WO1998027647A1

WO1998027647A1 - Surface wave device filters using resonant single phase unidirectional transducers

Info

Publication number: WO1998027647A1
Application number: PCT/CA1997/000386
Authority: WO
Inventors: Ji-Dong Dai; Petr G. Ivanov; Valery B. Shvets
Original assignee: Northern Telecom Limited
Priority date: 1996-12-18
Filing date: 1997-06-04
Publication date: 1998-06-25

Abstract

A surface wave device resonator filter comprises two SPUDTs - single phase unidirectional transducers - or RSPUDTs - resonant SPUDTs - (22, 24) on a piezoelectric substrate, coupled in line for propagation of surface waves via a resonant cavity, or via two cavities (C2, C3) one on each side of a central reflection grating (26), between the transducers. Reflectors (28, 30) at outer ends of the transducers reflect the propagated surface waves for resonance, and can be provided either by reflection gratings, optionally providing a further resonant cavity (C1, C4) between each grating and the adjacent transducer, or by weighting functions applied to the transducers. The weighting functions can comprise withdrawal weighting or apodization. The filter can provide a low insertion loss and other advantages of resonator filters.

Description

SURFACE WAVE DEVICE FILTERS USING RESONANT SINGLE PHASE UNIDIRECTIONAL TRANSDUCERS Technical Field and Industrial Application

This invention relates to surface wave device filters using SPUDTs (single phase unidirectional transducers) and RSPUDTs (resonant SPUDTs). The term "surface wave" is used herein to embrace surface acoustic waves (SAWs), including leaky SAWs, and surface skimming bulk waves. Background Art

For convenience, the following documents are referred to below by the following reference numbers:

1. Sawtek Incorporated, "Fundamentals of SAW Transversal Filters", "SAW Filters", and "Specifying A Custom SAW Filter", 1995 Product Catalog, pages 4-14.

2. C. Campbell, "Surface Acoustic Wave Devices and Their Signal Processing Applications", Academic Press, Inc., 1989, pages 125-126. 3. T. Morita et al., "Wideband Low Loss Double Mode SAW Filters", Proceedings of the IEEE 1992 Ultrasonics Symposium, pages 95-104.

4. P. Ventura et al., "A New Concept in SPUDT Design: the RSPUDT (Resonant

SPUDT)", Proceedings of the IEEE 1994 Ultrasonics Symposium, pages 1-6.

Reference 1 discusses various designs and characteristics of SAW filters, which are classified as either transversal filters or resonator filters. Transversal filters comprise bidirectional filters and low-loss filters; a preferred form of low-loss filter uses SPUDTs.

Resonator filters comprise waveguide coupled resonator filters and in-line coupled resonator filters. Reference 1 outlines the forms of these filters and compares their characteristics in Table 3 on page 14. The compared characteristics include fractional bandwidth (bandwidth as a percentage of center frequency), insertion loss (attenuation in the pass band), and close-in (near-in) rejection (attenuation in the stop band close to the filter pass band).

The necessity in surface wave device filters for trade-offs among these characteristics is known in the art, as discussed for example in Reference 2. Thus although it is desirable to provide a surface wave device filter with very low inseition loss and very high close-in rejection for any desired fractional bandwidth, in practice this is not possible and compromises must be made.

An important application of surface wave device filters is for filtering in wireless communications systems. By way of example, a GSM (Global System for Mobile communications) wireless communications system uses an IF (intermediate frequency) of

211 MHz and a channel bandwidth of 200 kHz, thereby requiring a nominal fractional bandwidth of about 0.1%, and hence a practical fractional bandwidth (allowing for tolerances) of the order of 0.14%. As shown by Table 3 of Reference 1, a waveguide coupled resonator filter using a quartz substrate can provide good close-in rejection and low insertion loss of the order of 3 dB for small fractional bandwidths, but for the relatively wide fractional bandwidth of 0.14% the insertion loss becomes undesirably high. Other known piezoelectric materials are not suitable for waveguide coupled resonator filters, at least in such an application. References 1 and 3 disclose use of in-line or longitudinally coupled resonator filters to provide surface wave device filtering with a low insertion loss of about 2 to 4 dB for a wide fractional bandwidth. However, as shown for example by Table 3 of Reference 1, such resonator filters do not have a good close-in rejection which is required for GSM IF filtering, and therefore have not been considered practical for such applications.

Thus for GSM IF filtering, where the requirements of a relatively wide fractional bandwidth and high close-in rejection are predetermined by the characteristics of the system, it has not been practical to use resonator filters. Consequently many potential advantages of resonator filters, such as low insertion loss, low pass band ripple, no TTI (triple transit interference), no need for acoustic absorbers, ease of frequency trimming, and relatively small size, have not been achieved.

In order to avoid the inherent 6 dB loss of bidirectional transversal filters, various forms of low-loss transversal filter have been developed using unidirectional transducers, in particular SPUDTs. For IF bandpass filtering, Reference 4 describes examples of

SPUDTs and an improvement which produces so-called RSPUDTs (resonant SPUDTs). Section IV of Reference 4 provides a comparison between classical SPUDT and RSPUDT designs of bandpass transversal filter, in each case using two identical transducers (SPUDTs or RSPUDTs) which are coupled in line with one another, the transducers having lengths of 352λ and a separation of 32λ where λ is the wavelength of the surface waves at the bandpass center frequency.

Such designs of bandpass filter in practice can provide a minimum insertion loss of about 7 dB. A lower insertion loss, consistent with the required fractional bandwidth, close-in rejection, and other filter characteristics, is still desirable. An object of this invention, therefore, is to provide an improved surface wave device filter that can facilitate providing a low insertion loss consistent with the other considerations discussed above. Disclosure of the Invention

According to one aspect of this invention there is provided a surface wave device resonator filter comprising two transducers on a piezoelectric substrate, the transducers being coupled in line with one another for propagation of surface waves therebetween, the resonator filter including two reflectors for reflecting the propagated surface waves to form at least one resonant cavity of the filter between the transducers, wherein each of the transducers comprises a single phase unidirectional transducer.

The resonator filter can include a reflection grating between the two transducers, said at least one resonant cavity between the transducers comprising two resonant cavities each between the reflection grating and a respective one of the two transducers, or the two transducers can be spaced, in the direction of propagation of the surface waves, by a gap determining a single resonant cavity, the gap being of the order of one or a few wavelengths, or less, of the propagated surface waves.

At least one of the reflectors can comprise a reflection grating in line with a respective one of the transducers adjacent an outer end thereof, and a gap between such a reflection grating and the respective one of the transducers can provide a further resonant cavity of the filter. Furthermore, at least one of the reflectors can be constituted by a weighting function applied to fingers of the single phase unidirectional transducer at an outer end thereof. The weighting function can comprise withdrawal weighting or apodization.

Preferably one or each of the two transducers comprises a resonant single phase unidirectional transducer providing at least one resonant cavity within the transducer.

Another aspect of this invention provides a resonator filter comprising two resonant single phase unidirectional transducers coupled in line with one another, via a reflection grating between the transducers, on a piezoelectric substrate for propagation of surface waves therebetween, the transducers having weighting functions to provide reflectors at their outer ends thereby to reflect propagated surface waves between the transducers, each transducer including at least one resonant cavity, and a gap between each transducer and the reflection grating providing a further resonant cavity, each gap being of the order of one or a few wavelengths, or less, of the propagated surface waves. A further aspect of the invention provides a resonator filter comprising two resonant single phase unidirectional transducers coupled in line with one another on a piezoelectric substrate for propagation of surface waves therebetween, each transducer including at least one resonant cavity, and a gap between the transducers providing a further resonant cavity, said gap being of the order of one or a few wavelengths, or less, of the propagated surface waves, the transducers having weighting functions to provide reflectors at their outer ends thereby to reflect surface waves between the transducers. Brief Description of the Drawings

The invention will be further understood from the following description with reference to the accompanying drawings, in which:

Fig. 1 schematically illustrates a known form of surface wave device transversal filter using SPUDTs or RSPUDTs;

Fig. 2 schematically illustrates a known form of in-line coupled resonator filter; Fig. 3 schematically illustrates a general form of a resonator filter in accordance with this invention;

Figs. 4 to 8 schematically illustrate various modified forms of resonator filter in accordance with different embodiments of this invention; Fig. 9 schematically illustrates a particular form of resonator filter in accordance with an embodiment of this invention; and

Figs. 10 to 12 illustrate details of the resonator filter of Fig. 9. Mode(s) of Carrying Out the Invention

Referring to the drawings, Fig. 1 illustrates a surface wave device transversal filter using SPUDTs or RSPUDTs as described in Reference 4. As is well known, the two transducers 10 and 12 are provided on a surface of a piezoelectric substrate, represented by the plane of the drawing, for propagation of a surface wave between them. Although the transducers 10 and 12 are referred to as being unidirectional, as is known they are more accurately bidirectional with a predominant surface wave propagation direction which is represented in Fig. 1 by an arrow for each transducer; this designation is used throughout the drawings for SPUDTs and RSPUDTs. Fig. 1 also illustrates unbalanced (i.e. signal and ground) connections to the transducers 10 and 12, and corresponding connections are illustrated throughout the drawings.

As described in Reference 4, each of the transducers 10 and 12 has a length, in the direction of surface wave propagation, of 352λ, where λ is the center frequency wavelength of the propagated surface waves, and there is a separation of 32λ between the transducers 10 and 12. This separation is necessary in order to reduce EMFT (electromagnetic feed-through) between the transducers to an acceptably low level. In order to limit surface wave diffraction over this separation distance, the aperture of each of the transducers 10 and 12 is desirably greater than about 40λ as illustrated in Fig. 1.

As discussed above, a transversal filter such as that of Fig. 1 in practice provides an insertion loss of about 7 dB. It is typically necessary in a GSM communications system to use two such filters in cascade to provide desired filter characteristics, resulting in a total insertion loss of at least 14 dB, which must be compensated by an amplifier connected between the two filters.

Fig. 2 illustrates a known in-line coupled resonator surface wave device filter, which comprises two IDTs (inter-digital transducers) 14 and 16 which are aligned in the direction of surface wave propagation and arranged between two reflection gratings (RGs) 18 and 20. As is well known, the IDTs are bidirectional; for example the IDT 14 generates surface waves both forwards towards the IDT 16 and backwards towards the RG 18. The distances between adjacent IDTs, and between the IDTs and the RGs, are small, typically of the order of λ or less, and are precisely determined to provide resonant cavities at transitions between the RGs and the IDTs. Such a resonator filter can have a low insertion loss of about 3 dB, but as discussed above has relatively poor close-in rejection which makes it unsuitable for use in applications such as an IF filter in a GSM communications system. As described in reference 1, the in-line coupled resonator filter can generally include a central leaky reflection grating (not shown) between the IDTs 14 and 16.

Fig. 3 illustrates a general form of a resonator filter in accordance with this invention, comprising two SPUDTs or RSPUDTs 22 and 24 which are aligned in the direction of surface wave propagation and are coupled via a central reflection grating (RG) 26 which provides partial reflection and partial transmission of surface waves, between two lateral RGs 28 and 30.

Assuming initially that the transducers 22 and 24 are SPUDTs, the resonator filter of Fig. 3 provides for four resonant cavities Cl to C4 at the transitions between the transducers and the RGs. Because it is a resonator filter, the filter of Fig. 3 can provide a low insertion loss as in the resonator filter of Fig. 2. The use of the SPUDTs 22 and 24 rather than IDTs enables the resonator filter of Fig. 3 to be designed (with input and/or output matching circuits provided in known manner) to have a good close-in rejection and a substantially flat pass band. In addition, the resonator filter of Fig. 3 can have a smaller aperture, for example about 26λ, than that of the transversal filter of Fig. 1 because diffraction is not a concern, so that the filter requires a relatively smaller substrate. The resonator filter of Fig. 3 also provides other desirable features of a resonator filter, such as no I^'ll and hence a potentially ripple-free pass band, no need for acoustic absorbers, and ease of frequency trimming.

The use of RSPUDTs for the transducers 22 and 24 enables a further improvement of the resonator filter to be provided. As is known from Reference 4, each RSPUDT has at least one internal resonant cavity, so that the design can be more flexible to enable the characteristics of the overall resonator filter to be more precisely defined. In this respect it is observed that the different resonant cavities can be designed to have slightly different resonant frequencies within the pass band of the resonator filter, to produce an overall desired filter response including a flat pass band and good close-in rejection. It is also observed that, contrary to the description in Reference 4, the RSPUDTs (or SPUDTs) 22 and 24 are not identical to one another but are deliberately designed to be slightly different in order to reduce spurious modes which may otherwise occur and which would impair the filter performance.

As the use of RSPUDTs rather than SPUDTs for the transducers 22 and 24 provides a desirable improvement in filter performance, the following description refers primarily to RSPUDTs, but it can be appreciated that generally SPUDTs may be used instead of the RSPUDTs referred to. The central RG 26 can be omitted from the resonator filter of Fig. 3 to produce an alternative form of the resonator filter which, as illustrated in Fig. 4, comprises the two RSPUDTs 22 and 24 between the two lateral RGs 28 and 30. This resonator filter provides three resonant cavities Cl, C3, and C5 at the transitions between the RGs and the RSPUDTs, and at least one resonant cavity C2, C4 in each RSPUDT 22, 24.

An improvement in performance of the resonator filters of Figs. 3 and 4 can be provided by replacing one or both of the lateral RGs 28 and 30 by weighting the RSPUDTs 22 and 24 to act as reflectors at their outer ends; the weighting can comprise apodization or withdrawal weighting as described further below. For example, Fig. 5 illustrates the resonator filter of Fig. 3, and Fig. 6 illustrates the resonator filter of Fig. 4, with the RG 28 replaced in this manner. Fig. 7 illusttates the resonator filter of Fig. 3, and Fig. 8 illustrates the resonator filter of Fig. 4, with both of the lateral RGs 28 and 30 replaced in this manner. In each case each RSPUDT is illustrated as having one resonant cavity, and the resonant cavities are identified by the references Cl, C2, etc. Thus the resonator filters of Figs. 5 and 6 are similar to the resonator filters of

Figs. 3 and 4 respectively, except that the RSPUDT 22 and RG 28 are replaced by an RSPUDT 32 which is weighted, at its left-hand or outer end as illustrated by vertical lines, to act as a reflector. With the RSPUDTs 32 and 24 each having one resonant cavity, the resonator filter of Fig. 5 thus provides five resonant cavities Cl to C5, and the resonator filter of Fig. 6 provides four resonant cavities Cl to C4.

The resonator filters of Figs. 7 and 8 are similar to the resonator filters of Figs. 5 and 6 respectively, except that the RSPUDT 24 and RG 30 are replaced by an RSPUDT 34 which is also weighted, at its right-hand or outer end as illustrated by vertical lines, to act as a reflector. With the RSPUDTs 32 and 34 each having one resonant cavity, the resonator filter of Fig. 7 thus provides four resonant cavities Cl to C4, and the resonator filter of Fig. 8 provides three resonant cavities Cl to C3.

It is important to appreciate that, although the arrangements of the filters of Figs. 1 and 8 appear to be similar, the filters are in fact completely different. The filter of Fig. 1 is a transversal filter, whereas the filter of Fig. 8 is a resonator filter in which the RSPUDTs 32 and 34 are designed not only to provide the internal resonant cavities Cl and C3 but also, through the weighting as discussed above and described further below, to provide reflectors for the resonant cavity C2 so that the overall filter acts as a resonator. For operation as a resonator, the gap between the RSPUDTs 32 and 34 in the resonator filter of Fig. 8 is small, typically being of the order of one or a few wavelengths (in any event less than about lOλ) and preferably being less than one wavelength λ, and is precisely determined for resonance, in comparison to the relatively arbitrary but much larger distance between the transducers in the transversal filter of Fig. 1. The same applies to other gaps between adjacent resonators and RGs in the resonator filters of Figs. 3 to 7. As discussed above, the RSPUDTs 32 and 34 can be weighted using apodization and/or withdrawal weighting. For example, both RSPUDTs 32 and 34 of a resonator filter as illustrated in Fig. 7 or 8 can be withdrawal weighted, or one can be withdrawal weighted and the other weighted by apodization. If each RSPUDT provides only one resonant cavity, each RSPUDT can be referred to as a WWSCR (withdrawal weighted single cavity RSPUDT) or as an ASCR (apodized single cavity RSPUDT). A resonator filter having four cavities as illustrated in Fig. 7 to constitute a four-pole filter, using two WWSCRs or one WWSCR and one ASCR, can provide an insertion loss of less than about 4 dB with excellent or good filter roll-off (and hence close-in rejection) and an ultimate rejection of about 70 dB. A resonator filter having three cavities as illustrated in Fig. 8 to constitute a three-pole filter, using two WWSCRs or one WWSCR and one ASCR, can provide a lower insertion loss of about 3 dB or less with good filter roll-off and an ultimate rejection of about 60 dB.

It is observed that resonator filter arrangements as illustrated in Figs. 3 to 8 need not provide as many resonant cavities as are identified and described above. For example, a resonator filter can be provided in the form illustrated in Fig. 4, for example using WWSCRs to constitute the RSPUDTs, with no discontinuity or change in polarity between each WWSCR and the respective lateral RG 28 or 30. In this case resonant cavities are not formed at these transitions (Cl and C5 in Fig. 4), so that the resonator filter has only three resonant cavities, i.e. the filter functions as shown in Fig. 8 but also has lateral reflection gratings. Similar comments apply in respect of the other transitions between RGs and transducers, and more than one cavity can be provided in each RSPUDT, so that in practice each resonator filter can be designed to have any desired number of cavities (and hence filter poles). Various forms of SPUDT are known, and each RSPUDT or SPUDT in the resonator filters described above can have any desired form. In particular, each elementary cell, occupying one wavelength of a SPUDT or RSPUDT in the direction of surface wave propagation, can comprise a conventional electrode width control (EWC) structure or an improved reflectivity EWC (IR-EWC) structure (also referred to as a DART or Distributed Acoustic Reflection Transducer) both of which are for example described in Reference 4. The EWC structure comprises, for each elementary cell, two electrodes each λ/8 wide with a gap of λ/8 between them, connected to ground and a signal line respectively, and an electrode λ/4 wide connected to ground, thereby defining transduction and reflection centers spaced by 3λ/8. In the IR-EWC structure the width of the wide electrode is increased to 3λ/8, and the gaps between adjacent electrodes are all λ/8. The latter is preferred for its improved reflectivity and equal gaps.

One particular form of resonator filter in accordance with an embodiment of this invention is further described by way of example below with reference to Figs. 9 to 12. Fig. 9 schematically illustrates the overall arrangement of the resonator filter, and Figs. 10 to 12 schematically illustrate details of this filter at regions marked by arrows A, B, and C respectively in Fig. 9.

Referring to Fig. 9, the resonator filter has the form described above with reference to Fig. 7, providing four resonant cavities using two WWSCRs 36, 38 and a central RG 40. As illustrated (not to scale) in Fig. 9, each WWSCR has a length, in the direction of surface wave propagation, of about 335λ and an aperture of about 26λ, and the RG 40 has a length of about lOOλ and a corresponding aperture. In the same manner as illustrated in Fig. 7, signal and ground connections are made to the WWSCR 36 via conductors 42 and 44 respectively, and to the WWSCR 38 via conductors 46 and 48 respectively. A ground connection is made to conductors 50 of the RG 40, or the RG 40 can instead be electrically floating. The two WWSCRs 36 and 38 have slightly different withdrawal weighting functions in order to reduce spurs in the filter response, and use the IR-EWC structure outlined above. The region A in Fig. 9 provides a resonant cavity (C3 in Fig. 7) between the

WWSCR 38 and the RG 40 and is shown in detail in Fig. 10; a similar resonant cavity (C2 in Fig. 7) is formed between the WWSCR 36 and the RG 40. The region B, which is offset from the center along the length of the WWSCR 36, provides a resonant cavity (Cl in Fig. 7) due to a sign change in the IR-EWC structure as is shown in detail in Fig. 11. A similar resonant cavity (C4 in Fig. 7) is provided along the length of the WWSCR 38. The region C, at the outer end of the WWSCR 36, provides reflection of surface waves by withdrawal weighting and is shown in detail in Fig. 12. A similar reflection by withdrawal weighting is provided at the outer end of the WWSCR 38. Consequently, lateral RGs are not provided in this resonator filter. In the IR-EWC structure used in the resonator filter of Figs. 9 to 12, an elementary cell occupying one wavelength λ in the direction of surface wave propagation comprises a 3λ 8 wide finger connected to the grounded conductor and forming a reflection center RC, a λ/8 wide finger connected to the signal conductor and forming a transduction center TC, and a further λ/8 wide finger connected to the grounded conductor, with gaps of λ/8 between adjacent fingers. Withdrawal weighting is achieved by replacing the TC finger by a finger connected to the grounded conductor. This and the adjacent λ/8 wide finger can be replaced by a single 3λ/8 wide finger connected to the grounded conductor to create an additional RC, and the same applies to each elementary cell. Consequently, transduction and reflection can be largely independently determined over the length of each WWSCR to provide a desired filter response.

Referring to Fig. 10, the RG 40 comprises only reflecting fingers each 3λ/8 wide with λ/8 gaps between them, the fingers being connected to ground via the conductors 50. The resonant cavity in the region A is formed by a gap of 3λ/8 between an end reflecting finger 52 of the RG 40 and an end RC finger 54 of the WWSCR 38. A TC finger 56 is connected to the signal conductor 46, and an adjacent further finger 58 is connected to the grounded conductor 48. Fig. 10 also shows in the next elementary cell a finger 60 that is withdrawn, i.e. converted from being a TC finger to a neutral finger, in accordance with a desired withdrawal weighting function, by connecting it to the grounded conductor 48 instead of the signal conductor 46.

Referring to Fig. 11, in the region B of the WWSCR 36 an additional λ/8 wide finger 62 is provided and connected to the grounded conductor 44 with λ/8 gaps between the further fingers 64 of cells of the WWSCR which have opposite predominant directions of surface wave transmission, in each case towards the additional finger 62. Thus to the left of the additional finger 62, in each elementary cell the RC finger 66 connected to the grounded conductor 44 is to the left of the TC finger 68 connected to the signal conductor 42, whereas to the right of the additional finger 62, in each elementary cell the RC finger 66 is to the right of the TC finger 68. In consequence, a resonant cavity is formed in the region B. The TC and RC functions of the overall WWSCR 36 are arranged so that there is a predominant direction of surface wave propagation to the right as shown by the arrow in Fig. 9.

Referring to Fig. 12, in the region C at the outer end of the WWSCR 36 nearly all of the TC fingers 68 connected to the signal rail 42 and the adjacent further fingers 64 connected to the grounded conductor 44 are replaced by 3λ/8 wide fingers 70 connected to the grounded conductor 44, thereby forming additional RCs between the original RC fingers 66, in accordance with a desired RC weighting function that makes the WWSCR 36 a good surface wave reflector in the region C.

Although the resonator filters described in detail above all use withdrawal weighting, it can be appreciated that weighting can alternatively be achieved by apodization of the transducers in accordance with a desired weighting function. Apodization of transducers is known in the art and need not be further described here; for example, IDTs using alternatively withdrawal weighting or apodization are known from Kodama et al. United States Patent No. 4,866,325 issued September 12, 1989 and entitled "Surface Acoustic Wave Transducer". However, it is observed that withdrawal weighting may be preferred because it avoids a so-called apodization loss which is inherent in apodized transducers.

Thus although particular embodiments of the invention have been described in detail, it should be appreciated that this and numerous other modifications, variations, and adaptations may be made without departing from the scope of the invention as defined in the claims.

Claims

WHAT IS CLAIMED IS:

1. A surface wave device resonator filter comprising two transducers on a piezoelectric substrate, the transducers being coupled in line with one another for propagation of surface waves therebetween, the resonator filter including two reflectors for reflecting the propagated surface waves to form at least one resonant cavity of the filter between the transducers, characterised in that each of the transducers comprises a single phase unidirectional transducer.

2. A resonator filter as claimed in claim 1 and including a reflection grating between the two transducers, said at least one resonant cavity between the transducers comprising two resonant cavities each between the reflection grating and a respective one of the two transducers.

3. A resonator filter as claimed in claim 1 wherein the two transducers are spaced, in the direction of propagation of the surface waves, by a gap determining said resonant cavity, the gap being of the order of one or a few wavelengths, or less, of the propagated surface waves.

4. A resonator filter as claimed in claim 1, 2, or 3 wherein at least one of said reflectors comprises a reflection grating in line with a respective one of the transducers adjacent an outer end thereof.

5. A resonator filter as claimed in claim 4 wherein a gap between the reflection grating constituting a reflector and the respective one of the transducers provides a further resonant cavity of the filter.

6. A resonator filter as claimed in any of claims 1 to 5 wherein at least one of said reflectors is constituted by a weighting function applied to fingers of the single phase unidirectional transducer at an outer end thereof.

7. A resonator filter as claimed in claim 6 wherein the weighting function comprises withdrawal weighting.

8. A resonator filter as claimed in claim 6 wherein the weighting function comprises apodization.

9. A resonator filter as claimed in any of claims 1 to 8 wherein at least one of the two transducers comprises a resonant single phase unidirectional transducer providing at least one resonant cavity within the transducer.

10. A resonator filter as claimed in claim 9 wherein each of the transducers comprises a resonant single phase unidirectional transducer.

11. A resonator filter comprising two resonant single phase unidirectional transducers coupled in line with one another, via a reflection grating between the transducers, on a piezoelectric substrate for propagation of surface waves therebetween, the transducers having weighting functions to provide reflectors at their outer ends thereby to reflect propagated surface waves between the transducers, each transducer including at least one resonant cavity, and a gap between each transducer and the reflection grating providing a further resonant cavity, each gap being of the order of one or a few wavelengths, or less, of the propagated surface waves.

12. A resonator filter comprising two resonant single phase unidirectional transducers coupled in line with one another on a piezoelectric substrate for propagation of surface waves therebetween, each transducer including at least one resonant cavity, and a gap between the transducers providing a further resonant cavity, said gap being of the order of one or a few wavelengths, or less, of the propagated surface waves, the transducers having weighting functions to provide reflectors at their outer ends thereby to reflect surface waves between the transducers.