GB1517371A - Acoustic wave devices - Google Patents

Abstract

1517371 Acoustic wave devices; frequency discriminators DEFENCE SECRETARY OF STATE FOR 6 Jan 1976 [15 Jan 1975] 1813/75 Headings H3U and H3A An acoustic wave device employing surface waves or surface skimming bulk waves comprises input and output transducer means at least one of which comprises a plurality of spaced transducers connected to an electromagnetic delay line so that acoustic waves are launched from different parts of the transducer means at different times. As shown in Fig. I transducer T, comprises spaced groups of fingers 3 1-6 interconnected by delay line elements 6 1-5 constituted by coaxial cables, which may be terminated by a resistance 7. The effect of the delay line is that acoustic waves of respective frequencies f 2 ,f 3 , both slightly shifted from the centre frequency f 0 of the device, are directed towards output transducers T 2 ,T 3 ; thus an input signal containing two binary-coded trains may be split into two binary outputs. In a modification the delay line is constituted by two spaced strips (21, 22, Fig. 4 not shown) on the substrate 1 with fingers (23) in the space between the strips; alternatively the delay line may be provided on another substrate, e.g. of a ferroelectric medium in the vicinity of a phase transition. In a further modification the delay line is formed by the bus-bars (26, 27 Fig. 5 not shown) of the transducer T 1 itself; the inductance of the line can be varied by adjusting the bus-bar thickness or printing flat coils and the capacitance can be varied by means of the transducer aperture. A variable delay may be obtained by providing PIN diodes or varactors between the groups of fingers. An oscillator incorporating the device connected to an amplifier 40 is shown in Fig. 7. By varying the electromagnetic delay, the acoustic phase shift is also varied and hence the frequency is shifted. Both transducers may have an associated electromagnetic delay line. Fig. 8 shows a discriminator circuit in which the outputs from the extreme finger-groups 3 of transducer 47 are fed via detector circuits 49, 55 to a differential amplifier 52 and output terminals 54, 58. The application of a swept frequency signal to transducer 46, produces responses at f2, f3 (Fig. 9 not shown). Specification 1,451,326 is referred to.