WO2000033461A1

WO2000033461A1 - Low ripple hybrid surface acoustic wave filter apparatus for improved signal-to-noise ratio

Info

Publication number: WO2000033461A1
Application number: PCT/US1999/028808
Authority: WO
Inventors: Puttagunta Sudhakar; Kushal K. Bhattacharjee
Original assignee: Cts Corporation
Priority date: 1998-12-03
Filing date: 1999-12-03
Publication date: 2000-06-08
Also published as: AU3111000A

Abstract

A surface acoustic wave filter (10) with a center transducer (16) having a first aperture encompassing two acoustic tracks (22, 24) on a piezoelectric substrate (12). Input transducers (26) having a second aperture about half that of the first aperture are disposed on a first acoustic track (22). When electrically energized the input transducers (26) cause in-phase acoustic waves (30) to propagate towards the center transducer (16) to be incident on a portion of the center transducer (16). Subsequently, the acoustic waves (30) change tracks (22, 24) and are scattered outwardly from the center transducer (16) along a second track (24) to be absorbed preventing acoustic echoes. This configuration of the filter (10) produces excellent ripple and insertion loss and contributes to an improved signal-to-noise ratio in a radio.

Description

LOW RIPPLE HYBRID SURFACE ACOUSTIC WAVE FILTER APPARATUS FOR IMPROVED SIGNAL-TO-NOISE RATIO

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Indian Patent Application No. 3655/DEL/98 filed December 3, 1998.

FIELD OF THE INVENTION

The present invention relates in general to surface acoustic wave devices, and in particular to surface acoustic wave filters.

BACKGROUND OF THE INVENTION

There is an ongoing need for filters in a variety of communications applications wherein small size, light weight and high performance are simultaneously required. Increasing numbers of products seek to employ fixed spectral resources, often to achieve tasks not previously envisioned. Examples include cellular telephones, computer linkages as well as a host of other, increasingly complex personal equipment information sharing requirements. The desire to render increasingly complicated communications devices places extreme demands on filtering technology in the context of increasingly crowded radio frequency resources.

Acoustic wave technology provides devices meeting stringent performance requirements which are (i) extremely robust, (ii) readily mass produced, (iii) adjustment-free over the life of the unit and which (iv) sharply increase the performance-to-size ratio achievable in the frequency range extending from a few tens of megahertz to about several gigahertz. However, the need for good signal-to-noise ratio (SNR) performance in a radio, low passband insertion loss simultaneously coupled with demand for good shape factor, low ripple, wide bandwidth, and high stopband attenuation pose filter design and performance requirements which have not been easily met in existing acoustic wave filters.

A typical acoustic wave filter configuration, used in a cellular phone for example, attempts to minimize passband ripple by reducing multiple transit echoes. However, this results in an increase in insertion loss and/or decrease in bandwidth in existing bidirectional interdigital transducer based configurations. Single phase unidirectional transducers attempt to solve this problem partially, but are limited to narrow bandwidths, less than a few percent fractional bandwidths, and constrained by small electrode sizes. In practice, low ripple filters using single phase unidirectional transducers still exhibit an insertion loss of several db. Insertion loss problems result in degraded SNR for radio applications.

What is needed is an acoustic filter configuration providing improved in-band insertion loss with low ripple while also providing a large fractional bandwidth. It is also desired to provide improved out-of-band rejection and good shape factor in a device realized in compact, robust and desirably in monolithic form. It would also be an advantage to realize improved insertion loss, ripple, bandwidth and shape factor in a smaller package.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims. However, a more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the figures, wherein like reference characters refer to similar items throughout the figures, and:

FIG. 1 is a simplified plan view showing a metallization pattern of an acoustic wave filter, in accordance with the present invention;

FIG. 2 is a schematic diagram of a preferred embodiment of the acoustic wave filter of FIG 1 , in accordance with the present invention; FIG. 3 is a schematic diagram of a first alternate embodiment of an acoustic wave filter, in accordance with the present invention;

FIG. 4 is a schematic diagram of a second alternate embodiment of an acoustic wave filter,, in accordance with the present invention; FIG. 5 is a schematic diagram of a third alternate embodiment of an acoustic wave filter with modified input transducers, in accordance with the present invention;

FIG. 6 is a schematic diagram of the third alternate embodiment of an acoustic wave filter with modified input transducers of FIG. 5, in accordance with the present invention; and

FIG. 7 is a block diagram of a portion of a communication device including an acoustic wave filter, in accordance with the present invention.

The exemplification set out herein illustrates a preferred embodiment of the invention in one form thereof, and such exemplification is not intended to be construed as limiting in any manner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a track changing, in-line surface acoustic wave filter with half-aperture side transducers on a first track and acoustic absorbers on a second track. The present invention provides a filter configuration with improved in-band insertion loss with low ripple while providing a large bandwidth and good shape factor. The addition of a high input impedance buffer amplifier results in an improved signal-to-noise ratio (SNR) in radio application. In addition, where signal gain is desired, a second cascaded amplifier can be provided. The present invention also provides improved out-of-band rejection in a device realized in compact, robust and desirably in monolithic form. Moreover, the improved insertion loss, ripple, bandwidth and shape factor can be provided in a reduced chip size. Generally, the present invention provides an apparatus for ripple and insertion loss in acoustic wave (e.g., SAW, surface skimming bulk wave, leaky wave etc.) filters. The present invention may be more fully understood with reference to FIG. 1 , which provides a simplified plan view of an acoustic wave filter 10 in accordance with the present invention. The acoustic wave filter 10 comprises a center interdigital acoustic transducer 16 and at least one side input transducer 26 collectively disposed in a multiple track, in-line configuration along a preferred axis of a piezoelectric substrate 12 and on a suitably-prepared, preferably polished, upper sur ace 14 thereof.

SAW filters are typically fabricated on a polished piezoelectric substrate by depositing and patterning a thin metal film, often comprising aluminum, in a thickness ranging from about 500A to about 6000A thick, by techniques similar to those employed in integrated circuit manufacturing. The substrate 12 can be of any suitable piezoelectric material such as ST-cut quartz, 41 ° rotated Y-cut X-propagating LiNb0₃, 64° rotated Y-cut X-propagating LiNb0₃, 36° rotated Y-cut Xpropagating LiTa0₃, and 45° rotated X-cut Z-propagating Li₂B₄0₇ (lithium tetraborate).

Typically, SAW transducers comprise a plurality of interdigitated electrodes 18 (also referred to herein as "fingers", "finger electrodes", etc.) disposed on one-half acoustic wavelength centers. As used herein, the term "acoustic wavelength" for each interdigital transducer is taken to mean an acoustic wavelength at the resonator synchronous frequency. The electrodes typically have a width of about one-fourth of a wavelength, i.e., measured along directions in accordance with design principles and performance objectives for the filter 10. However, it will be appreciated that other arrangements are possible and in some applications are preferred. It should also be recognized that other widths providing suitable acoustic wave generation characteristics may be usefully employed and this width may be chosen to accommodate the characteristics of the materials employed for the filter, including the substrate.

In a preferred embodiment, electrode widths of one-quarter wavelength were used for all transducers 16,26. Where bidirectional transducers 26 are utilized the present invention benefits by the use of reflective gratings (not shown) configured as is known in the art. However, it is preferred that configuration of the side input transducers 26 be unidirectional to minimize signal loss. The electrodes 18,28 are periodic and define a particular acoustic wavelength at which the transducers exhibit a characteristic acoustic center frequency together with a center frequency wavelength for the acoustic energy transduced in response to electrical stimulation of an appropriate frequency applied via the electrical terminal V

The present invention includes a center interdigital acoustic transducer 16 comprises a plurality of interdigitated electrodes 18 being substantially perpendicular to a direction of propagation 20 of the piezoelectric substrate 12. The center interdigital acoustic transducer 16 has a first aperture encompassing two acoustic tracks 22,24 on the piezoelectric substrate 12. At least one input interdigital acoustic transducer 26 is disposed on a first acoustic track 22 on the upper surface 14 of the piezoelectric substrate 12 on either side of the center interdigital transducer 16. Each transducer 26 comprises a plurality of interdigitated electrodes 28 having a second aperture about one-half, or slightly more than one-half, that of the first aperture of the center interdigital acoustic transducer 16. Preferably, two transducers 26 are disposed on either side of the center interdigital acoustic transducer 16. When electrically energized the input interdigital acoustic transducers

26 cause acoustic waves 30 to propagate along a common direction of propagation 20 substantially perpendicular to the interdigitated electrodes 28 of both transducers 26 and be substantially in phase and incident on a portion of the center interdigital acoustic transducer 16. Since the acoustic waves only impinge on a portion of the center transducer the center transducer causes the acoustic waves to change tracks, known in the art as a "track changer." Subsequently, the acoustic waves 30 are scattered outwardly from the center interdigital acoustic transducer 16 along a second track 24 on the upper surface 14 of the piezoelectric substrate 12 in the common direction of propagation 20. The transducers are separated from each other by gaps or delays which are typically integer multiples of one wavelength of the synchronous frequency of the associated transducer. This is known in the art as synchronous coupling. The delays between transducers are important to the present invention. The delay or spacing values for SAW filters of the in-line resonant type, employing one-quarter wavelength wide electrodes, is typically measured as a percentage of wavelength or as a portion of 2 radians. In the present invention, the gap between transducers is set to n 12 radians (synchronous coupling) where n=1 , 2, 3, etc. such that a predetermined acoustic wave phase relationship is obtained.

A desired aspect of the present invention is to provide acoustic waves which impinge in-phase on the center transducer. This can be accomplished by several basic configurations. Firstly, both transducers can be separated by an equal gap or delay from the center transducer and be electrically driven in parallel. Secondly, the transducers can have gaps differing by one-half wavelength and be electrically driven out-of-phase with each other. Thirdly, the transducers can have integer multiples of wavelengths delay differences for either of the above two conditions. Any of these conditions will work equally well in the present invention. In a preferred embodiment, the two input transducers 26 are identical unidirectional transducers that are spaced equidistant from the center transducer 16, and electrically driven in parallel. The two transducers 26, when energized, propagate acoustic waves towards the center interdigital acoustic transducer 16. The aperture of the input transducers 26 is about half that of the center transducer 16, and is preferably slightly more than one-half. The input transducers 26 act as an linput port, V for the filter 10 while the center transducer 16 provides an output port, V₀ .

It is preferred that the center transducer 16 is configured to be self resonant. The self resonant condition eliminates the need for tuning elements as are used in the prior art, which in turn improves insertion loss of the filter. All the transducers 16,26 are operable at the same center frequency. A novel aspect of the present invention is having acoustic absorption material 32 disposed on the upper surface 14 of the piezoelectric substrate 12 along the second acoustic track 24 and on either side of the center interdigital acoustic transducer 16. The acoustic absorption material 32 substantially absorbs any acoustic waves 30 propagated on the second acoustic track 24 so as to reduce multiple transit acoustic echoes reflected back to the center interdigital acoustic transducer 16. This configuration results in a substantially ripple free and lossless output for a high impedance load such as a buffer amplifier 34. In particular, it is preferred that the buffer amplifier 34 is a high input impedance amplifier is coupled to the output port, V₀ , from the center interdigital acoustic transducer 16 so as to provide an improved combined SNR in a radio communication device. The high input impedance condition maintains a low insertion loss characteristic. A second amplifier can be cascaded with the amplifier 34 in order to provide gain in the radio.

In addition, where the input transducers 26 are unidirectional, the input transducers 26 can be tuned to minimize reflections, thereby minimizing multiple transit echoes to a second order. The multiple transit echoes are substantially eliminated to a first order as explained previously, i.e. by removing the entire acoustic power from the first track and transferring into the second track and to a lesser extent into the static ohmic conductance, G₀ , of the center transducer as will be explained below.

Electrical stimulation of an appropriate frequency, which is supplied at electrical terminal V1 , at an input port of the input transducers 26 for example, and thence to the interdigitated or interleaved electrodes 28, results in acoustic waves 30 being generated from the transducers 26. Acoustic waves 30 of an appropriate frequency impinging upon the center transducer 16 along the first track 22 result in a substantially lossless electrical signal being manifested at the output terminal, V₀ . In addition, acoustic waves 30 are propagated outwardly from the center transducer 16 along a second track 24 to be absorbed within acoustic absorbers 32. This eliminates acoustic reflections which would degrade ripple performance of the filter 10.

Normally, it is not desired to dampen an acoustic signal in a filter. However, the present invention takes advantage of the nearly lossless performance of a track changer to produce a filter with excellent insertion loss performance. Surprisingly, by utilizing a high input impedance buffer amplifier at the filter output, the frequency response of the filter 10 demonstrates not only low insertion loss, but improved shape factor, bandwidth and improved selectivity over the prior art filters for a given chip size. These factors, utilized in a radio environment, provide for improved SNR in the radio, even where the high input impedance amplifier has a higher noise figure than standard low impedance amplifiers.

For example, a SNR in a front end of a radio is dependent on the noise figure of radio components in the signal path. The major contributors to noise degradation in a radio is the insertion loss of the filter and the noise figure of the amplifier being used. Conventional techniques for reducing noise have included the use of an amplifier with as low a noise figure as possible and a filter with low insertion loss. Typically, low input impedance amplifiers have better noise figures than high input impedance amplifiers, e.g. on the order of about 2 db versus about 5 db for a high input impedance amplifier such as Motorola's MRF IC 0916 amplifier. Therefore, conventional techniques have used low input impedance amplifiers due to their low noise figure.

However, performance constraints on the filter, i.e. wide passband and low ripple, have resulted in filters with high insertion loss. Typical insertion loss for filters with fractional passband widths of greater than 5% and ripple less than 0.5 db is about 8 db. Therefore, an expected SNR for a radio application using a low-noise, low-impedance amplifier is about 10 db total (8 db + 2 db). In contrast, the present invention uses a track changing filter specifically configured to drive a high impedance. In this configuration, although the high input impedance amplifier contributes about 5 db to the noise, the track changing filter contributes only about 2 db insertion loss due to efficient voltage transfer, for a total of about 7 db degradation in SNR (2 db + 5 db), This is an improvement over prior art SAW filters in radio applications where 10 db insertion loss is typically experienced. Moreover, the size of the transducers (number of electrode fingers and length of fingers) in a track changer acoustic device is much smaller than those in conventional SAW filters, which results in a smaller die size.

One of the novel aspects of the present invention is the use of acoustic absorbers in the second track of the filter. This is counterintuitive from prior art filters where it is not desired to dampen the signal in a filter. However, this aspect of the present invention provides the very performance improvement desired. In particular, prior art track changer devices are known to provide wide bandwidth with low insertion loss. However, ripple performance is degraded due to multiple transit echoes on the tracks of the filter. Ripple can be improved in the prior art but only at the expense of degraded insertion loss and/or bandwidth. The present invention, provides improved ripple performance in a track changer filter without sacrificing insertion loss or bandwidth performance by using acoustic absorbers in the second track. This configuration substantially eliminates multiple transit echoes thereby improving ripple performance. Also, the use of a high impedance load maintains near lossless voltage transfer. Advantageously, the insertion loss improvement provided by the present invention more than outweighs the noise figure degradation presented by a high impedance load such as a high input impedance amplifier. Therefore, although the use of acoustic absorbers actually cause a high impedance condition in the filter, this filter configuration ultimately results in improved SNR performance when matched to a high input impedance amplifier.

The transfer function of a self-resonant track changer device with an track aperture ratio (side transducer aperture versus center transducer aperture) of about 0.5 is given by:

H(ω) = -{[1 +G₀ /G_a(ω)] + j[B_t(ω)/G_a(ω]}-¹ where G₀ is the static conductance of the center transducer at the center frequency, G_a(ω) is the radiation conductance, and B_t(ω) is the radiation susceptance of the center transducer. In practice, G₀ is much less than G_a(ω) and the term containing G₀ can be neglected. In addition, B_t(ω) can be represented as:

B_t(ω) = B_a(ω) + B_t(ω)) + B_e(ω)

where B_a(ω) is the acoustic susceptance of the center transducer and B_e(ω) is the electrical susceptance of the center transducer's capacitance and tuning inductance (if any). In a self-resonant center transducer, which is preferred, there are no tuning elements to contribute to the electrical susceptance. The voltage transfer function defined by

Hv = 10 ^* log lv₀/V,l

is given by

where

G_aι is the radiation conductance of the input interdigital transducer, G_a01 is the radiation conductance of the part of the output center interdigital transducer in the region of track 1 , G_ao is the radiation conductance of the track changer center interdigital transducer, and G₀ and B_t are defined as previously.

The elimination of multiple transit echoes requires that no acoustic power is left in track 1. The necessary condition for this is ^Gao1 ^{= G}ao2 ⁺ Go

where

G_ao2 is the radiation conductance of the part of the output center interdigital transducer in the region of track 2 (G_ao G_ao1 + G_ao2). The apertures of tracks 1 and 2 are to be chosen such that the above condition is valid at the synchronous frequency and substantially valid over the entire passband, hence ensuring no multiple transit echoes. When the input interdigital transducer and the center interdigital transducer are identical, except in aperture, the loss in the voltage transfer is nearly 0 db at the center frequency and is nearly flat over a broad frequency region where:

B_t(ω) « G_a(ω)

Therefore, it is preferred that the center interdigital acoustic transducer has a radiation conductance having a magnitude that is much greater than a magnitude of the sum of an electrical and acoustical radiation susceptance of the center interdigital acoustic transducer.

Surprisingly, the above transfer function remains flat to within 0.5 db even though G_a(ω) itself can vary by up to 4 db. In addition, filter sidelobe suppression of better than -30 db can be achieved by choosing appropriate weighting for the transducers, although this is not required since the transition of the filter is reasonably sharp with 35 db to 3 db shape factors of 1.6 to 1 .7 being achievable. If the radiation conductance of G_ai(ω) is more than that of G_ao1(ω) it would result in a voltage gain and a flat passband would still result if the passband shapes of G_ai(ω) and G_a01(ω) closely match each other.

FIG. 3 shows a first alternate embodiment of the present invention which has all of the attributes of FIG. 1 which are hereby incorporated by reference with the exception of the using only one input transducer 26 that is unidirectional and having the center transducer 16 also being unidirectional. This first alternate embodiment has the advantage of substantially reducing the filter length while still substantially eliminating the multiple echoes to a second order. FIG. 4 shows a second alternate embodiment of the present invention which has all of the attributes of FIG. 3 which are hereby incorporated by reference with the addition of being resonantly loaded with an external tuning inductor 36 at the output, V₀. This second alternate embodiment has the advantage of substantially smaller size for the filter and being practical even on low coupling substrates.

FIG. 5 shows a third alternate embodiment of the present invention wherein an input transducer 26 of any of the variants of the present invention includes two structures, a bidirectional interdigital transducer 38 and a self resonant interdigital transducer 40. These two structures together comprise a unidirectional interdigital transducer which can be used in lieu of convention unidirectional interdigital transducers known in the art (e.g. DART, EWC, SPUDT, etc.) as a replacement for any of the input transducers 26 in any of the embodiments of the present invention. In particular, the self resonant interdigital transducer 40 is used a reflector on one side of a bidirectional interdigital transducer 38 making the combination unidirectional. In practice, the interdigital electrodes of the self resonant interdigital transducer 40 and the bidirectional interdigital transducer 38 are separated by one-quarter wavelength, and the bidirectional interdigital transducers 38 are matched externally. FIG. 6 shows the preferred embodiment of FIG. 2, which description is hereby incorporated by reference, including the input transducers 26 of FIG. 5 matched with a matching network 42. This third alternate embodiment provides the advantage of providing a broad band unidirectional interdigital transducer whereas prior are unidirectional interdigital transducer have a limited fractional bandwidth. A transducer that is self resonant, i.e. the reactive part of the impedance is canceled by the proper choice of k² (the square of the electromechanical coupling coefficient) and N, (the number of interdigital finger pairs), reflects a considerable portion of the energy incident upon it. Under ideal circumstances, it is possible to get a sizable reflectivity under self-resonant condition over a fractional bandwidth of nearly k²/2. For high coupling subtrates, k² can be from 11 to 17%, and the above fractional bandwidth can be fairly broad, e.g., 5 to 10%.

The filter of the present invention provides several advantages over the prior art. First, the filter tolerates a substantial variation (up to 4 db) in radiation conductance, G_a(ω). Therefore, since a flat passband is assured, the constraints in synthesizing a needed weighting on the transducers are substantially relaxed. Second, the track changer action effectively gives an extra 10 db in side lobe suppression. Third, rather large far-off sidelobes on either side of the passband, which can be a problem in withdrawal weighting, are not a problem in the present invention, as the track changer action suppresses these lobes by more than 5 db. Fourthly, the center transducer is self resonant, eliminating tuning elements and their contribution to signal degradation. Fifthly, the track changer configuration can be provided with smaller and fewer interdigitated electrodes than in the prior art.

FIG. 3 is a block diagram of portion 500 of a radio frequency receiver or other communication device including acoustic wave filters, in accordance with the present invention, The portion 500 of the radio apparatus includes an antenna 501 , by way of example, used to receive and/or transmit signals. Alternatively, the antenna 501 could be replaced by a fiber-optic link, cable or other signal transmissive media. A diplexer 503 is coupled to the antenna 501 and to a transmitter portion (not shown). The diplexer 503 is a special purpose filter which couples received signals (but not much larger signals from an attached transmitter) to a filter 505. The filter 505 is coupled to an amplifier 507. An output of the amplifier 507 is transmitted to a filter 509 according to the present invention. The filter 509 transmits a signal to a mixer 511. The signal from filter 509 is combined in the mixer 511 with another signal from a local oscillator 513 coupled via a filter 515. An output signal from the mixer 511 is then filtered by a filter 517 to provide an IF output signal. The arrangement of the present invention may be used to provide any or all of the filters 505, 509, 515, 517. An oscillator and filter analogous to the local oscillator 513 and filter 515 may be employed together with a suitable amplifier and modulator to provide the signal "FROM TRANSMITTER" and this filter (known as a "transmit clean-up filter") as well may be provided in accordance with the present invention. Thus, an acoustic filter has been described which overcomes specific problems and accomplishes certain advantages relative to prior art methods and mechanisms. The improvements over known technology are significant. Further, simple input and output impedances are realized for compact, lightweight, adjustment-free filters together with improved design flexibility. The present invention advantageously provides a reduction in size and weight, and an improvement in performance, over prior art acoustic wave filters by the novel utilization of incorporating dual transducers driving a self- resonant track changing transducer in phase and absorbing the scattered acoustic waves. In addition, insertion loss and subsequently SNR is improved. The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and therefore such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Accordingly, the invention is intended to embrace all such alternatives, modifications, equivalents and variations as fall within the broad scope of the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A surface acoustic wave filter apparatus, comprising: a piezoelectric substrate having an upper surface; a center interdigital acoustic transducer disposed on the upper surface, the center interdigital acoustic transducer comprising a plurality of interdigitated electrodes being substantially perpendicular to a direction of propagation of the piezoelectric substrate, the center interdigital acoustic transducer having a first aperture encompassing two acoustic tracks on the piezoelectric substrate; and at least one input interdigital acoustic transducer disposed on a first acoustic track on the upper surface of the piezoelectric substrate, each of the at least one input interdigital acoustic transducer comprising a plurality of interdigitated electrodes having a second aperture about one-half that of the first aperture; when electrically energized the at least one input interdigital acoustic transducer causing acoustic waves to propagate along a common direction of propagation substantially perpendicular to the interdigitated electrodes of the center and at least one input interdigital acoustic transducers and be substantially in phase and incident on a portion of the center interdigital acoustic transducer, wherein the acoustic waves are subsequently scattered outwardly from the center interdigital acoustic transducer along a second track on the upper surface of the piezoelectric substrate in a direction of propagation substantially perpendicular to the interdigitated electrodes of the center interdigital acoustic transducer.

2. The filter apparatus of claim 1 , wherein the at least one input interdigital acoustic transducer includes two interdigital transducers disposed on either side of the center transducer along the first acoustic track, the input interdigital acoustic transducers configured to be unidirectional transducers that, when energized, propagate acoustic waves towards the center interdigital acoustic transducer.

3. The filter apparatus of claim 1 , wherein the center interdigital acoustic transducer is configured to be self resonant.

4. The filter apparatus of claim 1 , further comprising acoustic absorption material disposed on the upper surface of the piezoelectric substrate along the second acoustic track, the acoustic absorption material substantially absorbing any acoustic waves propagated on the second acoustic track so as to reduce multiple transit acoustic echoes reflected back to the center interdigital acoustic transducer.

5. The filter apparatus of claim 1 , wherein the center interdigital acoustic transducer has a radiation conductance having a magnitude that is much greater than a magnitude of the sum of an electrical and acoustical radiation susceptance of the center interdigital acoustic transducer.

6. The filter apparatus of claim 1 , further comprising a high input impedance amplifier coupled to an output from the center interdigital acoustic transducer.

7. The filter apparatus of claim 1 , wherein the at least one input interdigital acoustic transducer includes one unidirectional input interdigital transducer disposed on one side of the center transducer along the first acoustic track, when energized the input interdigital acoustic transducer propagates acoustic waves towards the center interdigital acoustic transducer.

8. The filter apparatus of claim 7, further comprising a tuning inductor coupled to an output from the center interdigital acoustic transducer.

9. The filter apparatus of claim 1 , wherein each of the at least one input

> interdigital acoustic transducer includes one bidirectional interdigital transducer and one self resonant interdigital transducer separated by a gap of about one-quarter acoustic wavelength along the common direction of propagation, the self resonant interdigital transducer acting as a reflector for acoustic waves.

10. The filter apparatus of claim 9, further comprising the bidirectional interdigital transducer of the at least one input interdigital acoustic transducer being coupled to a matching network.

11. A surface acoustic wave filter apparatus for driving a high input impedance amplifier, comprising: a piezoelectric substrate having an upper surface; a center interdigital acoustic transducer disposed on the upper surface, the center interdigital acoustic transducer comprising a plurality of interdigitated electrodes being substantially perpendicular to a direction of propagation of the piezoelectric substrate, the center interdigital acoustic transducer having a first aperture encompassing two acoustic tracks on the piezoelectric substrate; two unidirectional interdigital acoustic transducers disposed on the upper surface of the piezoelectric substrate along a first acoustic track and on either side of the center interdigital acoustic transducer, each transducer comprising a plurality of interdigitated electrodes having a second aperture about one-half that of the first aperture; and acoustic absorption material disposed on the upper surface of the piezoelectric substrate along a second acoustic track and on either side of the center interdigital acoustic transducer, the acoustic absorption material substantially absorbing any acoustic waves propagated on the second acoustic track so as to reduce multiple transit acoustic echoes reflected back to the center interdigital acoustic transducer when electrically energized the two unidirectional interdigital acoustic transducers cause acoustic waves to propagate unidirectionally and substantially perpendicular to the interdigitated electrodes of both transducers towards the center interdigital acoustic tranducer so as to be substantially in phase and incident on a portion of the center interdigital acoustic transducer, wherein the acoustic waves are subsequently scattered outwardly from the center interdigital acoustic transducer along the second track on the upper surface of the piezoelectric substrate in a direction of propagation substantially perpendicular to the interdigitated electrodes.

12. The filter apparatus of claim 11 , wherein the center interdigital acoustic transducer has a radiation conductance having a magnitude that is much greater than a magnitude of the sum of an electrical and acoustical radiation susceptance of the center interdigital acoustic transducer.

13. The filter apparatus of claim 11 , wherein the center interdigital acoustic transducer is configured to be self resonant.

14. The filter apparatus of claim 11 , further comprising a high input impedance amplifier coupled to an output from the center interdigital acoustic transducer.

15. The filter apparatus of claim 11 , further comprising a tuning inductor coupled to an output from the center interdigital acoustic transducer.

16. The filter apparatus of claim 11 , wherein each of the at least one input interdigital acoustic transducer includes one bidirectional interdigital transducer and one self resonant interdigital transducer separated by a gap of about one-quarter acoustic wavelength along the common direction of propagation, the self resonant interdigital transducer acting as a reflector for acoustic waves.

17. The filter apparatus of claim 16, further comprising the bidirectional interdigital transducer of the at least one input interdigital acoustic transducer being coupled to a matching network.

18. A radio communication device having a high input impedance amplifier and at least one surface acoustic wave filter, the surface acoustic wave filter comprising: a piezoelectric substrate having an upper surface; a center interdigital acoustic transducer disposed on the upper surface, the center interdigital acoustic transducer comprising a plurality of interdigitated electrodes being substantially perpendicular to a direction of propagation of the piezoelectric substrate, the center interdigital acoustic transducer having a first aperture encompassing two acoustic tracks on the piezoelectric substrate; and at least one input interdigital acoustic transducer disposed on a first acoustic track on the upper surface of the piezoelectric substrate, each of the at least one input interdigital acoustic transducer comprising a plurality of interdigitated electrodes having a second aperture about one-half that of the first aperture; when electrically energized the at least one input interdigital acoustic transducer causing acoustic waves to propagate along a common direction of propagation substantially perpendicular to the interdigitated electrodes of the center and at least one input interdigital acoustic transducers and be substantially in phase and incident on a portion of the center interdigital acoustic transducer, wherein the acoustic waves are subsequently scattered outwardly from the center interdigital acoustic transducer along a second track on the upper surface of the piezoelectric substrate in a direction of propagation substantially perpendicular to the interdigitated electrodes of the center interdigital acoustic transducer.

19. A surface acoustic wave filter apparatus substantially as hereinbefore described with reference to the accompanying drawings.

20. A radio communication device substantially as herein described with reference to the accompanying drawings.