Patent GB-1070170-A (GB)
Title: Improvements in or relating to ultrasonic devices
IPC: H03H9/64
Filed: 1964-12-18
Abstract: 1070,170. Ultrasonic devices. WESTERN ELECTRIC CO. Inc. Dec. 18, 1964 [Dec. 24, 1963], No. 51515/64. Heading H3U. An ultrasonic device comprises a body of piezoelectric material arranged to couple between electrical signals and an ultrasonic Rayleigh surface wave having a component of particle displacement normal to one surface of the body, first electrode means including a plurality of conductive members adjacent said one surface of the body, and second electrode means spaced from the first electrode means and disposed in coupling relation to the body and to the first electrode means. The plurality of conductive members may be acoustically separated from each other according to various functions and electrically connected to each other to produce bandpass filter characteristics, delay versus frequency characteristics that may be uniform with frequency or may vary with frequency according to any desired function, and amplitude transfer characteristics that vary with frequency according to any desired function. In one embodiment (Fig. 1), a delay line 10 of any suitable ultrasonic propagation material having piezoelectric properties, e.g. a suitably cut quartz crystal, ADP (ammonium dihydrogen phosphate), cadmium sulphide, sodium potassium niobate or barium titanate, is provided at one end with a wedge 14 to the end face 13 of which a conventional piezoelectric crystal or ceramic transducer 16 is bonded, an electrical signal source 17 being coupled to the transducer 16. The wedge 14 is preferably formed from a medium having an elastic wave acoustical impedance which is significantly lower than that of the delay line 10, suitable media for this purpose being plastics and containers filled with liquids, but materials of elastic wave impedance more nearly that of the line 10 may also be used if an effective impedance discontinuity. is produced by interposing a viscous or liquid layer between the wedge 14 and the line 10. The angle # between the axis 19, perpendicular to face 13, and the opposite face 15 of the wedge 14 is selected so that the mode of ultrasonic propagation generated by the transducer 16 in the wedge 14 has a velocity component parallel to the upper surface 11 of the line 10 that is equal to the longitudinal velocity of the Rayleigh surface wave along line 10. The Rayleigh surface wave travels along surface 11 and is characterized by particle displacements in at least two perpendicular directions, i.e. normal to surface 11 and along the direction of wave propagation. A plurality of output electrodes 21 to 22, numbering from several hundred to several thousand, are located along surface 11 and comprise thin narrow conductive strips each extending transversely across surface 11 and having a width no greater than one-half wavelength. As the surface wave passes the electrodes 21 to 22, the strains which it sets up in the piezoelectric material of line 10 adjacent the electrodes cause electric fields to form between these electrodes and a ground electrode 24 on the lower surface 12 of the line 10. The remaining energy is dissipated in an acoustical wave absorber 18 located on the end of line 10 remote from the wedge 14. If all electrodes 21 to 22 are electrically connected by a low impedance connection, as by closing switches 23, the structure becomes a bandpass filter. The centre frequency of the band is that frequency for which the electrode spacing is one wavelength of the surface wave. Only for this frequency are the voltages from the electrodes in phase. In order to modify the delay time versus frequency arrangement of the components in a broadband signal, the electrodes are spaced from each other by amounts that vary with distance along the line according to the desired frequency versus delay characteristic and electrically connected together (Fig. 2, not shown). An amplitude transfer characteristic may be introduced merely by controlling the number and/or size of adjacent electrodes having a spacing of one wavelength of the surface wave at each frequency. The devices described are fully reciprocal and may be used in pairs, one member of the pair being used to provide a Rayleigh surface wave as shown in Fig. 3. A delay line 30 of piezoelectric material has two arrays of electrodes 31 to 32 and 34 to 35 arranged on one surface with a spacing between electrodes that varies according to the desired dispersion characteristic, each array being opposed by a ground electrode 33, 36 respectively. If a multifrequency electrical signal is applied between the input terminals 38, the low frequency components will be launched as an ultrasonic wave in the Rayleigh surface mode by electrodes 31 while successively higher frequencies will be launched by electrodes 32. In converse order, the high frequency components are detected by the electrodes 34 and lower frequency components are successively detected as electrodes 35 are approached.