GB2029148A

GB2029148A - Surface wave devices

Info

Publication number: GB2029148A
Application number: GB7923864A
Authority: GB
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1978-07-12
Filing date: 1979-07-09
Publication date: 1980-03-12

Abstract

A surface acoustic wave device comprising a first interdigital transducer (9) coupled with two further interdigital transducers (5 and 7) wherein one (7) of the further transducers is symmetric about its geometric centre (23) and the other (5) is antisymmetric about its geometric centre (23), the first transducer being either symmetric or antisymmetric. The arrangement introduces a 90 DEG phase difference between signals transmitted to the further transducers from the first transducer or vice versa, and can be used to provide frequency independent quadrature outputs by appropriate spacing of the geometric centres (15 and 23) of the first and further transducers. <IMAGE>

Description

SPECIFICATION Surface acoustic wave devices This invention relates to surface acoustic wave devices.

The invention relates particularly to surface acoustic wave devices of the kind comprising a first interdigital transducer coupled with two further interdigital transducers via respective surface acoustic wave propagation paths on the surface of a substrate.

It is an object of the present invention to provide such a device wherein a frequency independent 90 phase difference between signals transmitted to the further transducers from the first transducer or vice versa is introduced.

According to the present invention in a surface acoustic wave device comprising a first interdigital transducer coupled with two further interdigital transducers via respective surface acoustic wave propagation paths on the surface of a substrate, one of the further transducers is symmetric about its geometric centre, and the other further transducer is antisymmetric about its geometric centre, the first transducer being either symmetric or antisymmetric about its geometric centre.

In one particular embodiment of the invention suitable for use as a delay line providing quadrature outputs the geometric centres of the two further transducers are at the same distance from the geometric centre of the first transducer.

One surface acoustic wave device in accordance with the invention will now be described by way of example with reference to the accompanying drawing which is a schematic diagram of a delay line providing quadrature outputs.

Referring to the drawing the device comprises a substrate 1 of suitable piezoelectric material, such as lithium niobate, on one main face of which there are provided a launching interdigital transducer 3 and two detecting interdigital transducers 5 and 7.

The launching transducer 3 comprises two metal film electrodes 9 and 11 each comprising a set of parallel spaced apart fingers 1 3 which extend from a metallised area 1 5 which electrically connects the fingers at one end and constitutes a terminal of the transducer.

The fingers 13 each have a width of ""/8m where "" is the wavelength of an acoustic wave in the substrate at the centre frequency fo of the range of frequencies over which the device is required to operate. In each of the electrodes 9 and 11 the fingers 1 3 are arranged in pairs, the two electrodes in each pair being spaced by ""/8 and adjacent pairs being spaced by 5Ao/8. The two electrodes 9 and 11 are relatively positioned on the substrate so that the finger pairs are interdigitated in parallel spaced relation.The transducer fingers 1 3 are thus all equally spaced by A /8 and disposed in an antisymmetric arrange ment about the geometric centre of the trans ducer indicated by dotted line 1 7. In the drawing each electrode is shown as having two finger pairs only but in practice the num ber of finger pairs will normally be greater.

The detecting transducer 5 has electrodes 19 and 21 essentially of the same form as the launching transducer 3 except that its fingers are shorter in length. The transducer 5 is disposed on the substrate 1 along the path of surface acoustic waves propagated in the sub strate when the transducer 3 is excited with its geometric centre 23 at a distance from the geometric centre 1 7 of the transducer 3 de pendent on the required time delay of the outputs relative to the input.

The detecting transducer 7 is similarly posi tioned with respect to the transducer 3, along side the transducer 5. However, the transduc er 7 differs from the transducer 5 in that it is of symmetric form. Thus, in the transducer 7 one electrode 25 is of the same form as the electrodes 19 and 21 of transducer 5 but the other electrode 27 of transducer 7, while having the same number of fingers as elec trode 25, has at each end a single finger instead of a finger pair. The two electrodes 25 and 27 when interdigitated thus provide a finger arrangement which is symmetric about the geometric centre 29 of the transducer 7.

In operation an electrical input signal ap plied between the terminals of transducer 3 causes a corresponding surface acoustic wave to be propagated in the surface of the sub strate 1 in a direction perpendicular to the lengths of the fingers 13, towards the trans ducers 5 and 7. Between the terminals of each of the transducers 5 and 7 there is produced an electrical output signal corre sponding to the propagated surface wave, and hence to the input signal applied to the trans ducer 3.

As may be shown mathematically, the phase of the output signal of the transducer 5 relative to the input signal applied to the transducer 3 is 2 m f 1 /v where fis the frequency of the waves, vis the veolcity of propagation of the wave in the substrate, and 1 is the distance between the geometric centres of the two transducers. The phase of the output signal of the transducer 7 is 2 7r f1 /v + m /2 the sign being dependent on the sense of connection of the output termi nals of transducer 7.

The device thus provides outputs at the transducers 5 and 7 in quadrature relation, the quadrature relation remaining precise de spite variation of the frequency of the input signal within the range over which the device is required to operate.

In known comparable devices the transduc ers are either all symmetric or all antisymmetric and the required quadrature relation be tween output signals is obtained by disposing the detecting transducers at distances from the launching transducer which differ by A /4, With such an arrangement, at any frequency f other than fo, the two output signals are not precisely in quadrature, but have a phase difference of 90 x f/ > o f/f0 degrees.

It will be appreciated that the trandsucer 3 could be symmetric instead of antisymmetric without affecting the principle of operation as described above.

It is pointed out that whilst the device dscribed above by way of example employs transducers of the so-called 'split-finger' type, the invention may also be carried out using other types of interdigital transducer. For example, transducers where finger width and sapcing is A /4 may be used although in the case of the required symmetric transducer a finger of width ""/, will be required at each end of the transducer.

It is further pointed out that a device in accordance with the invention may employ more than three transducers. For example, the device described by way of example may be modified by the addition of two further transducers, one symmetric and one asymmetric, to provide four 90 phase shifted outputs for use in a quadrature phase demodulator.

It is further pointed out that in an arrangement according to the invention the geometric centres of the further transducers may be at different distances from the geometric centre of the first transducer, but of course the additional difference in phase shift over the two propagation paths thereby introduced is not independent of signal frequency.

Claims

1. A surface acoustic wave device comprising a first interdigital transducer coupled with two further interdigital transducres via respective surface acoustic wave propagation paths on the surface of the substrate wherein one of the further transducers is symmetric about its geometric centre, and the other further transducer is antisymmetric about its geometric centre, the first transducer being either symmetric or antisymmetric about its geometric centre.

2. A device according to Claim 1 wherein the geometric centres of the two further transducers are at the same distance from the geometric centre of the first transducer.

3. A device according to Claim 1 or Claim 2 wherein the transducers are of the splitfinger type.

4. A surface acoustic wave device substantially as hereinbefore described with reference to the accompanying drawing.