GB2139442A - Surface acoustic wave device - Google Patents

Abstract

A SAW device capable of providing a convolution between a SAW and a variable reference function. The device comprises a substrate (5) and means (9) for causing SAWs to propagate along a surface of the substrate. A convolver (1) is provided in the path of the waves comprising a number of electrically conductive strips (3) in spaced, parallel relationship on the substrate surface, with a common strip (7) connecting the conductive strips. Means (e.g. optical) are provided for varying the conductivity of the common strip (7) in dependence on the required reference function. Means (11, 13) are also provided for detecting SAWs generated by the convolver (1). <IMAGE>

Description

SPECIFICATION Surface Acoustic Wave Device This invention relates to surface acoustic wave (SAW) devices. In particular the invention relates to SAW devices for providing a convolution between a SAW, and a reference function.

Devices are known for providing a convolution between an input signal and a fixed reference function. Such devices find application in, for example, signal encoding in communication systems.

It is an object of the present invention to provide a SAW device capable of providing a convolution between an input signal and a variable reference function.

According to the present invention a surface acoustic wave device comprises: a substrate; first transducer means for causing surface acoustic waves representing an input signal to propagate along a surface of the substrate; a convolver in the path of said waves comprising a plurality of electrically conductive strips on the substrate surface in spaced parallel relationship and a common strip connecting said conductive strips; means for varying the conductivity of said common strip; and second transducer means for detecting surface acoustic waves generated by said convolver.

In one particular device in accordance with the invention said common strip is made of a photoconducting material and said means for varying said conductivity comprises a controllable light source arranged-to direct light onto said strip and control means for varying the intensity of the light from said source falling on said strip.

In one such particular device, said control means is arranged to momentarily switch off said light source at times dependent on said reference function.

One SAW device in accordance with the invention will now be described, by way of example only, with reference to the accompanying figure which is a schematic diagram of the device.

Referring to the figure the device includes a SAW convolver 1 consisting of a number of metal strips 3 in spaced, parallel relationship on a piezoelectric substrate 5. Connecting the metal strips 3 is a layer of a photoconductive material 7.

Also on the substrate 5, a distance away from the convolver 1 are input and output SAW interdigital transducers 9 and 11 positioned as shown in the figure. Between the two transducers 9, 11 and the convolver there is a multistrip coupler 1 3.

In use of the device, a surface acoustic wave representing an applied input signal is propagated in the substrate 5 by the input transducer 9, the wave travelling towards the convolver 1 via the multistrip coupler 1 3. Light from a light source (not shown) is shone onto the photoconductive layer 7, rendering it conductive. The propagated wave produces corresponding potentials at the substrate surface. Hence, as the wave passes beneath the metal strips 3 of the convolver 1, a redistribution of charge between the strips 3 will take place which reflects the profile of the wave. If the light is then suddenly switched off, the strips 3 are suddenly electrically isolated from each other, the charges on the individual strips thus being trapped and thus retaining a memory of the wave.If there are at least two strips per period, corresponding to the Nyquist rate, this memory will be unambiguous. If the light is switched on again, the strips 3 will again become electrically connected causing the charges on them to collapse and produce a wave in the substrate corresponding to the stored distribution.

Separation of the output SAW thus generated by the convolver 1 towards the transducers 9, 11 from the input SAW generated by the input transducer 9 is achieved by the multistrip coupler 13 which transfers some of the output SAW to a second track to be detected by the output transducer 11.

To obtain convolution of the input signal with a reference function the light source is modulated so as to generate a light signal which is normally on, but which is periodically switched off for a very short time interval, the time between the switching off intervals being variable in accordance with the desired reference function for the particular input signal. This produces an effective reference function R(t) which can be approximated to a series of delta functions S(tTn) where Tn corresponds to the times at which the light is switched off, if it is assumed that the photoconductive layer 7 has ideal properties in that it switches from zero to infinite resistance and back.If the input SAW signal generated by the transducer 9 is a function f(t), then ideally the output signal of the convolver 1 would be the convolution integral of R(t) and f(t) I.e.

The effect of the delta function light modulation is then to store a spatial image of the incoming wave in the convolver 1, and then to release it almost immediately so that an output is obtained which consists of a series of replicas of the original signal, each centred at time intervals corresponding to Tn. As the effect of the convolver is relatively weak, it may be assumed that the magnitude of the input signals which pass through the convolver are not significantly diminished by the process, and that second order effects, such as the subsequent storage of released signals may be neglected.

It will be appreciated that the convolver will in fact product two waves, one propagating in the same direction as the input wave, and one propagating in the reverse direction, back to the transducers 9, 1 The forward waves, however, are not useful in that they will all coincide with the input signal, but it can easily be seen that the backward waves will produce an output of the form

Thus the output of the transducer 11 is a convolution between the delta function sequence R(t) and the time reversed signal f(-t), i.e. each replica of the input signal in the series is a mirror image of the original input signal.By altering the time between the switching off of the light source, the form of R(t) can readily be varied, in correspondence to the required reference function for the particular input function f(t).

Considering now the selection of a photoconductive material for the layer 7 in a simple model of a photoconductive material, the excess carrier concentration of electrons or holes is described by an equation of the type: dn n +=G( 1 +Meiwt) dt T where n=excess carrier concentration T=free carrier recombination time G=average rate of generation of electron-hole pairs (proportional to light intensity) M=modulation index and w=modulation frequency.

The solution of this equation is readily shown to be: M n=lG(1 + & t)+Ce ( 1 +jut) where C is a constant of integration.

If T1 w then the steady state value of n is given approximately by n~TG(1 +MeiWt) and the modulation is exactly reproducible.

If however T-1 W then MG n~TGi- eiwt w and the effective modulation of n will decrease as w1. Thus the parameter T is very important, as a slow response time would degrade both the charge storage and release mechanisms of the strips 3 of the convolver 1. In practice speeds of around 10-8 secs. are desirable.

It will be appreciated that whilst in the particular example described herebefore, the reference function R(t) is a delta function sequence, other reference functions are equally possible these being chosen in correspondence to the required reference function for the particular input function f(t). In particular a delayed replica of an input signal may be achieved by leaving the light switched off for a period of time. For a general analogue reference signal the resistance of the connecting layer will be a continuous variable, but a second order mixing term of the required form would still be produced. Detailed analysis of this situation is obviously more complicated than for the special example given, but the principle is unaltered.

Claims (5)

1. A surface acoustic wave device comprising: a substrate; first transducer means for causing surface acoustic waves represer..ing an input signal to propagate along a surface of the substrate; a convolver in the path of said waves comprising a plurality of electrically conductive strips on the substrate surface in spaced parallel relationship and a common strip connecting said conductive strips; means for varying the conductivity of said common strip; and second transducer means for detecting surface acoustic waves generated by said convolver.

2. A surface acoustic wave device according to Claim 1 in which said common strip is made of a photoconducting material, and said means for varying said conductivity comprises a controllable light source arranged to direct light onto said strip and control means for varying the intensity of the light from said source falling on said strip.

3. A surface acoustic wave device according to Claim 2 in which said control means is arranged to momentarily switch off said light source at times dependent on said reference function.

4. A surface acoustic wave device according to any one of the preceding claims in which said second transducer means includes a multistrip coupler disposed between said first transducer means and said convolver, said coupler being effective to transfer surface acoustic waves generated by said convolver to a path on said substrate away from the path of waves produced by said first transducer means.

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