Acoustic wave devices

#The present invention relates to acoustic wave devices, and in particular to such devices including a track changing multistrip array arranged between input and output transducers on a substrate able to propagate acoustic waves at a surface thereof. Such devices are used, for example, as bandpass electrical filters.

#Surface acoustic wave (SAW) bandpass electrical filters are well known which consist of an input interdigital transducer and an output interdigital transducer with an interposed track changing multistrip array on a high coupling piezoelectric substrate; e.g. Y-Z lithium niobate. The major advantage of the multistrip array in these filters is that it provides relative suppression of bulk wave spuriae, and a further advantage is that it allows apodization weighting of both transducers.

#The amplitude-frequency response of each transducer in these filters has a fundamental passband which is repeated with a certain amplitude at particular harmonics dependent on the sampling configuration of the transducer, that is to say the centre-to-centre separation period and the mark-to-space ratio of the transducer electrodes. If the input and output transducers have different sampling configurations then this may result in suppression of certain of these harmonic passbands in the combined amplitude-frequency response of the filter. This suppression will not occur if the two transducers have the same sampling configuration. An interdigital transducer having electrodes with a centre-to-centre separation period of a quarter wavelength of surface acoustic waves at the centre frequency of the fundamental passband, known as a double or split electrode transducer, and with an electrode mark-to-space ratio of one-to-one has a third harmonic passband response which is of nearly equal amplitude to that of the fundamental passband. A major advantage of the double electrode configuration transducer is that interactions within the transducer which cause undesired ripples in the amplitude-frequency response are minimised, and for this reason it is common to use this configuration for both the input and output transducers. However, the unsuppressed third harmonic response resulting from this use can be a significant disadvantage in certain filter applications.

#The invention will now be described in more detail with reference to the accompanying drawings, in which:

#FIG. 1 shows schematically in plan view a known surface acoustic wave bandpass electrical filter incorporating double electrode input and output interdigital transducers with an interposed conventional track changing multistrip coupler,

#FIG. 2 shows the filter of FIG. 1 with a modified multistrip coupler according to the invention for suppression of a first frequency,

#Referring now to FIG. 1 there is shown a substrate SU able to propagate surface acoustic waves at a surface thereof, input transducing means IP arranged to launch surface acoustic wave energy along a first propagation track T1 at the surface, output transducing means OP arranged to receive surface acoustic wave energy from a second propagation track T2 at the surface and electrical coupling means MSA arranged to receive surface acoustic wave energy from the input transducing means IP in the first track T1 via an input port P1 and to relaunch at least part of that energy as surface acoustic wave energy towards the output transducing means OP in the second track T2 via an output port P2.

#The input transducing means IP and the output transducing means OP are both interdigital transducers comprising double electrodes, that is to say having electrodes with a center-to-center separation period of a quarter wavelength .lambda.o/4 of surface acoustic waves at the centre frequency f.sub.o of a fundamental passband of the amplitude-frequency response of the transducer, and the mark-to-space ratio of the electrodes of the transducers is one-to-one. The combined amplitude-frequency response of the transducers IP and OP has harmonically repeated passbands of predetermined shape including a fundamental passband centered at the frequency f.sub.o and a third harmonic passband at a frequency 3f.sub.o which is of nearly equal amplitude to that of the fundamental passband.

#The electrical coupling means MSA consists of an array of a number N of discrete parallel conducting strips ST electrically insulated from each other. Each strip ST may be considered as a pair of strips in which one strip of the pair is part of an array of strips across the track T1, the other strip of the pair is part of an array of strips across the track T2 and the two strips of each pair are electrically connected.