Patent GB-1330143-A (GB)
Title: Acoustic surface wave devices
IPC: H03H9/68
Filed: 1971-04-19
Abstract: 1330143 Acoustic surface wave devices DEFENCE SECRETARY OF STATE FOR 19 April 1971 [13 Feb 1970] 6972/70 Heading H3U An acoustic surface wave device includes a body of a material which has an electrostrictive tensor and/or an electro-elastic stiffness tensor such that the velocity of an acoustic wave travelling in the surface of the body in a first given direction can be varied by varying an electric field applied in a second given direction, and means for applying an electric field to the body in the second direction. The underlying theory is developed in order to define the electrostrictive tensor and electro-elastic stiffness tensor and it is shown that a material whose electrostrictive tensors or electro-elastic stiffness tensors have elements that are sufficiently large, behaves like a material in which the elastic stiffness constant and hence the velocity of acoustic waves, can be varied by means of an externally applied voltage. The material may also have piezoelectric properties but this effect is preferably rendered very small by suitable choice of orientation of field and crystal directions. Suitable materials include ferroelectric materials near their Curie temperature such as tri-glycine sulphate or potassium tantalum niobate. Fig. 1 shows an embodiment of the device functioning as a phase shifter, this device including a body 1 of the specified material supporting electrodes 3, 5 connected to a variable voltage source 7. An acoustic wave A propagates in the surface 2 of the body 1 in the direction of the arrow and since the velocity of the wave between the electrodes 3, 5 may be varied by the voltage source 7 the phase of the wave at electrode 5 may be varied relatively to that at electrode 3. In a modification (Fig. 2, not shown) the electrodes are parallel to the direction of travel of the wave. In another embodiment of the phase shifter, Fig. 3, electrodes 17, 19 are positioned on opposite faces of the body 1 and the acoustic wave A travels in the surface 2 beneath the electrode 17. A surface wave prism, Fig. 4, for deflecting an acoustic wave travelling along a path A into a path B comprises triangular electrodes 27, 29 situated on opposite faces of the body 1. A lens device, Fig. 5, for focusing parallel acoustic surface waves A to a point D comprises lenticular electrodes 31, 33 deposited on opposite surfaces of the body 1. In the devices of Figs. 4, 5 the amount of deflection or the focusing point may be varied by varying the voltage source 7. An acoustic wave-guide, Fig. 6, comprises parallel conducting strips 35, 37 on the surface 2 of the body 1 and Figs. 7, 8 show other waveguide embodiments. A device functioning as a variable coupling directional coupler, Figs. 9, 10, comprises waveguides 51, 53 arranged side-byside. The waveguide 53 is not straight but has a short portion 55 nearer to an isolated part 57 of the waveguide 51. Metal strips 59 are deposited on the opposite surface of body 1 exactly opposite to the waveguides 51, 53. The variable voltage source 7 is connected between the strip 57 and its opposite strip 61 and an additional voltage source is connected as shown. Voltage source 7 permits the local electric field between the strips 57, 61 to be different from that elsewhere along the waveguides 51, 53. Thus, the part of waveguide 51 at strip 57 may be made leaky through field dependent velocity changes in the waveguide wall in a controlled manner whereby the amount of acoustic flux transferred from one waveguide to the other may be varied. In a modification (Fig. 11, not shown) the strip 57 is common to both waveguides 51, 53. The device of Fig. 12 functions as a multiple (or Bragg) reflector and comprises a number of parallel conducting strips 69 arranged between electrodes 71, 73 connected to a voltage source 75. An acoustic wave propagated in the surface 2 in the direction A is reflected back in direction E, these directions making an angle 6 with the normal to the strips 69 and the wavelength X of the acoustic wave being related to the angle # and the distance d between the strips by the equation # = 2d cos #. Figs. 16, 17 show an embodiment of the device functioning as an interdigital comb transducer, the digital spacing of the comb electrodes 81, 83 being constant. The opposite surface of the body 1 carries strip electrodes 85 exactly opposite the electrodes 81, 83, the electrodes 85 being connected to tapping points of a potentiometer 87. The potentiometer causes the electric field to vary along the length of the transducer to give it dispersive properties. The degree of dispersion is variable by varying the voltage sources 89, 93. In a final embodiment, Fig. 18, which shows a phase shifter, a film 12 of the material specified is deposited on a substrate 10 which is not of the specified material. An electrode 17 is deposited on the substrate 10 and an electrode 19 is deposited on the film 12. An acoustic surface wave is propagated by means not shown in the surface of the substrate 10 and this wave meets the film 12 during travel. Because the film 12 is thin it picks up acoustic surface waves in the substrate and transfers them back to the substrate. The phase of the acoustic surface waves in the substrate 10 where they emerge from the film 12 relative to the phase where they enter the film depends upon the velocity of the waves in the film and this is variable by the variable voltage source 7.