GB2108800A - Surface acoustic wave devices - Google Patents

Abstract

A surface acoustic wave device includes an interdigital transducer designed to reduce anomalous responses in the high frequency side close-in stop band when the transducer forms part of a narrow pass band filter. Various means are provided for inhibiting coherent reflections of surface acoustic waves A, B, C, D from at least a portion 9, 11 of the outer edge of an electrode busbar 5,7 of the transducer towards a second transducer. <IMAGE>

Description

SPECIFICATION Surface acoustic wave devices This invention relates to surface acoustic wave (SAW) devices.

More particularly the invention relates to SAW devices which include transducers of the kind comprising two electrodes each comprising a set of fingers extending in spaced parallel relationship from a common bus-bar, the fingers of the two electrodes being interdigitated. Such a transducer is hereafter referred to as a surface acoustic wave interdigital transducer (SAW IDT).

Such devices find particular application as surface acoustic wave filters, the desired filter characteristic being obtained by appropriate design of the geometry of the transducer electrodes, in particular the fingers of the electrodes.

In the case of narrow band with fast cut-off filters, it is well known that anomalous responses are often obtained especially in the high frequency side close-in stop band.

It is an object of the present invention to provide a surface acoustic wave device including an interdigital transducer for use in a filter wherein this problem is alleviated.

According to the present invention there is provided a surface acoustic wave device including at least one surface acoustic wave interdigital transducer wherein in respect of at least a portion of that longitudinal edge of at least one electrode bus-bar which is remote from the electrode fingers which extend from said one bus-bar, coherent reflections of surface acoustic waves towards a second transducer are inhibited.

In one particular device in accordance with the invention said portion is formed with a profile such that reflections resulting from surface acoustic waves incident at said portion are randomly scattered.

In one such particular device said portion is formed with a profile such that reflections resulting from acoustic waves incident on said portion of the edge are randomly scattered.

In such a particular device in which said one transducer forms part of a narrow pass band filter said portion has a profile in the form of a regular series of rectangular projections having a pitch of half the wavelength of oscillations at the centre of the pass-band of the filter.

In another such particular device said portion has a diffuse profile.

In another particular device said portion extends generally along the length of said one electrode bus-bar at an angle of other than 900 to the direction of length of said one electrode busbar at an angle of other than 900 to the direction of length of said electrode fingers.

In such a device preferably the edge of said one electrode bus-bar from which said electrode fingers extend is also at an angle of other than 90" to the direction of length of said electrode fingers, and preferably said portion and said other edge are non-parallel.

In a further particular device at least part of said one bus-bar has a pattern of holes in it. Preferably said pattern of holes is randomly distributed over said part. When said one transducer forms part of a narrow pass band filter, preferably said holes are rectangular with sides having a length less than the wavelength of the oscillations at the centre of the pass band of the filter.

Four devices in accordance with the invention suitable for use as a narrow band cut-off filter will now be described, by way of example only, with three of the devices being described with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a SAW IDT for use in the first device; Figure 2 represents the frequency response characteristic of a filter incorporating the device of Figure 1; Figure 3 represents the frequency response characteristic of a comparable conventional filter.

Figure 4 is a schematic diagram of a SAW IDT for use in the third device; and Figure 5 is a schematic diagram of a SAW IDT for use in the fourth device.

Referring firstly to Figure 1, the transducer consists df two sets of interdigitated metal fingers 1, 3 deposited on a quartz substrate 2. The sets of fingers 1, 3 are connected by bus-bars 5, 7 respectively. The longer edges 9, 11 of the bus-bars 5, 7 which are more remote from the fingers 1, 3 have a regular series of rectangular projections having a pitch of half the wavelength of oscillations at the centre of the passband of the filter. The lines A and B represent the passage of the surface acoustic waves transmitted by the transducer to an output transducer (not shown) to provide the filter output characteristic shown in Figure 2.The lines C and D represent the passage of surface acoustic waves which would, in the absence of the series of rectangular projections on the edges 9, ii of the bus-bars 5,7 as in a comparable conventional filter 1 be reflected by the edges 9, 1 1 of the bus-bars 5, 7 to give rise to the anomalous peak A in the high frequency side close-in stop band, shown in Figure 3. In the transducer shown in Figure 1, however, the half wavelength pitch series of rectangular projections on the edges 9, 1 1 acts as a strong scatterer for surface acoustic waves impinging on the series at glancing incidence.The resulting reflected waves are, therefore, randomly scattered so that a coherent signal due to the waves C, D is not produced at the output transducer, and the anomalous peak in the high frequency side closein stop band shown in Figure 3 is substantially reduced, to produce the filter output characteristic shown in Figure 2.

In the second embodiment of the invention the SAW IDT which is again designed for use in a narrow band cut-off filter, is of simiiar form to the SAW IDT depicted in Figure 1 with the exception of the profiled bus-bars 5, 7. These are replaced by substantially conventional rectangular bus-bars which have, however, been fabricated with diffuse edges on the sides of the bus-bars remote from the metal fingers. These diffuse edges will scatter incident surface acoustic waves in random directions, rather than simply reflecting them, and will thus prevent waves reflected from these bus-bar edges from giving rise to the anomalous peak in the high frequency side close-in stop band as shown at A in Figure 3.

It will be appreciated that many other edge profiles may be provided on the bus-bars to form scattering profiles, for example a sawtooth profile.

Although the two examples described herebefore both describe the use of scattering profiles to reduce coherent reflections from the edges of the bus-bars remote from the metal fingers,.it will also be appreciated that means for reducing coherent SAW reflections may also be provided in accordance with the invention on the edges of the bus-bars adjacent the metal fingers, although calculations indicate that these reflections have less effect on the filter characteristic of the IDT.

Referring now to Figure 4, the SAW IDT for the third device to be described consists of two sets of interdigitated metal fingers 21,23 deposited on a quartz substrate 25. The sets of fingers 21,23 are connected by bus-bars 27, 29 respectively. The bus-bar 27 is formed as a trapezium with its edge 31 adjacent to the metal fingers 21 at an acute angle to the direction of the corresponding edge 33 of the bus-bar 29, the bus-bar 29 being of substantially rectangular form.

The lines E and F represent the passage of the surface acoustic waves transmitted by the transducer to an output transducer (not shown) to generate the narrow band cut-off filter characteristics. The lines G and H represent the passage of surface acoustic waves which are generated in a given direction and are reflected off the outer edge 35 of the bus-bar 29 to give an anomalous response in the filter characteristic at a frequency dependent on the finger spacing, and the velocity of the waves in the given direction.

Similar waves, represented by the lines J and K, are reflected off the outer edge 37 of the bus-bar 31. This edge serves to direct the waves away from a direction in which they would be picked up by the output transducer as in the case of the waves E, F, G and H. The anomalous response in the output characteristic which would have resulted from all the waves reflected off the edge 37 is thus substantially reduced. The anomalous response resulting from the reflection of surface acoustic waves off the edge 31 of the bus-bar 27 is similarly reduced. It will be noticed that in order to accommodate the angling of the edge 31 in a direction away from a line perpendicular to the metal fingers 21,23, i.e. the line XX' it is necessary to lengthen the fingers accordingly.As calculations indicate that reflections from the inner edges of the bus-bars have less effect on the filter characteristic than the reflections from the outer edges, in some circumstances it may prove expedient not to angle the edge 31 and thus avoid this lengthening of the fingers.

It will be appreciated that in practice at least one edge of the transducer 29 will normally also be angled with respect to the line XX'. It will also be appreciated that whilst the bus-bar 27 is in the form of a trapezium, a rectangular bus-bar which is angled with respect to the line XX' will also fulfil the same function of reducing coherent reflections of the surface acoustic waves from the edges of the bus-bar although slightly less effectively.

Referring now to Figure 5, the SAW IDT for the fourth device to be described again comprises two sets of interdigitated metal fingers 39, 41 deposited on a quartz substrate 43. The sets of fingers 39,41 are connected by bus-bars 45,47 respectively. The bus-bars 45, 47 are both of substantially conventional form, i.e. rectangular, and are arranged parallel to each other with the metal fingers extending in the direction of the normal to each bus-bar. Each bus-bar has a number of square holes 49 formed through it in a random pattern over the length of the bus-bar.

Each hole has dimensions less than wavelength of the oscillations at the centre of the passband of the filter. The number of holes is chosen to be a maximum within the constraint that the electrical resistance of the bus-bar should not be significantly increased. Each of the holes serves to scatter incident surface acoustic waves in random directions, thus preventing the waves from being reflected from the outer edges of the bus-bars and giving use to an anomalous response in the output characteristic of the filter generated at the output transducer (not shown). It will be appreciated that although the holes described herebefore are chosen to be square, any shape of hole will suffice.

It will be appreciated that a transducer combining all or some of the features of the four transducers described herebefore is possible.

It will also be appreciated that other means for reducing SAW reflections from the edges of the bus-bars may be employed, e.g. the application of an acoustic absorbent material such as black wax on the edges of the bus-bars, although it has been found that consistent results are difficult to obtain by this method.

Claims (12)

1. A surface acoustic wave device including at least one surface acoustic wave interdigital transducer wherein in respect of at least a portion of that longitudinal edge of at least one electrode bus-bar which is remote from the electrode fingers which extend from said one bus-bar, coherent reflections of surface acoustic waves towards a second transducer are inhibited.

2. A device according to Claim 1 in which said portion is formed with a profile such that reflections resulting from surface acoustic waves incident at said portion are randomly scattered.

3. A device according to Claim 2 in which said one transducer forms part of a narrow pass band filter and in which said portion has a profile in the form of a regular series of rectangular projections having a pitch of half the wavelength of oscillations at the centre of the passband of the filter.

4. A device according to Claim 2 in which said portion has a diffuse profile.

5. A device according to Claim 2 in which said portion has a saw tooth profile.

6. A device according to Claim 1 in which said portion carries an acoustic absorbent material.

7. A device according to any one of the preceding claims in which said portion extends generally along the length of said one electrode bus-bar at an angle of other than 900 to the direction of length of said electrode fingers.

8. A device according to Claim 7 in which the edge of said one electrode bus-bar from which said electrode fingers extend is also at an angle of other than 900 to the direction of length of said electrode fingers.

9. A device according to Claim 8 in which said portion and said other edge are non-parallel.

10. A device according to any one of the preceding claims in which at least part of said one bus-bar has a pattern of holes in it.

11. A device according to Claim 10 in which said pattern of holes is randomly distributed over said part.

12. A device according to Claim 10 or Claim 11 in which said one transducer forms part of a narrow pass band filter and said holes are rectangular with sides having a length less than the wavelength of the oscillations at the centre of the pass band of the filter.

1 3. A surface acoustic wave device substantially as hereinbefore described with reference to Figures 1, 4 or 5 of the accompanying drawings.