GB2249681A - Surface acoustic wave device - Google Patents

Abstract

A surface acoustic wave ring filter includes input and output transducers (11, 12) disposed between reflective arrays (13a, 13b) on a piezoelectric substrate. Each reflector comprises a repetitive array of groups of isolated electrode strips. In a preferred construction each electrode group comprises a first, generally straight electrode, and second and third electrodes each having end portions parallel to the first electrode, said end portions being coupled by a central offset portion. <IMAGE>

Description

SURFACE ACOUSTIC WAVE DEVICE This invention relates to surface acoustic wave (SAW) devices, and in particular to SAW filters e.g. for radio frequency applications.

Surface acoustic wave filters have been use a number of applications. Considerable work has been directed to reducing the losses in suc filters thereby improving their performance. A recent development i this area has been the SAW rIng filter in which a pair of bidirectional transducers are disposed between reflective coupler structures. This structure is described bv Feldmann and Henaff in Ultrasonics Symposium Proceedings 1974 pages 157 to 160. That arrangement uses two reflecting multistrip couplers and two bidirectional interdigital transducers in a ring configuration. The reflectors of this arrangement have a multistrip fan configuration.This has proved difficult to manufacture and has also displayed significant resistive loss, thus impa rinc the performance of the filter. A further ring filter structure which reduces these disadvantages has been described by Pollock et a in IEEE Ultrasonics Symposium 1983 pages 8 to 92. This document proposes a coupler or reflective structure comprising an array oboth isolated and shorter electrode strips. however, the shorted strips in this arrangement are relatively long and thin and they introduce significant resistive losses particularly at higher frequencies.Furthermore, the reflection and coupling coefficients of isolated and shorted strips differ by a factor depending on the substrate material and the thickness of the metallisation. This renders the filter performance somewhat unpredictable thus making the filter design a difficult task.

An object of the invention is to minimise or to overcome this disadvantage.

A further object of the invention is to provide an SAW ring fIlter having â low loss and a high selectivity, and which is relatively simple to manufacture.

According to the invention there is provided a surface acoustic wave (SAX) ring filter, including a first (input) transducer and a second (output) transducer disposed on a piezoelectric substrate between first and second reflector arrays, said arrays being adapted to reflect acoustic signals from the first transducer to the second transducer, wherein each said reflector array comprises a repetitive array of isolated electrode units.

According to the invention there is further provided a surface acoustic wave (SAW) ring filter, including a first (input) transducer and a second (output transducer disposed on a piercelectric substrate beyween first and second reflector arrays, said arrays being adapted to reflect acoustic signals from the first transducer to the second transducer, wherein each reflector array comprises a repetitive array of similar groups of three electrode strips, the electrodes of each group comprising a first generally straight electrode, and second and third electrodes each having first and second mutually offset end portions parallel to the first electrode and each having a central portion coupling the end protions, and disposed at an angle to the first electrode, the second and third electrodes being dispose one on each side of the first electrode.

We have found that the omission of shorted electrode strips from the reflector array results in an improved filter performance particularly at higher frequencies. Further, the structure is simplified thus reducing manufacturing costs.

Typically the filter frequency is in the range 400 MHz to 1700 MHz.

Embodiments of the invention will now be desoribed with reference to the accompanying drawings in which: Fig. 1 is a general schematic view of a ring filter construction; Fig. 2 shows one form of reflector array for use in te ring fIlter of Fig. 1; Figs. 3 to 5 show alternative reflector arrays; Fig. 6 illustrates a typical forward transfer characeristic of a filter employing the reflector structure of Fig. 4; Fig. 6a shows the central pass-band part of @ characteristic of Fig. 6, and Fig. 6b illustrates the filter phase shift across the pass-band.

Referring to Fig. 1, the ring filter includes linearly arrange input (11) and output (12) bidirectional transducers disposed on a piescelectric substrate (10) between first and second reflective arrays (13a, 13b).

Typically, the substrate comprises lithium niobate (LiNbO3) or lithium tantalate (LiTaO3).

Typically eac transducer 11, 12 comprises te sets of interdigitated electrodes. Contact to one electrode set of each transducer is effected via contact pads 141 and 142 respectively. The other electrode set of each transducer 11, 12 may be grounded via a

contact pad 151, 152.

Acoustic signals generated by the input transducer 11 are transmitted across the substrate, and are then reflected by the reflective arrays through 180 and simultaneously shifted transversely so as to direct the reflected signals to the output transducer 12. The pass-band frequency of the filter is determined by the cevice dimensions and by the characteristics of the piezoelectric substrate on which the device is disposed.

Typically the filter dimensions corresponding to a particular frequency are determined to a first order by computer simulation. The final design is then determined experimentally.

In a preferred construction the electrode arrays and the transducer regions are formed from a surface film of aluminium which is masked and etched to the desired pactern.

Fig. 2 shows one form of reflector construction for use in the ring filter of Fig. 2. The reflector comprises an array of similar electrode strips 31. Each electrode strip 31 comprises two offset parallel end portions, 31a, 31b, joined by a central protion 31a disposed at an angle to the to end portions. The electrode strips 31 are arranged as an array of mirror image or right ended and let handed pairs, the lateral spacing of the electrode strips being such that there are two strips per accoustic wavelength at the filter frequency. Typically the width of each electtode strip 31 is about one carter or the accoustic wavelength of the filter frequency.Typically each reflector array comprises between 50 and 100 pairs of electrode strips.

A variant of this reflector construction is shown in Fig. 3. In this arrangement alternate pairs of electrode strips 41 are inverted so as to provide a generally symmetrical arrangement. In this construction these electrode strips may be one quarter wavelength in width. Each set of four strips occupies a width of two wavelengths. Again each reflector may corprise between 50 and 100 pairs of electrode strips.

Fig. 4 shows a reflector structure in which eac repeating nit comprises three electrodes.

centre electrode 51 of eec unit is strait while the outer electrodes 52c, 52b comprise each two offset parallel portions linked by an angled centre portion. The reflector structure comprises a repetItive arrav of generally symmetrical units of this form. In this arrangement each electrode strip is typically one sixth or a wavelength in width, and each set of three electrodes occupies a total width of two wavelengths.

Fig. 5 shows a variant of the structure of Fig.

4. In this arrangement alternate electrode units are inverted.

Referring now to Fig. 6, this illustrates a typical forward transfer characteristic or frequency response of a ring filter employing the electrode construction of Fig. 4. s can be seen from Fig. 6 and from Fig. 6a, the device has a generally flat pass-band response with a sharp out-off at either edge of the pass-band. Fig. 6b shows the phase relationship across the pass-band of Fig. 6a. As can be seen from Fig. 6b, the phase relationship Is substantially linear across tne filter pass band. The apparent 360 phase transistor in the centre of the band is a function of the phase measurement system and is not an inherent property of the device.

In many applications it may be advantageous to design the transducer so as to present a real input admittance which matches the source and load impedance o external circuitry. This approach minimises the overall filter @ loss ano suppresses triple-transit rIpple effects without recourse to external tuning and matching components that are normally required for this purpose.

The ring filter constructions detailed above are of particular application, but are not limited t front end filtering in a radio communications device such as a radio pager or a cordless telephone.

Claims (9)

1. A surface acoustic wave ring filter, including a first (input) transducer and a second (output) transducer disposed on a piezoelectric substrate between first and second reflector arrays, said arrays being adapted to reflect acoustic signals from the first transducer to the second transducer, wherein each said reflector array comprises a repetitive array of isolate electrode strips.

2. A ring filter as claimed in claim 1, wherein each electrode unit comprises a pair of similar electrodes.

3. A ring filter as claimed in claim 2, wherein each electrode strip has a first and second offset parallel end portion linked by a central portion disposed at an angle to the end portions.

4. A ring filter as claimed in claim 3, wherein each electrode strip has a width equivalent to one quarter of the operating wavelength of the filter.

5. A surface acoustic wave (SAW) ring filter, including a first (input) transducer and a second (output) transducer disposed on a piezoelectric substrate beyween first and second reflector arrays, said arrays being adapted to reflect acoustic signals from the first transducer to the second transducer, wherein each reflector array comprises a repetitive array of similar groups of three electrode strips, the electrodes of each group comprising a first generally straight electrode, and second end thlr electrodes each having first ano second mutually offset end portions parallel to the first electrode and each having a central portion coupling t end portions, end disposed a an angle to te first electrode, the second and third electrodes being disposed one on eac sloe of the first electrode.

6. x filter as claimed in claim 5, wherein each said group of tree electrode strips is disposed in an inverted configuration in relation to an immediately adjacent group of electrode strips.

7. A filter as claimed in claim 5 or 6, wherein each electrode strip has a width equivalent to one sixth of the operating wavelength of the filter.

8. A surface acoustic wave ring filter substantially as described herein with reference to and as shown in Fig. 1, together with any one of Figs 2 to 5 of the accompanying drawings.

9. A communicatIons device incorporating a ring filter as claimed in any one of claims 1 to 8.