GB2168869A

GB2168869A - A surface acoustic wave ladder transducer

Info

Publication number: GB2168869A
Application number: GB08432074A
Authority: GB
Inventors: Adrian Nigel Farr
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1984-12-19
Filing date: 1984-12-19
Publication date: 1986-06-25
Also published as: GB2168869B

Abstract

A surface acoustic wave transducer comprises two sets of rungs. The rungs 2 of the first set operate in a conventional way and result in the transducer having peak frequency responses at a number of discrete spaced frequencies. The invention provides additional peak responses mid-way between the frequencies of the first mentioned responses. This is done using a second set of transducers 9A, 9B, alternate transducers of which are arranged to operate in antiphase. The transducer may utilise surface skimming bulk waves. <IMAGE>

Description

SPECIFICATION A surface acoustic wave ladder transducer This invention relates to a surface acoustic wave ladder transducer.

A conventional surface acoustic wave ladder transducer is shown in Figure 1 and comprises a number (r) of rungs 1 each comprising a number (n) of finger pairs. The rungs are separated by a distance equal to an integral number (m) of wavelengths at a centre frequency fO. The frequency response of such a transducer is shown in continuous lines on Figure 2 from which it is apparent that there are a number of different amplitude peaks at different spaced frequencies distributed to either side of the centre frequency f0.

The frequency spacing Af, called the "mode spacing", between these amplitude peaks is inversely proportional to the rung separation mA. Where small mode spacing is required it is therefore necessary to have a large rung spacing. To obtain a useful bandwidth (i.e. the width of the envelope enclosing the peaks of Figure 2) the number of finger pairs in each rung has to be small. Therefore the number of rungs has to be large in order to obtain a low insertion loss i.e., sufficient efficiency in coupling electric energy into surface acoustic wave energy. The combination of a large rung separation and a large number of rungs leads to the device becoming larger than is desirable. It also means that the transducer has a high Q i.e., each of the frequency peaks of Figure 2 is narrow.This is undesirable in situations when the transducer forms part of a device such as an oscillator which needs to be tuned to a particular frequency near but not necessarily at the centre of one of the amplitude peaks of Figure 2. There is therefore a conflict between the requirements for wide bandwidth, narrow mode spacing, low insertion loss and low Q. This invention arose in an endeavour to resolve the aforementioned conflict.

The invention provides a surface acoustic wave ladder transducer comprising two notional sets of rungs arranged and connected to a radio frequency signal source so that, at frequencies of the source where surface acoustic waves generated by adjacent rungs of the first set are in phase, surface acoustic waves generated by adjacent rungs of the second set are in antiphase.

The term "notional sets of rungs" is intended to indicate that the rungs can be considered to comprise two sets and is not intended to imply that each rung of one set is different from each rung of the other set.

By employing the invention the conflict of requirements previously described can be reduced.

This is because the modes of one set of rungs will lie between the modes of the other set thereby increasing the number of modes for a given rung spacing or reducing the rung spacing for a given number of modes; whilst simultaneously increasing the total number of rungs and thereby reducing the insertion loss.

The two sets of rungs can be arranged side by side so that they both propagate surface acoustic waves in the same direction out of adjacent parts of a common aperture. Alternatively the two sets of rungs can be superimposed. This can be done by interposing each rung of one set between adjacent rungs of the other set.

Two ways in which the invention may be performed will now be described by way of example with reference to Figures 3 and 4 of the accompanying drawings in which: Figure 3 illustrates a surface acoustic wave ladder transducer constructed in accordance with the invention and in which the two sets of rungs are superimposed; and Figure 4 illustrates another surface acoustic wave ladder transducer constructed in accordance with the invention and in which the two sets of rungs are arranged side by side.

Referring firstly to Figure 3 the illustrated transducer comprises a first notional set of identical rungs 2 similar to the rungs 1 of Figure 1. Each rung 2 comprises electrode fingers 3, connected to a bus bar 4, interdigital with fingers 5, connected to a bus bar 6. A radio frequency source 7 is connected between the bus bars 4 and 6. The fungs 2 are separated by a distance mA as in Figure 1 and the fingers 3 are separated by a distance X as are the fingers 4: being the wavelength of the surface acoustic wave in the substrate material at the desired centre frequency fO.

The effect of the rungs 2 along would be to give the transducer the characteristics shown by the peaks P of Figure 2 and to propagate surface acoustic waves in the direction shown by arrow 7 from an aperture 8 of the transducer.

The transducer of Figure 3 also includes a second notional set of rungs 9A and 9B also spaced by a distance mX. Each of these rungs 9A, 9B also comprises fingers 3, connected to bus 4, interdigitated with fingers 5, connected to bus 6. The fingers 3 are, like those of rungs 2, spaced by a wavelength A as are the fingers 5. The arrangement of fingers of alternate rungs 9A of the second set 9A, 9B are identical to the arrangement of the fingers of the rungs 2. The arrangement of fingers in the other rungs 9B of the second set are however different in that they are connected to opposite bus bars.This results in rungs 9A generating waves which are in antiphase with those generated by rungs 9B at the frequencies of peaks P of Figure 2; but which are in phase with the waves generated by rungs 9B at frequencies mid-way between the peaks P of Figure 2. Consequently the second set of rungs 9A, 9B produces peaks as shown at P' on Figure 2. Thus, the combined frequency response of the first set of rungs 2 and or the second set of rungs 9A, 9B contains all the peaks P and P' shown in Figure 2. This is twice as good as for the known arrangement of Figure 1. Furthermore, the increase in the number of rungs as compared with the arrangement of Figure 1 significantly reduces insertion loss.

The arrangement shown in Figure 4 is similar to that of Figure 3 and corresponding components are denoted by identical reference numerals. In Figure 4 the first set of rungs 2 is arranged laterally with respect to the second set of rungs 9A, 9B. The two sets of rungs sharing common bus bars 4 and 5 between which they extend. Alternate fingers of each rung of the first set are formed by extensions of alternate fingers of a rung of the other set and vice versa. In the case of rungs 9B these extensions are straight linear extensions whereas in the case of rungs 9A the extensions are offset to form a stepped configuration. This achieves the desired phase displacement of 180" between the rungs 9A and 9B whilst retaining the spacing mA between adjacent rungs of each set. In an alternative configuration the rungs 2 could be separate from the rungs 9A and 9B with each set of rungs having its own bus bars.

The term "surface acoustic waves" is to be construed for the purpose of this specification as including surface skimming bulk waves which travel just below the surface of a material in addition to the more commonly empioyed waves which travel on the surface.

Claims

1. A surface acoustic wave ladder transducer comprising two notional sets rungs arranged and connected to a radio frequency signal source so that, at frequencies of the source where surface acoustic waves generated by adjacent rungs of the first set are in phase, surface acoustic waves generated by adjacent rungs of the second set are in antiphase.

2. A surface acoustic wave transducer according to claim 1 in which the two sets of rungs are arranged laterally with respect to an aperture of the transducer.

3. A surface acoustic wave transducer according to claim 1 in which each set of rungs is located between and connected to a common pair of bus bars and in which the fingers of alternate rungs of one of the sets are connected by a step to the fingers of an adjacent rung of the other set in order to achieve a phase reversal whilst maintaining equal separation between rungs.

4. A surface acoustic wave transducer substantially as described with reference to Figure 3 of the accompanying drawings and substantially as illustrated therein.

5. A surface acoustic wave transducer substantially as described with reference to Figure 4 of the accompanying drawings and substantially as illustrated therein.