GB2368988A - Floating electrode SAW device of the FEUDT type - Google Patents

Abstract

A surface acoustic wave transducer comprising an upper and lower bus bar and four electrodes within one period substantially equal to the surface acoustic wavelength, where at least one electrode is attached to the upper bus bar, at least one electrode is attached to the lower bus bar and at least one electrode is floating. Further embodiments are shown in figs 6 to 14.

Description

TRANSDUCERS FOR SURFACE ACOUSTIC WAVE DEVICES

The present invention relates to transducers for surface acoustic wave devices.

Technology employing surface acoustic waves (SAW) has found extensive applications in devices, for example, such as electronic filters. These electronic filters can be utilised in systems including, for example, television, communications and radar. These devices typically operate in the frequency range 20 to 3000 MHz.

Arrays of metal strips, such as aluminium, are deposited on the surface of a substrate. The substrate is usually a piezoelectric material such as lithium niobate, lithium tantalate or quartz, on which SAWs can be generated or detected by the metal strips. Such arrays are called Interdigital Transducers (IDTs).

Figure 1 illustrates a simple conventional SAW device consisting of two IDTs on a substrate 101. The IDTs consist of electrodes 102 connected alternately to bus bars 103. The bus bars are electrically connected to terminals 105. An electrical signal applied across the terminals on the left causes the left IDT to generate SAWs, which then travel to the right IDT where they are converted back to an electrical signal appearing across the output terminals at the right. The transducers shown are periodic structures, with two electrodes per period 104. The period is of length A.

There are many variants of this basic device. The IDTs may have more than two electrodes per period. They may be weighted, so that the electrode length or spacing varies along the length of the transducer in order to modify the electrical performance of the device. The electrodes can also be connected to the bus bars in an irregular sequence for the same purpose.

Typically, devices can have up to several hundred electrodes, leading to a substantial versatility in design, and a wide range of practical devices can be realised.

One limitation of the devices described above is the reflection of SAWs by the transducers. Each transducer reflects incident SAWs to some extent, and in consequence the device response includes a series of unwanted echoes due to multiple transits of the waves between the two transducers. This phenomenon introduces unwanted ripples into the frequency response of the device.

It is usually possible to reduce the reflections by adjusting the electrical source or load impedances connected to the transducers. However, for adequate suppression of the unwanted echoes the required impedances normally cause the device insertion loss to be quite large, typically 15 dB or more. Thus, conventional devices are limited in that low insertion loss cannot generally be obtained without the presence of unacceptable echoes.

A variety of techniques have been used in SAW devices in an attempt to overcome the problem of enabling low insertion loss to be obtained without substantial distortion due to multiple-transit echoes. In particular, these

techniques include various types of unidirectional transducer, that is, transducers that generate waves with different amplitudes in the two directions when a voltage is applied. The ratio of the wave amplitudes is called the directivity, and is a function of frequency.

Most unidirectional transducers are called'single-phase unidirectional transducers', or SPUDTs (these have two terminals, allowing only one voltage signal to be applied).

For a SPUDT to have any directivity, the SPUDT must have internal reflections, that is, it reflects incident SAWs when it is electrically shorted.

For a given reflectivity, there is an optimum relationship between the phases of SAW generation and reflection, in order to maximise the directivity. The phase relationship needs to be close to this condition for good directivity to be obtained.

As the SPUDT is directional, it is always possible to devise an electrical source or load impedance such that, for a specified frequency, the SPUDT does not reflect SAWs incident on its'forward'side. The forward side is the side where the larger amplitude is produced when a voltage is applied.

If the directivity is large (e. g. 10 dB or more), the load impedance is close to the transducer impedance, and therefore provides an efficient

conversion of electrical signals to SAW's, or vice versa.

A SAW device can typically consist of two SPUDT's, but may consist of more. The devices are oriented so that their'forward'ports are facing each

other. Hence SPUDTs enable low-loss devices to be produced with very little distortion from multiple-transit echoes.

Generally, reflections of surface waves arise from either mechanical or electrical loading (perturbation) of the surface, or perhaps both. Electrical loading is characterised by the piezoelectric coupling factor of the substrate, expressed as the quantity v/v == (v0-vm)/v0, where vo and Vm are the SAW velocities for a free and electrically shorted surface, respectively. Mechanical loading is dependent on the mechanical properties of the substrate and electrode materials, and on the film thickness of the electrodes.

One type of SPUDT is the FEUDT (Floating Electrode Unidirectional Transducer) introduced by Yamanouchi et al ("New low-loss SAW filter using internal floating electrode reflection types of single-phase unidirectional transducer", Electronics Lett. , 20,1984, p. 989-990 & "Couple mode analysis of SAW floating electrode type unidirectional transducers", IEEE Trans.

Ultrasonics, Ferroelectrics and Freq. Control, 40,1993, p. 648-658). The FEUDT uses one or more electrodes that are not connected to either bus-bar during each period of the transducer. The electrode arrangement is asymmetrical, leading to directivity.

The SAW reflections arise from the voltages of the floating electrodes, and mechanical loading has little effect. Therefore, the FEUDT has the following advantages over types of SPUDTs.

(a) FEUDTs are effective on strong piezoelectric substrates such as lithium niobate.

(b) Since FEUDTs do not depend on mechanical loading, their performance is hardly affected by the film thickness. This means that FEUDTs are easier to fabricate.

Figs. 2 and 3 show two known types of FEUDT using regular electrodes.

Figure 2 shows a type with six electrodes per period. The drawing refers to a repetitive electrode pattern. Only one period of the pattern, between the broken lines, is shown completely. The period consists of six electrodes. Typically, the actual FEUDT would have say 10 to 20 periods, so that it has 60 to 120 electrodes, with each period having the pattern between the broken lines in Figure 2. In each period the 1st electrode (102) is connected to the upper bus bar (103) and the 4th to the lower bus bar. The other 4 electrodes are floating (not connected to either bus bar). The 2nd and 5th electrodes (107) are floating electrodes connected to each other, while the 3rd and 6th electrodes (106) are floating electrodes with no connections.

Reflections arise from the presence of the floating electrodes; when a surface wave is incident on the FEUDT with its bus bars connected together, the wave induces voltages on the floating electrodes. These voltages have the effect of generating additional surface waves in both directions, including a reflected wave travelling back toward the source of the incident wave. Also, when a voltage is applied across the bus bars (with no incident SAW), the FEUDT generates SAWs in a way similar to a conventional IDT, though the floating electrodes give rise to mathematical complications. The floating

electrodes have the effect of emphasising the SAW generated to the right (the 'forward'end), so that the wave emerging to the right has larger amplitude than that emerging at the left.

Figure 3 shows another known type of FEUDT, this time with 5 electrodes per period. The principles of this are similar to those of the 6electrode FEUDT in Figure 2.

The analysis used here is described in a previous publication by the inventor in"Quasi-static analysis of floating-electrode unidirectional SAW transducers (FEUDTs) ", IEEE Ultrasonics Symp. , 1999, p. 107-111. In common with previous analysis carried out, the analysis here considers the behaviour of the electrodes in one period, assuming that the number of periods is infinite. For a finite length FEUDT the behaviour of the electrodes is slightly modified by end effects, but this is not significant if the FEUDT is long enough, for example 10 periods or more. From the behaviour of the electrodes in one period, the behaviour of a finite-length FEUDT can be calculated by a standard method, namely, the well-known coupling-of-modes (COM) method.

Throughout this document it is assumed that the substrate material behaves in a symmetrical fashion, so that the asymmetry of the FEUDT arises only from the asymmetry of the electrode geometry. There are known cases in which the substrate behaves asymmetrically, but this does not occur for the usual standard SAW substrates.

Transduction and Reflection Centres A FEUDT design can be specified by the configuration of the electrodes in each period. In order to assess the effectiveness of a particular design, the main consideration is the directivity to be expected for a given length (i. e. number of periods). Other important factors are the strength of the transduction (the surface wave amplitude produced for a given applied voltage) and the capacitance. For simplicity, we consider operation at the centre frequency.

From general analysis of SPUDTs, it is known that the directivity depends on the following factors: (a) The magnitude of the reflection coefficient for each period of the FEUDT (b) The length (number of periods) (c) A factor that expresses the relative phase difference between transduction and reflection.

To clarify the relative phase difference between transduction and reflection, it is conventional to define transduction and reflection centres.

To define the transduction centre, it is assumed a FEUDT has a unit voltage applied across the bus bars and the reflections are ignored. The waves generated by one period of the FEUDT are then studied. Two waves are travelling in opposite directions, denoted as + x, with the same amplitude. The phases of these waves depend on the reference position used for the zero of the x-axis. It is possible to choose a location for the zero of the x-axis such

that the waves have the same phase when referring to that point. The location of the zero for the x-axis is the transduction centre. The location of the transduction centre, relative to the centre of the 1 st electrode in the period, is denoted by XTC To define the reflection centre, it is imagined that the FEUDT bus bars are connected together so that the above transduction process doesn't occur. An incident surface wave is assumed, and the voltages on the floating electrodes are calculated. The waves generated by these voltages are then found. The wave generated toward the source of the incident wave gives the reflection coefficient. This is done twice, taking the incident wave to be firstly from the left and secondly from the right. The resulting reflection coefficients are of the same magnitude (a fundamental result from the physics), but generally of different phases. However, by referring to an appropriate reference point on the x-axis the phases can be made the same. This reference point is the reflection centre, and its distance from the centre of the 1st electrode is denoted by xRC.

For a SPUDT, the directivity depends on the magnitude of the reflection coefficient and the distance between the transduction and reflection centres. The latter distance is written in terms of an angle \"defined as Aqf = 2 (xRC-xTC)/A.. (1)

which is simply the phase change, expressed in radians, for a wave propagating this distance. Here, A is the length of one period of the SPUDT, which is equal to the wavelength of the SAW under consideration. The optimum value for As is + z #/4 plus an arbitrary multiple of # (corresponding to a distance ~ #/8 + n. A/2).

Figure 4 shows the directivity in dB, as a function of FEUDT length, AR and Ay, withay expressed in degrees. An ideal value of is 45 , but it can be seen that quite large errors can be tolerated. For example, with an error of 10 (## = 55 ) the behaviour is quite close to ideal for directivities up to about 8 dB. Thus, in assessing a chosen design for practical value, the value of ## does not have to be very close to 45 . The directivity obtainable with a particular design depends on both the magnitude of AR and the error in

. The effect of reducing Ay instead of increasing it is the same. So, for example, with = 350 the curve for directivity is the same as the curve in Figure 4 for ## = 55 .

The positions of the transduction and reflection centres are not unique.

For transduction, the position of the centre can be changed by half the SAW wavelength, i. e. by A/2. This changes the phases of both waves by 1800 so that they remain the same, and corresponds to changing the sign of AT. For the reflection centre, the position can be changed by A/4. This causes a 180 change in the phase of both reflection coefficients, so that they remain the

same. Thus there are two transduction centres and four reflection centres in each period of length.

Transducer Data

TABLE 1. Data for known transducer types (prior art)

Device ## (deg) xTC/p xRC/p aT aR C Fwddir Figure 2 49.1 2.82 3.50 -1.043 -1.243 .625 R Figure 3 40.4 0.0609 2.00 .958 .905 1.214 R

Table 1 shows data for two known transducers shown in Figure 2 and Figure 3. The data refer to transducers with electrode width a equal to half the electrode pitch p, so that a/p = 0.5.

The transduction centre and reflection centre are expressed in terms of XTC and XRC, as defined previously. These are also normalised to the electrode pitch, p. For example, for the 6-electrode case shown in Figure 2, the transduction centre is 2.82p to the right of the centre of the first electrode in the period, as shown in Figure 2. The transduction centre is therefore between the centres of the 3rd and 4th electrodes, and close to the centre of the 4th electrode.

The parameters aT and aR are essentially transduction and reflection constants and are proportional to the piezoelectric coupling parameter Av/v mentioned earlier. In the Table, aT and aR are normalised with respect to

Av/v. The parameter C is a normalised capacitance, W is the FEUDT aperture and Np is the number of periods. The definition is such that a conventional single-electrode transducer with a/p = 0.5 has C = 1. The last column, headed 'Fwd dir'has entries R or L to show whether the forward end of the FEUDT is at the right or left, respectively.

Summary of invention According to the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and four electrodes within one period substantially equal to the surface acoustic wavelength, where at least one electrode is attached to the upper bus bar, at least one electrode is attached to the lower bus bar and at least one electrode is floating.

According to a further aspect of the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 1 1 2 0 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

According to yet a further aspect of the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 10203 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a first floating electrode, and numeral 3 defines a second floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

According to yet a further aspect of the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 100002 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

According to yet a further aspect of the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 1 1 0 2 0 3 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a first floating electrode, and numeral 3 defines a second floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

According to yet a further aspect of the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 1 0 2 0 3 3 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a first floating electrode, and numeral 3 defines second and third floating electrodes linked together, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents,

and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

According to yet a further aspect of the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 1 0 2 0 0 3 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a first floating electrode, and numeral 3 defines a second floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

According to yet a further aspect of the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 10202 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines first and second floating electrodes linked together, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in

the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

According to yet a further aspect of the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 12032 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines first and second floating electrodes linked together, and numeral 3 defines a third floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

According to yet a further aspect of the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 1 0 0 2 2 0 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines first and second floating electrodes linked together, and where one set of equivalents are defined by displacing the code cyclically, a

second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

According to yet a further aspect of the present invention, there is provided a surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 102434 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a first floating electrode, and numeral 3 defines a second floating electrode, and numeral 4 defines third and fourth floating electrodes linked together, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

Further objects and advantages will be apparent to the skilled reader from the following description and claims.

Brief Description of the Drawings Specific embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a layout of a basic SAW device.

Figure 2 shows a known FEUDT.

Figure 3 shows another known FEUDT.

Figure 4 shows a graph depicting directivity of SPUDTs.

Figure 5 shows a FEUDT according to one aspect of the present invention.

Figure 6 shows a FEUDT according to another aspect of the present invention.

Figure 7 shows a FEUDT according to yet another aspect of the present invention.

Figure 8 shows a FEUDT according to yet another aspect of the present invention.

Figure 9 shows a FEUDT according to yet another aspect of the present invention.

Figure 10 shows a FEUDT according to yet another aspect of the present invention.

Figure 11 shows a FEUDT according to yet another aspect of the present invention.

Figure 12 shows a FEUDT according to yet another aspect of the present invention.

Figure 13 shows a FEUDT according to yet another aspect of the present invention.

Figure 14 shows a FEUDT according to yet another aspect of the present invention.

Description of the Preferred Embodiments The preferred embodiments described below have been selected by defining various configurations of unknown FEUDTs, and analysing the devices using methods described in the inventor's previous publication, which is included herein by reference. The devices may be periodic FEUDTs, that is, FEUDTs consisting of a number of identical periods with each period having length A. Alternatively each period within the FEUDT may have a different layout enabling the frequency response of the device to be tailored to a specific need.

The FEUDT properties were calculated at the centre frequency, i. e. the frequency at which A is equal to the SAW wavelength. The properties at frequencies near to this value can be assumed to be similar.

In order to define the embodiments below, a special coding is used.

The electrodes in each period are assigned a set of integers, one integer to each electrode, in sequence. The integer values are defined as for an electrode connected to the upper (live) bus 0 for an electrode connected to the lower (ground) bus any other number for a floating electrode.

In addition, floating electrodes with the same index indicates they are connected together. For example, taking the known FEUDT of Figure 2 with 6 electrodes per period, the code can be written 142043 This indicates that in each period the 1 st electrode is connected to the upper bus and the 4th electrode is connected to the lower bus. The other four electrodes are floating. The 2nd and 5th electrodes are floating but connected together. The 3rd and 6th electrodes are floating and not connected to any other floating electrodes. The values used for the floating electrodes are not significant except for the facts that they are not equal to 1 or 0 and that identical values indicate connected electrodes.

It should be noted that for any specified code, there are several other codes giving FEUDT devices that are equivalent. For example, since the FEUDT is periodic, an equivalent structure can be obtained by displacing the code cyclically. Thus for example 1 4 2 0 4 3 becomes 4 2 0 4 3 1. This process can be repeated, giving N codes, where N is the number of electrodes per period. Another modification is to interchange the numbers 1 and 0, which corresponds to reversing the connections to the bus bars. Also, for all these codes, new equivalent codes can be obtained by reversing the order, which corresponds to reversing the electrode sequence so that it gives the same directivity but in the reverse direction. In total, any one design is

actually a member of a set of 4N designs, all with equivalent behaviour. The following list shows the equivalent cases for the 6-electrode FEUDT with code 142043.

142043 042143 340241 341240 420431 421430 134024 034124 204314 214304 413402 403412 043142 143042 241340 240341 431420 430421 024134 124034 314204 304214 402413 412403 The first column above is obtained by displacing the code. The second column is obtained from the first column by interchanging the l's and 0's. The third column is obtained from the first column by reversing the order. The fourth column is obtained from the third column by interchanging the l's and 0's.

In the Tables shown below, each code is to be taken as representing the set of 4N equivalent codes. Only one code from the set is shown. When a code indicates a new device, the other codes in the set can also be regarded as the same device, as it effectively responds in the same manner.

A SAW device will typically be structured as a series of transducers linked together on a substrate. The structure may be of a repetitive kind, where all the devices are the same, alternatively, the structure may be of a

non-repetitive structure, utilising various different transducer structures within a single device.

The FEUDT is most effective for frequencies at and near its centre frequency, that is, the frequency at which the length A of each period is equal to the SAW wavelength. It is sufficient to consider operation at the centre frequency, because the behaviour at nearby frequencies is similar.

The calculations given in the Tables below refer to the behaviour of the electrodes in one period of the FEUDT, assuming that the FEUDT has a large number of periods. For theoretical convenience an infinite number of periods was assumed, but this gives results approximately valid for a finitelength FEUDT and these results are adequate to establish the utility of a particular design.

Data for FEUDT devices, according to the present invention, are shown in the following Tables, using the same convention as previously described. As stated previously there are four reflection centres per period, and only one of these is shown in the Tables. In many cases the reflection centre is simply related to electrode positions because of symmetry considerations, and the reflection centre has been chosen to emphasise this so that xpc/P has an integer or half-integer value. Also, because Ay can be changed by 900 without having any fundamental effect on the behaviour, the values of Ay have been reduced by subtracting multiples of 90 , such that the final value shown is in the range 0 to 90 . The optimum value of this final

angle is therefore 45 . This facilitates comparison between the designs, and comparison with Figure 4.

The Tables include values of the transduction parameter aT and the capacitance parameter C, defined earlier. These are included for convenience.

The differences in these values, for different FEUDTs, are not likely to be very important when considering the merit of particular FEUDTs.

In some cases the locations of reflection or transduction centres can be deduced by visual inspection, making use of symmetry. To locate a reflection centre, it is imagined the FEUDT is shorted across the bus bars, which is equivalent to replacing the'1'electrode code by'0'. If the structure appears symmetrical about some point when this is done, then that point must be a reflection centre. In doing this, it must be remembered that the structure of one period is repeated indefinitely.

A transduction centre can sometimes be found by considering the symmetry, and taking account of the applied voltage. If the FEUDT is symmetrical about some point in these circumstances, then that point is a transduction centre. In transducers with floating electrodes it can be seen that, if some point is a transduction centre by virtue of the symmetry, then it will also be a reflection centre. With the two centres coincident, it follows that there is no directivity.

As explained earlier, the main criterion for a FEUDT is the directivity obtainable. This depends on two factors, the reflectivity per period as specified by the parameter aR and the value of,. The latter is the transit

angle between the two centres, ideally equal to 450 ion the reduced form on the Tables. However, from the directivity curves in Figure 4 it can be seen that a deviation from 450 oftypically 100 in this angle is likely to be acceptable.

First Embodiment TABLE 2.

Code xTC/p xRC/p aT aR C Fwd dir (deg) 1 1 2 0 22.5 .750 2.00 1.350 1.290 1.000 R

The 4-electrode device 1120, shown in Table 2 and Figure 5, has a relatively large deviation from the ideal value of 45 (22. 5 ) for Ay. This tends to limit its directivity, but this FEUDT has the advantage of having wider electrodes than other types (for the same operating frequency) so that it is easier to fabricate. Hence, it would be most advantageous to use this in a device when only moderate directivity is required.

Second Embodiment TABLE 3.

Code ## xTC/p xRC/p At aR C Fwd air (deg) 1 0 2 0 3 38.0 2.28 3.00 -1.087 -1.2678 1.018 R

The 5-electrode device 1 0 2 0 3, shown in Table 3 and Figure 6, is notable for relatively strong reflectivity and value of close to 450.

Third Embodiment

TABLE 4.

Code ## (deg) xTC/p xRC/p aT aR CFwd dir 1 0 0 0 0 2 45.0 2.75 5.00 -1.115 .6661.077 R

The 6-electrode device 100002, shown in Table 4 and Figure 7, has the ideal value of 45'for Ay, making it suitable when a long transducer is needed with high directivity.

Fourth Embodiment

TABLE 5.

Code ##(deg) xTC/p xRC/p aT aR C Fwd air 1 1 0 2 0 3 46.3 2.71 4.00 1.447 -.541 1.251 R

The 6-electrode device 1 10203, shown in Table 5 and Figure 8, also has a value close to the ideal value of 45O for Z 46. 3 ).

Fifth Embodiment TABLE 6.

Code ## (deg) XTC/ xRC/p aT AR C Fwd dir p 1 0 2 0 3 3 52.8 2. 50 3. 12 -.965 -1.735 1.000 R

The six-electrode device 1 02033, shown in Table 6 and Figure 9, has relatively strong reflectivity, giving larger directivity for a given length

provided the length is not large enough to make the deviation from the ideal value of 45 in A\ (/ significant. Sixth Embodiment TABLE 7.

Code As (deg) xTC/P xRC/P aT AR C Fwddir 102003 49. 1 2. 82 3. 50-1. 043-1. 243 1. 058 R The six-electrode device 1 02003, shown in Table 7 and Figure 10, has similar properties to the prior art device 1 42043, shown in Table 1 previously. However, this device has the advantage that there are no bridges between the floating electrodes, and is therefore easier to manufacture. Seventh Embodiment TABLE 8.

Code ## (deg) xTC/p xRC/p aT AR C Fwd air 1 0 2 0 2 47.2 2.40 3.00 -0.912 -0.196 1.057 L The five electrode device 1 0 2 0 2, shown in Table 8 and Figure 11 would be a suitable device for a physically long transducer as the reflectivity is relatively weak. A long transducer could consist of a series of these devices, or in a combination of other devices, linked together, connecting the output of one device to the input of the next device, to form one transducer.

Eighth Embodiment TABLE 9.

Code ## (deg) xTC/P xRC/p ay AR C Fwddir 12032 41. 4 2. 324 3.00 -1.104 .826 .663 L The five electrode device 12032, shown in Table 9 and Figure 12 has a value for ## relatively close to the ideal value of 45 . The capacitance of this device is also relatively small. Ninth Embodiment TABLE 10.

Code As (deg) xTC/P xRC/P aT AR C Fwddir 1 0 0 2 2 0 51.7 2.86 3.50 -. 735 1. 576 1. 216 L The six electrode device 1 0 0 2 2 0, shown in Table 10 and Figure 13 has strong reflectivity. Tenth Embodiment TABLE 11.

Code AW (deg) xTC/P xRC/P aT AR C Fwd dir 102434 59. 0 1.86 0.88 -. 605 1. 735. 823 R The six electrode device 1 0 2 4 3 4, shown in Table 11 and Figure 14 also shows a characteristic of having strong reflectivity.

These embodiments provide novel devices that give good directivity in relation to the number of electrodes within a single period.

All of the above devices could be used, for example, in mobile phone manufacturing, televisions or radar where low loss filters are required, and the problem of unwanted ripples needs to be minimised. However, all these devices could be used in any application where a directional transducer is required.

Table 12 below shows data for new FEUDTs with 6 electrodes per period, and 1 or 2 floating electrodes. TABLE 12.

Code AW (deg) xTC/P XRC/P AT aR C Fwddir 111002 15. 0 0. 750 5. 00 1. 632. 666 1. 309 L 112100 56. 9 1. 449 2. 00 1. 445. 666 1. 541 L 1 1 0 1 0 2 85.9 .432 5.00 .790 .666 2.006 R 1 0 0 0 2 0 24.9 2.91 4.00 -8.69 .666 1. 232 L 100023 60. 0 2. 50 4. 50-1. 154 1.332 1.000 R 100203 13. 2 2. 72 4. 00-1. 027-. 541 1. 063 R 100230 51. 2 2. 85 3. 50-1. 741 1. 332 1. 215 L 100022 57. 6 2. 46 4. 50-1. 078 1. 576 1. 033 R 100202 16. 5 2. 78 4. 00-. 822. 369 1. 112 L 110202 53. 0 0. 38 4. 00 1. 310. 369 1. 300 L

Table 13 below shows data for FEUDTs with 6 electrodes per period and 3 floating electrodes.

TABLE 13.

code Ay (deg) XTC/P XRC/P AT aR C Fwd dir 1 0 0 2 3 4 75.0 2.25 4.0 -1.035 1.333 .928 R 102034 53. 8 2. 54 3. 14-1. 044-1. 496. 968 R 120034 83. 6 2. 75 2. 86-1. 247-1. 496. 764 R 102304 27. 7 2. 82 2. 36-. 854 1. 496 1. 023 R 100233 62. 3 2. 18 2. 64-. 986-1. 496. 945 R 120033 80. 9 2. 73 2. 88-1. 156-1. 735. 799 R 1 0 2 3 0 3 58.9 2.92 3.44 -.673 -.361 1.054 R 123003 50. 4. 339 1. 00 1. 236. 754. 773 L 102330 80.5 2.98 3.14 -.543 1.496 1. 177 L 120330 64. 0 0. 187. 620. 852 1. 735 1. 023 L 1 2 3 0 3 0 55.2 .361 2.44 .990 -.361 .979 L 1 2 3 3 0 0 4.6 .720 2.14 .966 1.496 .941 L 1 0 0 2 3 2 71.2 2.19 4.00 -.833 2.243 .977 R 102032 16. 6 2. 78 4. 00-. 823. 754 1. 035 L 1 2 0 0 3 2 22.2 2.81 3.941 -1.232 .361 .769 L 1 0 0 2 2 2 70.9 2.18 4.00 -.834 1.989 .977 R 102 022 13. 5 2. 73 1. 00-. 811. 346 1. 040 L 120022 42. 9 2. 72 2. 00-1. 157. 346. 799 R

Table 14 below shows data for FEUDTs with 6 electrodes per period and 4 floating electrodes.

TABLE14.

Code AW (deg) XTC/xRC/P aT aR C Fwd dir P 1 0 2 3 4 4 54.9 1.91 1.00 -.705 .541 .812 R 1 2 0 3 4 4 79.7 2.45 2.62 -.937 -1.735 .649 R 1 2 3 4 0 4 37.1 0.62 1.50 .957 1.175 .646 L 102333 59. 8 1. 85. 86-. 609 1. 496. 823 R 1 0 3 8. 2 2. 02 1. 88-. 461-1. 735. 849 L 123033 37. 0 0. 12 1. 00. 987. 754. 641 L 130233 58. 9 2. 54 3. 06-. 919. 361. 653 L 122303 39. 3. 655 1. 50. 911 1. 464. 657 L

Table 15 below shows further data for FEUDTs with 4 or 5 electrodes per period.

TABLE 15.

Code ## XTC/p xRC/p aT aR C Fwd dir (deg) 1 0 0 0 2 54.0 2.25 4.00 -1.241 .905 1.052 R 1 0 0 2 3 72.0 2.00 1.00 -1.180 .464 .951 R 1 0 0 2 2 70.0 1.97 3.50 -1.068 1.917 .982 R

10233 67. 6 1.689 2.00 -.789 -1.268 .824 R

It will be apparent to the skilled reader, that various modifications and variations may be employed in relation to the above-described embodiments without departing from the scope of the present invention.

For example, it will be apparent to the skilled man that weighting techniques could be applied to these devices so that the electrode length and spacing can be varied along the length of a transducer.

It will also be apparent to the skilled man that different disclosed devices may be used in combination with each other to provide a required frequency characteristic.

Claims (15)

1. A surface acoustic wave transducer comprising an upper and lower bus bar and four electrodes within one period substantially equal to the surface acoustic wavelength, where at least one electrode is attached to the upper bus bar, at least one electrode is attached to the lower bus bar and at least one electrode is floating.

2. A surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 1120 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

3. A surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 10203 and its equivalents, where numeral I defines an electrode attached to

the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a first floating electrode, and numeral 3 defines a second floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

4. A surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 100002 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

5. A surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 1 10203 and its equivalents, where numeral 1 defines an electrode attached

to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a first floating electrode, and numeral 3 defines a second floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

6. A surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 102033 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a first floating electrode, and numeral 3 defines second and third floating electrodes linked together, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

7. A surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code

102003 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a first floating electrode, and numeral 3 defines a second floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

8. A surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 10202 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines first and second floating electrodes linked together, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

9. A surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to

the surface acoustic wavelength, the electrodes are arranged according to code 12032 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines first and second floating electrodes linked together, and numeral 3 defines a third floating electrode, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

10. A surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 100220 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines first and second floating electrodes linked together, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

11. A surface acoustic wave transducer comprising an upper and lower bus bar and a pattern of electrodes, where in one period substantially equal to the surface acoustic wavelength, the electrodes are arranged according to code 102434 and its equivalents, where numeral 1 defines an electrode attached to the upper bus bar, numeral 0 defines an electrode attached to the lower bus bar, numeral 2 defines a first floating electrode, and numeral 3 defines a second floating electrode, and numeral 4 defines third and fourth floating electrodes linked together, and where one set of equivalents are defined by displacing the code cyclically, a second set of equivalents are defined by interchanging the numbers 1 and 0 in the first set of equivalents, and a third set of equivalents are defined by reversing the order of the electrodes defined in both the first and second sets of equivalents.

12. A surface acoustic wave device comprising any transducer according to any of claims 1 to 11 where the transducer sections are arranged in a nonrepetitive manner.

13. A surface acoustic wave device comprising any transducer according to any of claims 1 to 11 where the transducer sections are arranged in a repetitive structure.

14. A mobile telephone comprising a transducer according to any of claims I to 11.

15. An electronic filter comprising a transducer according to any of claims 1 to 11.