GB2239144A

GB2239144A - Surface acoustic wave transducer

Info

Publication number: GB2239144A
Application number: GB8928448A
Authority: GB
Inventors: John Maxwell Deacon; Christopher Eric Brice
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1989-12-16
Filing date: 1989-12-16
Publication date: 1991-06-19
Also published as: GB8928448D0

Abstract

A surface acoustic wave (SAW) transducer having two metal contact pads 6 arranged on a surface of piezoelectric material with narrow lateral fingers 4 extended inwardly therebetween to form a ladder-like structure, in which variable tap weights along the lengths of the pads are provided by a finger length weighting arrangement such that the length of overlap between the fingers 4 of opposite polarity shows a variation along the pad length, in which the positions of the overlap portions are arranged in order to generate a surface wave having a uniform cross-section whereby any apodisation loss in operation will be minimised. The transducer can be used to construct a SAW filter or delay line. <IMAGE>

Description

ACOUSTIC TRANSDUCER This invention relates to an acoustic transducer. It relates particularly to a surface acoustic wave (SAW) transducer type of device in which interdigital fingers forming a ladder-like structure are used.

Most SAW transducers consist of two interdigital transducer units on a polished surface of a piezoelectric material. The simplest interdigital transducer contains interleaved metal fingers having alternate electrical polarity and which are positioned with a pitch equal to one half of the expected SAW wavelength at centre frequency. However, because of the piezoelectric and mass loading of the substrate material by the fingers a small amount of the available SAW energy will be reflected by each finger. At centre frequency, all the reflected waves will add in phase, thus leading to an increasingly large distortion of the frequency response as the number of fingers is increased, that is, as the bandwidth of the filter is narrowed.

A similar problem in large transducers is that of regeneration.

Here, a wave is detected and reradiated before leaving the interdigital transducer. The errors in performance associated with this effect are very similar to those due to reflection.

The reflection problem is most commonly avoided by the use of transducers with more than two fingers per wavelength at centre frequency. However, this provision reduces finger width and it also decreases the maximum operating frequency which would be available to the device designer. In addition, this construction provides no improvement in the distortions due to regeneration.

A configuration which is often used to increase the length of a transducer without increasing the number of fingers is the ladder structure. In this construction, blocks of fingers are removed at equal intervals along the interdigital transducer leaving a ladder-like structure. The first order response of such transducer near centre frequency is practically identical to that of a solid interdigital transducer of the same length but the -interaction problems described above will be reduced since the total numbed , lingers in the transducer has been reduced.

Some examples of the ladder transducer configuration are disclosed in European Patent Application No. 0140618 (M.F. Lewis).

To use a SAW transducer to make a transversal filter having variable tap weights, the most popular and flexible method is to use a finger length weighting or 'apodisation' technique. In order that the frequency response of the whole device may be the product of the frequency responses of the two individual transducers, one of the transducers must have fingers of uniform length.

This provision in turn introduces other problems, one of which particularly concerns low insertion loss devices. The problem is that the minimum possible insertion loss is limited by virtue of an apodisation profile mismatch which is present between the two transducers. For most tap weighting functions this 'apodisation loss' can be small, but for low loss devices however it can be a significant factor which limits the expected device performance.

One object of the present invention is to provide an apodised transducer construction in which any apodisation loss is able to be kept to a minimum.

According to the invention, there is provided a surface acoustic wave (SAW) transducer having two metal contact pads arranged on a surface of piezoelectric material with narrow lateral fingers extended inwardly therebetween to form a ladder-like. structure, in which variable tap weights along the lengths of the pads are provided by a finger length weighting arrangement such that the length of overlap between the fingers of opposite polarity shows a variation along the pad length, in which the positions of the overlap portions are arranged in order to generate a surface wave having a uniform crosssection whereby any apodisation loss in operation will be minimised.

In the transducer, the positions of the overlap portions on the first half length of a said contact pad may be arranged to be a mirror-image of the positions of the said portions on a second half of the said pad length. In a different embodiment, the positions of the overlap portions on a first half length of a said contact pad may be arranged to be an image rotated through 1800 of the positions of the said portions on a second half of the pad length.

By way of example, some particular embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a basic layout for a SAW filter, Figure 2 shows a SAW ladder transducer, Figure 3 depicts a triangular weighting function arrangement, Figure 4 is a conventionally apodised ladder transducer which has been designed to exhibit the weighting function of Figure 3, Figure 5 shows a ladder transducer having rungs which form symmetrical rotational apodisation sources, Figure 6 shows a ladder transducer in which the rungs have been fitted to the sources by channelisation, Figure 7 shows a ladder transducer having rungs fitted to the apodisation sources by means of a meandering inner busbar connection, and, Figure 8 shows a complete SAW device incorporating two transducers.

Most surface acoustic wave (SAW) devices, as shown in Figure 1, consist of two interdigital transducers 1, 2 mounted on a polished surface of a body 3 of piezoelectric material. Each interdigital transducer contains interleaved metal fingers 4 having alternate electrical polarity and which are positioned at a pitch equal to one half of the expected SAW wavelength at centre frequency. The metal fingers 1 extend inwardly from contact pads 6 and the contact pads then serve for making the electrical connections to the fingers 4.

However. because of the piezoelectric and mass loading of the substrate material by the fingers, a small amount of the available SAW energy will be reflected by each finger 4. At centre frequency, all the reflected waves will add in phase thus leading to an increasingly large distortion of the frequency response as the number of fingers is increased (that is, as the bandwidth of the filter is narrowed).

A similar problem in large transducers is that of regeneration.

Here a wave is detected and reradiated before leaving the interdigital transducer. The errors in performance associated with this effect are very similar to those due to reflection.

The reflection problem is most commonly avoided by the use of transducers with more than two fingers per wavelength at centre frequency. However, this provision reduces the finger width and it also decreases the maximum operating frequency which is available to the device designer. Unfortunately, the modification does not mitigate the distortions due to regeneration.

One configuration which is frequently used to increase the length of a transducer without increasing the total number of fingers present is the ladder structure (Figure 2). In this arrangement, blocks of the fingers 4 are seen to have been removed at equal intervals along the length of the interdigital transducer 1 to form inter-rung spaces 7 so that a ladder-like structure is formed. A first order response of such a transducer near centre frequency is practically identical to that of a solid interdigital transducer of the same length but the total number of fingers in the construction has clearly been reduced.

To use a SAW transducer to make a transversal filter having variable tap weights, the most popular and flexible method is by a finger length weighting or 'apodisation' arrangement. This is shown, in the case of a ladder transducer, in Figures 3 and 4.

In order that the frequency response of the whole device may be the product of the frequency responses of the two individual transducers, one of the transducers must have fingers of uniform length. This in turn introduces other problems, one of which particularly concerns low insertion loss devices. This is that the minimum possible insertion loss is limited by virtue of the apodisation profile mismatch between the two transducers. For most tap weighting functions this 'apodisation loss' amounts to a decibel or so, and this for conventional SAW filters having insertion losses of 15-20 dB or more can be negligible. In the case of low loss devices however, the apodisation loss can be a significant factor which limits the device performance.

The triangular weighting function shown in Figure 3 will then be expected to be produced by the conventionally apodised ladder transducer of Figure 4.

As Figure 4 shows. there are thirteen tap positions spaced from the left to the right hand end of each contact pad. The upper contact pad carries thirteen individual fingers which extend inwards.

Similarly. the lower contact pad carries thirteen pairs of inwardly extended fingers. The amount of weighted finger overlap at each of the tap positions is arranged to vary in accordance with the weighting function at that position as depicted in Figure 3.

The finger length weighting provision is used to obtain a specific transducer characteristic and in this instance the generated wave profile would be of a triangular shape.

In contrast to this. a 'rotational apodisation' arrangement can allow the design of an apodised transducer which generates a surface acoustic wave having uniform cross-section, and which therefore would exhibit no apodisation loss when used with a uniform transducer. By splitting a tap into two parts where necessary, the set of taps can be made to be spread evenly in the lateral direction.

This arrangement can be seen in the ladder transducer of Figure 5 where the ladder rungs form symmetrical apodisation sources. The figure shows a transducer having seven channels, each of which contains an equal number (seven) of taps whereas each column contains a different number of taps, defining the weighting function. The tap weight, from the left to the right hand side of the transducer can be seen to follow the sequence: l, 2, 3, 4, 5, 6 7, 6, 5, 4, 3, 2, 1.

A similar arrangement can be seen in Figure 6 where the ladder transducer has rungs fitted to sources by channelisation.

Figure 6 is essentially just seven uniform profile transducers connected electrically in parallel. Hence. the order of the 'subtransducers' can be shuffled at will, for example in order to place the longest at the centre. The sub-transducers could alternatively be connected in series, if more convenient. or in a series/parallel combination.

A further embodiment is shown in Figure 7 where the use of a meandering inner busbar connection arrangement serves to reduce the number of interconnections needed for constructing the circuit.

Figure 8 shows a complete SAW device of the invention. This has two interdigital transducers carried on a piezoelectric body 3 as in the Figure 1 construction. The left hand transducer is similar to that of Figure 6 whilst the right hand transducer has the ladder structure of Figure 2. The two transducers are arranged to be of similar heights as depicted in Figure 8.

The foregoing description of embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, one or more reflector strips might be arranged in the inter-rung positions to enhance acoustic wave generation in one direction. Additional parallel reflector strips might be placed externally of the rungs. If necessary, portions of absorbent material could be provided on the piezoelectric substrate to reduce unwanted acoustic waves.

Claims

CLAIMS:

1. A surface acoustic wave (SAW) transducer having two metal contact pads arranged on a surface of piezoelectric material with narrow lateral fingers extended inwardly therebetween to form a iadder-like structure, in which variable tap weights along the lengths of the pads are provided by a finger length weighting arrangement such that the length of overlap between the fingers of opposite polarity shows a variation along the pad length. in which the positions of the overlap portions are arranged in order to generate a surface wave having a uniform cross-section whereby any apodisation loss in operation will be minimised.

2. A transducer as claimed in Claim 1 in which the positions of the overlap portions on a first half length of a said contact pad is arranged to be a mirror-image of the positions of the said portions on a second half of the pad length.

3. A transducer as claimed in Claim 1. in which the positions of the overlap portions on a first half length of a said contact pad is arranged to be an image rotated through 1800 of the positions of the said portions on a second half of the pad length.

4. A surface acoustic wave transducer substantially as hereinbefore described with reference to any one of Figures 5 to 7 of the accompanying drawings.

5. A surface acoustic wave filter or delay line, including a SAW transducer as claimed in any one of Claims 1 to 4.

6. A surface acoustic wave filter or delay line substantially as hereinbefore described.