GB1372235A - Acoustic surface wave devices - Google Patents

Abstract

1372235 Surface wave devices DEFENCE SECRETARY OF STATE FOR 4 May 1972 [5 May 1971 7 Jan 1972] 13125/71 and 749/72 Heading H3U Acoustic surface or interface wave devices include couplers comprising spaced conducting filaments electrically insulated one from another, so that surface waves may thereby be transferred from one region of a substrate to another. Disclosed theory indicates that, by varying the coupler length in the direction of wave propagation, surface waves may be entirely transferred from one track to another, Fig. 1, or an input wave may be split either into two equal amplitude output waves in quadrature, Fig. 4, or unequally split. The tracks may be of equal width or otherwise. The substrate is quartz, Y cut lithium niobate with propagation in the Z-direction, lithium germanate, aluminium nitride or layers of such material may be on an acoustic surface wave supporting, non-piezoelectric substrate such as glass. Alternatively a glass substrate may be used, with zinc oxide sputtered adjacent, and either over or under the electrodes. To avoid the radiation of acoustic surface waves by those portions of the couplers removed from any wave track, such connecting portions may be mounted on a layer of non- piezo-electric material such as silica supported on the substrate via a metal layer, or on a non-piezo-electric portion of the substrate. Alternatively there may be velocity mismatch, or acoustic surface wave attenuating material, under connecting portions, or they may be mounted in such direction that they do not couple to the substrate. As an alternative to piezo-electric couplers and electrodes, electrostrictive with bias in the track regions magnetostrictive, or closed loops of filaments with a normal biasing magnetic field types of couplers or electrodes may be used. Coupler details.-The coupler 5, Fig. 1, of length L for complete beam transfer from track A to track B may have the filaments in the latter track curved, so that the beam converges to a point (0), Fig. 2 (not shown), the input to an acoustic surface waveguide. The coupler may bridge a pair of disjoint substrates (11, 13), Fig. 3 (not shown), cemented to a base, the length of the coupler being different on the two substrates if the piezo-electric properties of each are distinct. The coupler 5 comprises a plurality of vapour deposited filaments, whose conductor spacing is regular, random, or monotonically varying. To avoid spurious reflections, coupler matching portions, Figs. 32, 33 (not shown), may be provided, comprising filaments of progressively changing angled portions or varying length. Conductor widths may be varied. Couplers may have a J-shape leading to track change and direction reversal, tracks (A, B) Figs. 11, 12 (not shown) being the radiating portions of the couplers. Applications. (I) Filter, Fig. 26, Amplification.-An acoustic surface wave recirculates in tracks B, C structures 189, 191 including track changers, and folded couplers, Figs. 18, 19 (not shown), which reflect a wave impinging on them. Input and output electrodes 187, 195, receive energy coupled out by units 185, 193, the output having spikes at resonance frequencies of the recirculating wave, an auxiliary output electrode 187 having corresponding notches. Input and output transducers may have a undirectional receiving/ transmitting characteristic obtained by enclosing an interdigital transducer (105), Figs. 16, 17 (not shown), within a set of U or 0-shaped filaments forming a quadrature coupler such that a pair of waves in quadrature are generated on respective sides of the transducer (105), and resulting in the emission of waves on one side only. The beam width may be changed by such an arrangement, Fig. 20 (not shown), or by feeding a pair of such transducers via a tunnel diode negative resistance circuit, and combining them with a track changer (163, Fig. 25) an amplifying, reflecting track changer is produced. (II) Delay lines disclosed include the tapped type, Fig. 9; Figs. 10, 21, 22 (not shown) a reflecting delay line, Fig. 24 (not shown), and one with LR tuned output electrodes to avoid triple echo, Fig. 23 (not shown), Fig. 14 discloses a reoirculating delay line with angle couplers, and may be modified to provide the folded path of Fig. 15 (not shown). In Fig. 13 (not shown) the recirculation involves two tracks (C, D). The recirculating delay lines may be used as a resonator, if the energy tapped out is restricted. (III) Beam splitting, hybridizing.-In Fig. 4, a beam from A is split into two beams in quadrature received at transducers 7, 21. In Fig. 5 (not shown) inputs to transducers (3, 23) appears in one track or the other depending on the phase sequence of a pair of inputs in quadrature. By displacing the filaments of the coupler in one of the tracks, see (19) Fig. 8 (not shown). The sum and difference of signals received at (3) and (23) appear, respectively at (21), (7). By providing a region (211), Fig. 27 (not shown), between a pair of couplers (207, 209) which can be electrically or magnetically controlled to vary the velocity of propagation beneath it, variable admixture is possible. Should a pair of waves in phase, respectively antiphase, in adjacent tracks impinge on a coupler of variable filament length, Figs. 30, 31 (not shown), the former, inducing no coupler current, pass undeflected (Fig. 31), the latter being refracted (Fig. 30). Facility may be provided for enabling conductive interconnection of the filaments to inhibit surface wave energy transfer, Figs. 34-39 (not shown), using a photoconductive layer (Fig. 34), integrated circuit transistors (Figs. 36, 37), or variable capacitance diodes fed by controllable D.C. (Figs. 38, 39) to change the beam splitting and track changing characteristics. (IV) Beam width compressing is achieved by projecting a pair of waves in quadrature, from (25), Fig. 6, on to a quadrature coupler (35) to provide a restricted width output at (37). By displacing portions of successive couplers (43, 45, 47) Fig. 7 (not shown) so that they themselves produce the quadrature effect, iterative beam width compression is possible.