential input and / or output (VIN +, VIN- and / or Wants +, Wants-).

6. Surface wave filter according to claim 5, characterized in that it comprises at least one acoustic channel connected to a differential input and / or a differential output (VIN +, VIN-and / or Vout +, Vaut -), said acoustic path comprising a central transducer split into two parts and comprising a continuous series of interdigitated electrodes with two continuous and adjacent series of electrodes connected to the input voltages (VIN +, VIN -) and / or

audits output voltages (Vaut +, Want-). 7. Surface wave filter according to one of claims 1 to 6, characterized in that the ith input acoustic channel (VA,) and / or the i + 1 st output acoustic channel (VA, AI + 1 ) comprises a series of central transducers (TC ,, j and / or TC ,. j) interposed with on the one hand a series of first lateral transducers (TlL ,,, and / or T1 L,) and on the other hand a series of second lateral transducers (T2L ,,, and / or T2L, + ,, J. 8. Surface wave filters according to one of claims 1 to 7, characterized in that it comprises a series of pair of acoustic channels comprising an input acoustic channel (VAi) and an output acoustic channel (VA, + i). 9. Surface wave filters according to one of claims 1 to 8, characterized in that the transducers are separated from one another in the same acoustic path by an electrode of width greater than the width of the other electrodes. 10. Surface wave filters, according to one of claims 1 to 8, characterized in that the transducers are separated from each other by several electrodes having between their axes distances less than the period in the transducer. <Desc / Clms Page number 21> 11. Surface wave filter according to one of claims 1 to 8, characterized in that it comprises a lithium tantalate substrate with a cutting angle of between Y + 410 or Y + 43, with Y crystallographic axis. 12. Surface wave filter according to claim 11, characterized in that the thickness of the electrodes is between approximately 5 and 10% of the acoustic wavelength of the filter. 13. Surface wave filter according to one of claims 11 or 12, characterized in that the number of electrodes of the lateral transducers is between approximately 50% and 100% of the number of electrodes of the central transducer, in each of the acoustic pathways.